xref: /linux/drivers/platform/surface/aggregator/ssh_request_layer.c (revision 24bce201d79807b668bf9d9e0aca801c5c0d5f78)

// SPDX-License-Identifier: GPL-2.0+
/*
 * SSH request transport layer.
 *
 * Copyright (C) 2019-2021 Maximilian Luz <luzmaximilian@gmail.com>
 */

#include <asm/unaligned.h>
#include <linux/atomic.h>
#include <linux/completion.h>
#include <linux/error-injection.h>
#include <linux/ktime.h>
#include <linux/limits.h>
#include <linux/list.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/types.h>
#include <linux/workqueue.h>

#include <linux/surface_aggregator/serial_hub.h>
#include <linux/surface_aggregator/controller.h>

#include "ssh_packet_layer.h"
#include "ssh_request_layer.h"

#include "trace.h"

/*
 * SSH_RTL_REQUEST_TIMEOUT - Request timeout.
 *
 * Timeout as ktime_t delta for request responses. If we have not received a
 * response in this time-frame after finishing the underlying packet
 * transmission, the request will be completed with %-ETIMEDOUT as status
 * code.
 */
#define SSH_RTL_REQUEST_TIMEOUT			ms_to_ktime(3000)

/*
 * SSH_RTL_REQUEST_TIMEOUT_RESOLUTION - Request timeout granularity.
 *
 * Time-resolution for timeouts. Should be larger than one jiffy to avoid
 * direct re-scheduling of reaper work_struct.
 */
#define SSH_RTL_REQUEST_TIMEOUT_RESOLUTION	ms_to_ktime(max(2000 / HZ, 50))

/*
 * SSH_RTL_MAX_PENDING - Maximum number of pending requests.
 *
 * Maximum number of requests concurrently waiting to be completed (i.e.
 * waiting for the corresponding packet transmission to finish if they don't
 * have a response or waiting for a response if they have one).
 */
#define SSH_RTL_MAX_PENDING		3

/*
 * SSH_RTL_TX_BATCH - Maximum number of requests processed per work execution.
 * Used to prevent livelocking of the workqueue. Value chosen via educated
 * guess, may be adjusted.
 */
#define SSH_RTL_TX_BATCH		10

#ifdef CONFIG_SURFACE_AGGREGATOR_ERROR_INJECTION

/**
 * ssh_rtl_should_drop_response() - Error injection hook to drop request
 * responses.
 *
 * Useful to cause request transmission timeouts in the driver by dropping the
 * response to a request.
 */
static noinline bool ssh_rtl_should_drop_response(void)
{
	return false;
}
ALLOW_ERROR_INJECTION(ssh_rtl_should_drop_response, TRUE);

#else

static inline bool ssh_rtl_should_drop_response(void)
{
	return false;
}

#endif

static u16 ssh_request_get_rqid(struct ssh_request *rqst)
{
	return get_unaligned_le16(rqst->packet.data.ptr
				  + SSH_MSGOFFSET_COMMAND(rqid));
}

static u32 ssh_request_get_rqid_safe(struct ssh_request *rqst)
{
	if (!rqst->packet.data.ptr)
		return U32_MAX;

	return ssh_request_get_rqid(rqst);
}

static void ssh_rtl_queue_remove(struct ssh_request *rqst)
{
	struct ssh_rtl *rtl = ssh_request_rtl(rqst);

	spin_lock(&rtl->queue.lock);

	if (!test_and_clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &rqst->state)) {
		spin_unlock(&rtl->queue.lock);
		return;
	}

	list_del(&rqst->node);

	spin_unlock(&rtl->queue.lock);
	ssh_request_put(rqst);
}

static bool ssh_rtl_queue_empty(struct ssh_rtl *rtl)
{
	bool empty;

	spin_lock(&rtl->queue.lock);
	empty = list_empty(&rtl->queue.head);
	spin_unlock(&rtl->queue.lock);

	return empty;
}

static void ssh_rtl_pending_remove(struct ssh_request *rqst)
{
	struct ssh_rtl *rtl = ssh_request_rtl(rqst);

	spin_lock(&rtl->pending.lock);

	if (!test_and_clear_bit(SSH_REQUEST_SF_PENDING_BIT, &rqst->state)) {
		spin_unlock(&rtl->pending.lock);
		return;
	}

	atomic_dec(&rtl->pending.count);
	list_del(&rqst->node);

	spin_unlock(&rtl->pending.lock);

	ssh_request_put(rqst);
}

static int ssh_rtl_tx_pending_push(struct ssh_request *rqst)
{
	struct ssh_rtl *rtl = ssh_request_rtl(rqst);

	spin_lock(&rtl->pending.lock);

	if (test_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state)) {
		spin_unlock(&rtl->pending.lock);
		return -EINVAL;
	}

	if (test_and_set_bit(SSH_REQUEST_SF_PENDING_BIT, &rqst->state)) {
		spin_unlock(&rtl->pending.lock);
		return -EALREADY;
	}

	atomic_inc(&rtl->pending.count);
	list_add_tail(&ssh_request_get(rqst)->node, &rtl->pending.head);

	spin_unlock(&rtl->pending.lock);
	return 0;
}

static void ssh_rtl_complete_with_status(struct ssh_request *rqst, int status)
{
	struct ssh_rtl *rtl = ssh_request_rtl(rqst);

	trace_ssam_request_complete(rqst, status);

