nova-core/gsp/cmdq.rs

// SPDX-License-Identifier: GPL-2.0

use core::{
    cmp,
    mem,
    sync::atomic::{
        fence,
        Ordering, //
    }, //
};

use kernel::{
    device,
    dma::{
        CoherentAllocation,
        DmaAddress, //
    },
    dma_write,
    io::poll::read_poll_timeout