	/* rtl/ptl may not be set if we're canceling before submitting. */
	rtl_dbg_cond(rtl, "rtl: completing request (rqid: %#06x, status: %d)\n",
		     ssh_request_get_rqid_safe(rqst), status);

	rqst->ops->complete(rqst, NULL, NULL, status);
}

static void ssh_rtl_complete_with_rsp(struct ssh_request *rqst,
				      const struct ssh_command *cmd,
				      const struct ssam_span *data)
{
	struct ssh_rtl *rtl = ssh_request_rtl(rqst);

	trace_ssam_request_complete(rqst, 0);

	rtl_dbg(rtl, "rtl: completing request with response (rqid: %#06x)\n",
		ssh_request_get_rqid(rqst));

	rqst->ops->complete(rqst, cmd, data, 0);
}

static bool ssh_rtl_tx_can_process(struct ssh_request *rqst)
{
	struct ssh_rtl *rtl = ssh_request_rtl(rqst);

	if (test_bit(SSH_REQUEST_TY_FLUSH_BIT, &rqst->state))
		return !atomic_read(&rtl->pending.count);

	return atomic_read(&rtl->pending.count) < SSH_RTL_MAX_PENDING;
}

static struct ssh_request *ssh_rtl_tx_next(struct ssh_rtl *rtl)
{
	struct ssh_request *rqst = ERR_PTR(-ENOENT);
	struct ssh_request *p, *n;

	spin_lock(&rtl->queue.lock);

	/* Find first non-locked request and remove it. */
	list_for_each_entry_safe(p, n, &rtl->queue.head, node) {
		if (unlikely(test_bit(SSH_REQUEST_SF_LOCKED_BIT, &p->state)))
			continue;

		if (!ssh_rtl_tx_can_process(p)) {
			rqst = ERR_PTR(-EBUSY);
			break;
		}

		/* Remove from queue and mark as transmitting. */
		set_bit(SSH_REQUEST_SF_TRANSMITTING_BIT, &p->state);
		/* Ensure state never gets zero. */
		smp_mb__before_atomic();
		clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &p->state);

		list_del(&p->node);

		rqst = p;
		break;
	}

	spin_unlock(&rtl->queue.lock);
	return rqst;
}

static int ssh_rtl_tx_try_process_one(struct ssh_rtl *rtl)
{
	struct ssh_request *rqst;
	int status;

	/* Get and prepare next request for transmit. */
	rqst = ssh_rtl_tx_next(rtl);
	if (IS_ERR(rqst))
		return PTR_ERR(rqst);

	/* Add it to/mark it as pending. */
	status = ssh_rtl_tx_pending_push(rqst);
	if (status) {
		ssh_request_put(rqst);
		return -EAGAIN;
	}

	/* Submit packet. */
	status = ssh_ptl_submit(&rtl->ptl, &rqst->packet);
	if (status == -ESHUTDOWN) {
		/*
		 * Packet has been refused due to the packet layer shutting
		 * down. Complete it here.
		 */
		set_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state);
		/*
		 * Note: A barrier is not required here, as there are only two
		 * references in the system at this point: The one that we have,
		 * and the other one that belongs to the pending set. Due to the
		 * request being marked as "transmitting", our process is the
		 * only one allowed to remove the pending node and change the
		 * state. Normally, the task would fall to the packet callback,
		 * but as this is a path where submission failed, this callback
		 * will never be executed.
		 */

		ssh_rtl_pending_remove(rqst);
		ssh_rtl_complete_with_status(rqst, -ESHUTDOWN);

		ssh_request_put(rqst);
		return -ESHUTDOWN;

	} else if (status) {
		/*
		 * If submitting the packet failed and the packet layer isn't
		 * shutting down, the packet has either been submitted/queued
		 * before (-EALREADY, which cannot happen as we have
		 * guaranteed that requests cannot be re-submitted), or the
		 * packet was marked as locked (-EINVAL). To mark the packet
		 * locked at this stage, the request, and thus the packets
		 * itself, had to have been canceled. Simply drop the
		 * reference. Cancellation itself will remove it from the set
		 * of pending requests.
		 */

		WARN_ON(status != -EINVAL);

		ssh_request_put(rqst);
		return -EAGAIN;
	}

	ssh_request_put(rqst);
	return 0;
}

static bool ssh_rtl_tx_schedule(struct ssh_rtl *rtl)
{
	if (atomic_read(&rtl->pending.count) >= SSH_RTL_MAX_PENDING)
		return false;

	if (ssh_rtl_queue_empty(rtl))
		return false;

	return schedule_work(&rtl->tx.work);
}

static void ssh_rtl_tx_work_fn(struct work_struct *work)
{
	struct ssh_rtl *rtl = to_ssh_rtl(work, tx.work);
	unsigned int iterations = SSH_RTL_TX_BATCH;
	int status;

	/*
	 * Try to be nice and not block/live-lock the workqueue: Run a maximum
	 * of 10 tries, then re-submit if necessary. This should not be
	 * necessary for normal execution, but guarantee it anyway.
	 */
	do {
		status = ssh_rtl_tx_try_process_one(rtl);
		if (status == -ENOENT || status == -EBUSY)
			return;		/* No more requests to process. */

		if (status == -ESHUTDOWN) {
			/*
			 * Packet system shutting down. No new packets can be
			 * transmitted. Return silently, the party initiating
			 * the shutdown should handle the rest.
			 */
			return;
		}

		WARN_ON(status != 0 && status != -EAGAIN);
	} while (--iterations);

	/* Out of tries, reschedule. */
	ssh_rtl_tx_schedule(rtl);
}

/**
 * ssh_rtl_submit() - Submit a request to the transport layer.
 * @rtl:  The request transport layer.
 * @rqst: The request to submit.
 *
 * Submits a request to the transport layer. A single request may not be
 * submitted multiple times without reinitializing it.
 *
 * Return: Returns zero on success, %-EINVAL if the request type is invalid or
 * the request has been canceled prior to submission, %-EALREADY if the
 * request has already been submitted, or %-ESHUTDOWN in case the request
 * transport layer has been shut down.
 */
int ssh_rtl_submit(struct ssh_rtl *rtl, struct ssh_request *rqst)
{
	trace_ssam_request_submit(rqst);

	/*
	 * Ensure that requests expecting a response are sequenced. If this
	 * invariant ever changes, see the comment in ssh_rtl_complete() on what
	 * is required to be changed in the code.
	 */
	if (test_bit(SSH_REQUEST_TY_HAS_RESPONSE_BIT, &rqst->state))
		if (!test_bit(SSH_PACKET_TY_SEQUENCED_BIT, &rqst->packet.state))
			return -EINVAL;

	spin_lock(&rtl->queue.lock);

	/*
	 * Try to set ptl and check if this request has already been submitted.
	 *
	 * Must be inside lock as we might run into a lost update problem
	 * otherwise: If this were outside of the lock, cancellation in
	 * ssh_rtl_cancel_nonpending() may run after we've set the ptl
	 * reference but before we enter the lock. In that case, we'd detect
	 * that the request is being added to the queue and would try to remove
	 * it from that, but removal might fail because it hasn't actually been
	 * added yet. By putting this cmpxchg in the critical section, we
	 * ensure that the queuing detection only triggers when we are already
	 * in the critical section and the remove process will wait until the
	 * push operation has been completed (via lock) due to that. Only then,
	 * we can safely try to remove it.
	 */
	if (cmpxchg(&rqst->packet.ptl, NULL, &rtl->ptl)) {
		spin_unlock(&rtl->queue.lock);
		return -EALREADY;
	}

	/*
	 * Ensure that we set ptl reference before we continue modifying state.
	 * This is required for non-pending cancellation. This barrier is paired
	 * with the one in ssh_rtl_cancel_nonpending().
	 *
	 * By setting the ptl reference before we test for "locked", we can
	 * check if the "locked" test may have already run. See comments in
	 * ssh_rtl_cancel_nonpending() for more detail.
	 */
	smp_mb__after_atomic();

	if (test_bit(SSH_RTL_SF_SHUTDOWN_BIT, &rtl->state)) {
		spin_unlock(&rtl->queue.lock);
		return -ESHUTDOWN;
	}

	if (test_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state)) {
		spin_unlock(&rtl->queue.lock);
		return -EINVAL;
	}

	set_bit(SSH_REQUEST_SF_QUEUED_BIT, &rqst->state);
	list_add_tail(&ssh_request_get(rqst)->node, &rtl->queue.head);

	spin_unlock(&rtl->queue.lock);

	ssh_rtl_tx_schedule(rtl);
	return 0;
}

static void ssh_rtl_timeout_reaper_mod(struct ssh_rtl *rtl, ktime_t now,
				       ktime_t expires)
{
	unsigned long delta = msecs_to_jiffies(ktime_ms_delta(expires, now));
	ktime_t aexp = ktime_add(expires, SSH_RTL_REQUEST_TIMEOUT_RESOLUTION);

	spin_lock(&rtl->rtx_timeout.lock);

	/* Re-adjust / schedule reaper only if it is above resolution delta. */
	if (ktime_before(aexp, rtl->rtx_timeout.expires)) {
		rtl->rtx_timeout.expires = expires;
		mod_delayed_work(system_wq, &rtl->rtx_timeout.reaper, delta);
	}

	spin_unlock(&rtl->rtx_timeout.lock);
}

static void ssh_rtl_timeout_start(struct ssh_request *rqst)
{
	struct ssh_rtl *rtl = ssh_request_rtl(rqst);
	ktime_t timestamp = ktime_get_coarse_boottime();
	ktime_t timeout = rtl->rtx_timeout.timeout;

	if (test_bit(SSH_REQUEST_SF_LOCKED_BIT, &rqst->state))
		return;

	/*
	 * Note: The timestamp gets set only once. This happens on the packet
	 * callback. All other access to it is read-only.
	 */
	WRITE_ONCE(rqst->timestamp, timestamp);
	/*
	 * Ensure timestamp is set before starting the reaper. Paired with
	 * implicit barrier following check on ssh_request_get_expiration() in
	 * ssh_rtl_timeout_reap.
	 */
	smp_mb__after_atomic();

	ssh_rtl_timeout_reaper_mod(rtl, timestamp, timestamp + timeout);
}

static void ssh_rtl_complete(struct ssh_rtl *rtl,
			     const struct ssh_command *command,
			     const struct ssam_span *command_data)
{
	struct ssh_request *r = NULL;
	struct ssh_request *p, *n;
	u16 rqid = get_unaligned_le16(&command->rqid);

	trace_ssam_rx_response_received(command, command_data->len);

	/*
	 * Get request from pending based on request ID and mark it as response
	 * received and locked.
	 */
	spin_lock(&rtl->pending.lock);
	list_for_each_entry_safe(p, n, &rtl->pending.head, node) {
		/* We generally expect requests to be processed in order. */
		if (unlikely(ssh_request_get_rqid(p) != rqid))
			continue;

		/* Simulate response timeout. */
		if (ssh_rtl_should_drop_response()) {
			spin_unlock(&rtl->pending.lock);

			trace_ssam_ei_rx_drop_response(p);
			rtl_info(rtl, "request error injection: dropping response for request %p\n",
				 &p->packet);
			return;
		}

		/*
		 * Mark as "response received" and "locked" as we're going to
		 * complete it.
		 */
		set_bit(SSH_REQUEST_SF_LOCKED_BIT, &p->state);
		set_bit(SSH_REQUEST_SF_RSPRCVD_BIT, &p->state);
		/* Ensure state never gets zero. */
		smp_mb__before_atomic();
		clear_bit(SSH_REQUEST_SF_PENDING_BIT, &p->state);

		atomic_dec(&rtl->pending.count);
		list_del(&p->node);

		r = p;
		break;
	}
	spin_unlock(&rtl->pending.lock);

	if (!r) {
		rtl_warn(rtl, "rtl: dropping unexpected command message (rqid = %#06x)\n",
			 rqid);
		return;
	}

	/* If the request hasn't been completed yet, we will do this now. */
	if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state)) {
		ssh_request_put(r);
		ssh_rtl_tx_schedule(rtl);
		return;
	}

	/*
	 * Make sure the request has been transmitted. In case of a sequenced
	 * request, we are guaranteed that the completion callback will run on
	 * the receiver thread directly when the ACK for the packet has been
	 * received. Similarly, this function is guaranteed to run on the
	 * receiver thread. Thus we are guaranteed that if the packet has been
	 * successfully transmitted and received an ACK, the transmitted flag
	 * has been set and is visible here.
	 *
	 * We are currently not handling unsequenced packets here, as those
	 * should never expect a response as ensured in ssh_rtl_submit. If this
	 * ever changes, one would have to test for
	 *
	 *	(r->state & (transmitting | transmitted))
	 *
	 * on unsequenced packets to determine if they could have been
	 * transmitted. There are no synchronization guarantees as in the
	 * sequenced case, since, in this case, the callback function will not
	 * run on the same thread. Thus an exact determination is impossible.
	 */
	if (!test_bit(SSH_REQUEST_SF_TRANSMITTED_BIT, &r->state)) {
		rtl_err(rtl, "rtl: received response before ACK for request (rqid = %#06x)\n",
			rqid);

		/*
		 * NB: Timeout has already been canceled, request already been
		 * removed from pending and marked as locked and completed. As
		 * we receive a "false" response, the packet might still be
		 * queued though.
		 */
		ssh_rtl_queue_remove(r);

		ssh_rtl_complete_with_status(r, -EREMOTEIO);
		ssh_request_put(r);

		ssh_rtl_tx_schedule(rtl);
		return;
	}

	/*
	 * NB: Timeout has already been canceled, request already been
	 * removed from pending and marked as locked and completed. The request
	 * can also not be queued any more, as it has been marked as
	 * transmitting and later transmitted. Thus no need to remove it from
	 * anywhere.
	 */

	ssh_rtl_complete_with_rsp(r, command, command_data);
	ssh_request_put(r);

	ssh_rtl_tx_schedule(rtl);
}

static bool ssh_rtl_cancel_nonpending(struct ssh_request *r)
{
	struct ssh_rtl *rtl;
	unsigned long flags, fixed;
	bool remove;

	/*
	 * Handle unsubmitted request: Try to mark the packet as locked,
	 * expecting the state to be zero (i.e. unsubmitted). Note that, if
	 * setting the state worked, we might still be adding the packet to the
	 * queue in a currently executing submit call. In that case, however,
	 * ptl reference must have been set previously, as locked is checked
	 * after setting ptl. Furthermore, when the ptl reference is set, the
	 * submission process is guaranteed to have entered the critical
	 * section. Thus only if we successfully locked this request and ptl is
	 * NULL, we have successfully removed the request, i.e. we are
	 * guaranteed that, due to the "locked" check in ssh_rtl_submit(), the
	 * packet will never be added. Otherwise, we need to try and grab it
	 * from the queue, where we are now guaranteed that the packet is or has
	 * been due to the critical section.
	 *
	 * Note that if the cmpxchg() fails, we are guaranteed that ptl has
	 * been set and is non-NULL, as states can only be nonzero after this
	 * has been set. Also note that we need to fetch the static (type)
	 * flags to ensure that they don't cause the cmpxchg() to fail.
	 */
	fixed = READ_ONCE(r->state) & SSH_REQUEST_FLAGS_TY_MASK;
	flags = cmpxchg(&r->state, fixed, SSH_REQUEST_SF_LOCKED_BIT);

	/*
	 * Force correct ordering with regards to state and ptl reference access
	 * to safe-guard cancellation to concurrent submission against a
	 * lost-update problem. First try to exchange state, then also check
	 * ptl if that worked. This barrier is paired with the
	 * one in ssh_rtl_submit().
	 */
	smp_mb__after_atomic();

	if (flags == fixed && !READ_ONCE(r->packet.ptl)) {
		if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
			return true;

		ssh_rtl_complete_with_status(r, -ECANCELED);
		return true;
	}

	rtl = ssh_request_rtl(r);
	spin_lock(&rtl->queue.lock);

	/*
	 * Note: 1) Requests cannot be re-submitted. 2) If a request is
	 * queued, it cannot be "transmitting"/"pending" yet. Thus, if we
	 * successfully remove the request here, we have removed all its
	 * occurrences in the system.
	 */

	remove = test_and_clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &r->state);
	if (!remove) {
		spin_unlock(&rtl->queue.lock);
		return false;
	}

	set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
	list_del(&r->node);

	spin_unlock(&rtl->queue.lock);

	ssh_request_put(r);	/* Drop reference obtained from queue. */

	if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
		return true;

	ssh_rtl_complete_with_status(r, -ECANCELED);
	return true;
}

static bool ssh_rtl_cancel_pending(struct ssh_request *r)
{
	/* If the packet is already locked, it's going to be removed shortly. */
	if (test_and_set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state))
		return true;

	/*
	 * Now that we have locked the packet, we have guaranteed that it can't
	 * be added to the system any more. If ptl is NULL, the locked
	 * check in ssh_rtl_submit() has not been run and any submission,
	 * currently in progress or called later, won't add the packet. Thus we
	 * can directly complete it.
	 *
	 * The implicit memory barrier of test_and_set_bit() should be enough
	 * to ensure that the correct order (first lock, then check ptl) is
	 * ensured. This is paired with the barrier in ssh_rtl_submit().
	 */
	if (!READ_ONCE(r->packet.ptl)) {
		if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
			return true;

		ssh_rtl_complete_with_status(r, -ECANCELED);
		return true;
	}

	/*
	 * Try to cancel the packet. If the packet has not been completed yet,
	 * this will subsequently (and synchronously) call the completion
	 * callback of the packet, which will complete the request.
	 */
	ssh_ptl_cancel(&r->packet);

	/*
	 * If the packet has been completed with success, i.e. has not been
	 * canceled by the above call, the request may not have been completed
	 * yet (may be waiting for a response). Check if we need to do this
	 * here.
	 */
	if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
		return true;

	ssh_rtl_queue_remove(r);
	ssh_rtl_pending_remove(r);
	ssh_rtl_complete_with_status(r, -ECANCELED);

	return true;
}

/**
 * ssh_rtl_cancel() - Cancel request.
 * @rqst:    The request to cancel.
 * @pending: Whether to also cancel pending requests.
 *
 * Cancels the given request. If @pending is %false, this will not cancel
 * pending requests, i.e. requests that have already been submitted to the
 * packet layer but not been completed yet. If @pending is %true, this will
 * cancel the given request regardless of the state it is in.
 *
 * If the request has been canceled by calling this function, both completion
 * and release callbacks of the request will be executed in a reasonable
 * time-frame. This may happen during execution of this function, however,
 * there is no guarantee for this. For example, a request currently
 * transmitting will be canceled/completed only after transmission has
 * completed, and the respective callbacks will be executed on the transmitter
 * thread, which may happen during, but also some time after execution of the
 * cancel function.
 *
 * Return: Returns %true if the given request has been canceled or completed,
 * either by this function or prior to calling this function, %false
 * otherwise. If @pending is %true, this function will always return %true.
 */
bool ssh_rtl_cancel(struct ssh_request *rqst, bool pending)
{
	struct ssh_rtl *rtl;
	bool canceled;

	if (test_and_set_bit(SSH_REQUEST_SF_CANCELED_BIT, &rqst->state))
		return true;

	trace_ssam_request_cancel(rqst);

	if (pending)
		canceled = ssh_rtl_cancel_pending(rqst);
	else
		canceled = ssh_rtl_cancel_nonpending(rqst);

	/* Note: rtl may be NULL if request has not been submitted yet. */
	rtl = ssh_request_rtl(rqst);
	if (canceled && rtl)
		ssh_rtl_tx_schedule(rtl);

	return canceled;
}

static void ssh_rtl_packet_callback(struct ssh_packet *p, int status)
{
	struct ssh_request *r = to_ssh_request(p);

	if (unlikely(status)) {
		set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);

		if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
			return;

		/*
		 * The packet may get canceled even though it has not been
		 * submitted yet. The request may still be queued. Check the
		 * queue and remove it if necessary. As the timeout would have
		 * been started in this function on success, there's no need
		 * to cancel it here.
		 */
		ssh_rtl_queue_remove(r);
		ssh_rtl_pending_remove(r);
		ssh_rtl_complete_with_status(r, status);

		ssh_rtl_tx_schedule(ssh_request_rtl(r));
		return;
	}

	/* Update state: Mark as transmitted and clear transmitting. */
	set_bit(SSH_REQUEST_SF_TRANSMITTED_BIT, &r->state);
	/* Ensure state never gets zero. */
	smp_mb__before_atomic();
	clear_bit(SSH_REQUEST_SF_TRANSMITTING_BIT, &r->state);

	/* If we expect a response, we just need to start the timeout. */
	if (test_bit(SSH_REQUEST_TY_HAS_RESPONSE_BIT, &r->state)) {
		/*
		 * Note: This is the only place where the timestamp gets set,
		 * all other access to it is read-only.
		 */
		ssh_rtl_timeout_start(r);
		return;
	}

	/*
	 * If we don't expect a response, lock, remove, and complete the
	 * request. Note that, at this point, the request is guaranteed to have
	 * left the queue and no timeout has been started. Thus we only need to
	 * remove it from pending. If the request has already been completed (it
	 * may have been canceled) return.
	 */

	set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
	if (test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
		return;

	ssh_rtl_pending_remove(r);
	ssh_rtl_complete_with_status(r, 0);

	ssh_rtl_tx_schedule(ssh_request_rtl(r));
}

static ktime_t ssh_request_get_expiration(struct ssh_request *r, ktime_t timeout)
{
	ktime_t timestamp = READ_ONCE(r->timestamp);

	if (timestamp != KTIME_MAX)
		return ktime_add(timestamp, timeout);
	else
		return KTIME_MAX;
}

static void ssh_rtl_timeout_reap(struct work_struct *work)
{
	struct ssh_rtl *rtl = to_ssh_rtl(work, rtx_timeout.reaper.work);
	struct ssh_request *r, *n;
	LIST_HEAD(claimed);
	ktime_t now = ktime_get_coarse_boottime();
	ktime_t timeout = rtl->rtx_timeout.timeout;
	ktime_t next = KTIME_MAX;

	trace_ssam_rtl_timeout_reap(atomic_read(&rtl->pending.count));

	/*
	 * Mark reaper as "not pending". This is done before checking any
	 * requests to avoid lost-update type problems.
	 */
	spin_lock(&rtl->rtx_timeout.lock);
	rtl->rtx_timeout.expires = KTIME_MAX;
	spin_unlock(&rtl->rtx_timeout.lock);

	spin_lock(&rtl->pending.lock);
	list_for_each_entry_safe(r, n, &rtl->pending.head, node) {
		ktime_t expires = ssh_request_get_expiration(r, timeout);

		/*
		 * Check if the timeout hasn't expired yet. Find out next
		 * expiration date to be handled after this run.
		 */
		if (ktime_after(expires, now)) {
			next = ktime_before(expires, next) ? expires : next;
			continue;
		}

		/* Avoid further transitions if locked. */
		if (test_and_set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state))
			continue;

		/*
		 * We have now marked the packet as locked. Thus it cannot be
		 * added to the pending or queued lists again after we've
		 * removed it here. We can therefore re-use the node of this
		 * packet temporarily.
		 */

		clear_bit(SSH_REQUEST_SF_PENDING_BIT, &r->state);

		atomic_dec(&rtl->pending.count);
		list_move_tail(&r->node, &claimed);
	}
	spin_unlock(&rtl->pending.lock);

	/* Cancel and complete the request. */
	list_for_each_entry_safe(r, n, &claimed, node) {
		trace_ssam_request_timeout(r);

		/*
		 * At this point we've removed the packet from pending. This
		 * means that we've obtained the last (only) reference of the
		 * system to it. Thus we can just complete it.
		 */
		if (!test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
			ssh_rtl_complete_with_status(r, -ETIMEDOUT);

		/*
		 * Drop the reference we've obtained by removing it from the
		 * pending set.
		 */
		list_del(&r->node);
		ssh_request_put(r);
	}

	/* Ensure that the reaper doesn't run again immediately. */
	next = max(next, ktime_add(now, SSH_RTL_REQUEST_TIMEOUT_RESOLUTION));
	if (next != KTIME_MAX)
		ssh_rtl_timeout_reaper_mod(rtl, now, next);

	ssh_rtl_tx_schedule(rtl);
}

static void ssh_rtl_rx_event(struct ssh_rtl *rtl, const struct ssh_command *cmd,
			     const struct ssam_span *data)
{
	trace_ssam_rx_event_received(cmd, data->len);

	rtl_dbg(rtl, "rtl: handling event (rqid: %#06x)\n",
		get_unaligned_le16(&cmd->rqid));

	rtl->ops.handle_event(rtl, cmd, data);
}

static void ssh_rtl_rx_command(struct ssh_ptl *p, const struct ssam_span *data)
{
	struct ssh_rtl *rtl = to_ssh_rtl(p, ptl);
	struct device *dev = &p->serdev->dev;
	struct ssh_command *command;
	struct ssam_span command_data;

	if (sshp_parse_command(dev, data, &command, &command_data))
		return;

	if (ssh_rqid_is_event(get_unaligned_le16(&command->rqid)))
		ssh_rtl_rx_event(rtl, command, &command_data);
	else
		ssh_rtl_complete(rtl, command, &command_data);
}

static void ssh_rtl_rx_data(struct ssh_ptl *p, const struct ssam_span *data)
{
	if (!data->len) {
		ptl_err(p, "rtl: rx: no data frame payload\n");
		return;
	}

	switch (data->ptr[0]) {
	case SSH_PLD_TYPE_CMD:
		ssh_rtl_rx_command(p, data);
		break;

	default:
		ptl_err(p, "rtl: rx: unknown frame payload type (type: %#04x)\n",
			data->ptr[0]);
		break;
	}
}

static void ssh_rtl_packet_release(struct ssh_packet *p)
{
	struct ssh_request *rqst;

	rqst = to_ssh_request(p);
	rqst->ops->release(rqst);
}

static const struct ssh_packet_ops ssh_rtl_packet_ops = {
	.complete = ssh_rtl_packet_callback,
	.release = ssh_rtl_packet_release,
};

/**
 * ssh_request_init() - Initialize SSH request.
 * @rqst:  The request to initialize.
 * @flags: Request flags, determining the type of the request.
 * @ops:   Request operations.
 *
 * Initializes the given SSH request and underlying packet. Sets the message
 * buffer pointer to %NULL and the message buffer length to zero. This buffer
 * has to be set separately via ssh_request_set_data() before submission and
 * must contain a valid SSH request message.
 *
 * Return: Returns zero on success or %-EINVAL if the given flags are invalid.
 */
int ssh_request_init(struct ssh_request *rqst, enum ssam_request_flags flags,
		     const struct ssh_request_ops *ops)
{
	unsigned long type = BIT(SSH_PACKET_TY_BLOCKING_BIT);

	/* Unsequenced requests cannot have a response. */
	if (flags & SSAM_REQUEST_UNSEQUENCED && flags & SSAM_REQUEST_HAS_RESPONSE)
		return -EINVAL;

	if (!(flags & SSAM_REQUEST_UNSEQUENCED))
		type |= BIT(SSH_PACKET_TY_SEQUENCED_BIT);

	ssh_packet_init(&rqst->packet, type, SSH_PACKET_PRIORITY(DATA, 0),
			&ssh_rtl_packet_ops);

	INIT_LIST_HEAD(&rqst->node);

	rqst->state = 0;
	if (flags & SSAM_REQUEST_HAS_RESPONSE)
		rqst->state |= BIT(SSH_REQUEST_TY_HAS_RESPONSE_BIT);

	rqst->timestamp = KTIME_MAX;
	rqst->ops = ops;

	return 0;
}

/**
 * ssh_rtl_init() - Initialize request transport layer.
 * @rtl:    The request transport layer to initialize.
 * @serdev: The underlying serial device, i.e. the lower-level transport.
 * @ops:    Request transport layer operations.
 *
 * Initializes the given request transport layer and associated packet
 * transport layer. Transmitter and receiver threads must be started
 * separately via ssh_rtl_start(), after the request-layer has been
 * initialized and the lower-level serial device layer has been set up.
 *
 * Return: Returns zero on success and a nonzero error code on failure.
 */
int ssh_rtl_init(struct ssh_rtl *rtl, struct serdev_device *serdev,
		 const struct ssh_rtl_ops *ops)
{
	struct ssh_ptl_ops ptl_ops;
	int status;

	ptl_ops.data_received = ssh_rtl_rx_data;

	status = ssh_ptl_init(&rtl->ptl, serdev, &ptl_ops);
	if (status)
		return status;

	spin_lock_init(&rtl->queue.lock);
	INIT_LIST_HEAD(&rtl->queue.head);

	spin_lock_init(&rtl->pending.lock);
	INIT_LIST_HEAD(&rtl->pending.head);
	atomic_set_release(&rtl->pending.count, 0);

	INIT_WORK(&rtl->tx.work, ssh_rtl_tx_work_fn);

	spin_lock_init(&rtl->rtx_timeout.lock);
	rtl->rtx_timeout.timeout = SSH_RTL_REQUEST_TIMEOUT;
	rtl->rtx_timeout.expires = KTIME_MAX;
	INIT_DELAYED_WORK(&rtl->rtx_timeout.reaper, ssh_rtl_timeout_reap);

	rtl->ops = *ops;

	return 0;
}

/**
 * ssh_rtl_destroy() - Deinitialize request transport layer.
 * @rtl: The request transport layer to deinitialize.
 *
 * Deinitializes the given request transport layer and frees resources
 * associated with it. If receiver and/or transmitter threads have been
 * started, the layer must first be shut down via ssh_rtl_shutdown() before
 * this function can be called.
 */
void ssh_rtl_destroy(struct ssh_rtl *rtl)
{
	ssh_ptl_destroy(&rtl->ptl);
}

/**
 * ssh_rtl_start() - Start request transmitter and receiver.
 * @rtl: The request transport layer.
 *
 * Return: Returns zero on success, a negative error code on failure.
 */
int ssh_rtl_start(struct ssh_rtl *rtl)
{
	int status;

	status = ssh_ptl_tx_start(&rtl->ptl);
	if (status)
		return status;

	ssh_rtl_tx_schedule(rtl);

	status = ssh_ptl_rx_start(&rtl->ptl);
	if (status) {
		ssh_rtl_flush(rtl, msecs_to_jiffies(5000));
		ssh_ptl_tx_stop(&rtl->ptl);
		return status;
	}

	return 0;
}

struct ssh_flush_request {
	struct ssh_request base;
	struct completion completion;
	int status;
};

static void ssh_rtl_flush_request_complete(struct ssh_request *r,
					   const struct ssh_command *cmd,
					   const struct ssam_span *data,
					   int status)
{
	struct ssh_flush_request *rqst;

	rqst = container_of(r, struct ssh_flush_request, base);
	rqst->status = status;
}

static void ssh_rtl_flush_request_release(struct ssh_request *r)
{
	struct ssh_flush_request *rqst;

	rqst = container_of(r, struct ssh_flush_request, base);
	complete_all(&rqst->completion);
}

static const struct ssh_request_ops ssh_rtl_flush_request_ops = {
	.complete = ssh_rtl_flush_request_complete,
	.release = ssh_rtl_flush_request_release,
};

/**
 * ssh_rtl_flush() - Flush the request transport layer.
 * @rtl:     request transport layer
 * @timeout: timeout for the flush operation in jiffies
 *
 * Queue a special flush request and wait for its completion. This request
 * will be completed after all other currently queued and pending requests
 * have been completed. Instead of a normal data packet, this request submits
 * a special flush packet, meaning that upon completion, also the underlying
 * packet transport layer has been flushed.
 *
 * Flushing the request layer guarantees that all previously submitted
 * requests have been fully completed before this call returns. Additionally,
 * flushing blocks execution of all later submitted requests until the flush
 * has been completed.
 *
 * If the caller ensures that no new requests are submitted after a call to
 * this function, the request transport layer is guaranteed to have no
 * remaining requests when this call returns. The same guarantee does not hold
 * for the packet layer, on which control packets may still be queued after
 * this call.
 *
 * Return: Returns zero on success, %-ETIMEDOUT if the flush timed out and has
 * been canceled as a result of the timeout, or %-ESHUTDOWN if the packet
 * and/or request transport layer has been shut down before this call. May
 * also return %-EINTR if the underlying packet transmission has been
 * interrupted.
 */
int ssh_rtl_flush(struct ssh_rtl *rtl, unsigned long timeout)
{
	const unsigned int init_flags = SSAM_REQUEST_UNSEQUENCED;
	struct ssh_flush_request rqst;
	int status;

	ssh_request_init(&rqst.base, init_flags, &ssh_rtl_flush_request_ops);
	rqst.base.packet.state |= BIT(SSH_PACKET_TY_FLUSH_BIT);
	rqst.base.packet.priority = SSH_PACKET_PRIORITY(FLUSH, 0);
	rqst.base.state |= BIT(SSH_REQUEST_TY_FLUSH_BIT);

	init_completion(&rqst.completion);

	status = ssh_rtl_submit(rtl, &rqst.base);
	if (status)
		return status;

	ssh_request_put(&rqst.base);

	if (!wait_for_completion_timeout(&rqst.completion, timeout)) {
		ssh_rtl_cancel(&rqst.base, true);
		wait_for_completion(&rqst.completion);
	}

	WARN_ON(rqst.status != 0 && rqst.status != -ECANCELED &&
		rqst.status != -ESHUTDOWN && rqst.status != -EINTR);

	return rqst.status == -ECANCELED ? -ETIMEDOUT : rqst.status;
}

/**
 * ssh_rtl_shutdown() - Shut down request transport layer.
 * @rtl: The request transport layer.
 *
 * Shuts down the request transport layer, removing and canceling all queued
 * and pending requests. Requests canceled by this operation will be completed
 * with %-ESHUTDOWN as status. Receiver and transmitter threads will be
 * stopped, the lower-level packet layer will be shutdown.
 *
 * As a result of this function, the transport layer will be marked as shut
 * down. Submission of requests after the transport layer has been shut down
 * will fail with %-ESHUTDOWN.
 */
void ssh_rtl_shutdown(struct ssh_rtl *rtl)
{
	struct ssh_request *r, *n;
	LIST_HEAD(claimed);
	int pending;

	set_bit(SSH_RTL_SF_SHUTDOWN_BIT, &rtl->state);
	/*
	 * Ensure that the layer gets marked as shut-down before actually
	 * stopping it. In combination with the check in ssh_rtl_submit(),
	 * this guarantees that no new requests can be added and all already
	 * queued requests are properly canceled.
	 */
	smp_mb__after_atomic();

	/* Remove requests from queue. */
	spin_lock(&rtl->queue.lock);
	list_for_each_entry_safe(r, n, &rtl->queue.head, node) {
		set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
		/* Ensure state never gets zero. */
		smp_mb__before_atomic();
		clear_bit(SSH_REQUEST_SF_QUEUED_BIT, &r->state);

		list_move_tail(&r->node, &claimed);
	}
	spin_unlock(&rtl->queue.lock);

	/*
	 * We have now guaranteed that the queue is empty and no more new
	 * requests can be submitted (i.e. it will stay empty). This means that
	 * calling ssh_rtl_tx_schedule() will not schedule tx.work any more. So
	 * we can simply call cancel_work_sync() on tx.work here and when that
	 * returns, we've locked it down. This also means that after this call,
	 * we don't submit any more packets to the underlying packet layer, so
	 * we can also shut that down.
	 */

	cancel_work_sync(&rtl->tx.work);
	ssh_ptl_shutdown(&rtl->ptl);
	cancel_delayed_work_sync(&rtl->rtx_timeout.reaper);

	/*
	 * Shutting down the packet layer should also have canceled all
	 * requests. Thus the pending set should be empty. Attempt to handle
	 * this gracefully anyways, even though this should be dead code.
	 */

	pending = atomic_read(&rtl->pending.count);
	if (WARN_ON(pending)) {
		spin_lock(&rtl->pending.lock);
		list_for_each_entry_safe(r, n, &rtl->pending.head, node) {
			set_bit(SSH_REQUEST_SF_LOCKED_BIT, &r->state);
			/* Ensure state never gets zero. */
			smp_mb__before_atomic();
			clear_bit(SSH_REQUEST_SF_PENDING_BIT, &r->state);

			list_move_tail(&r->node, &claimed);
		}
		spin_unlock(&rtl->pending.lock);
	}

	/* Finally, cancel and complete the requests we claimed before. */
	list_for_each_entry_safe(r, n, &claimed, node) {
		/*
		 * We need test_and_set() because we still might compete with
		 * cancellation.
		 */
		if (!test_and_set_bit(SSH_REQUEST_SF_COMPLETED_BIT, &r->state))
			ssh_rtl_complete_with_status(r, -ESHUTDOWN);

		/*
		 * Drop the reference we've obtained by removing it from the
		 * lists.
		 */
		list_del(&r->node);
		ssh_request_put(r);
	}
}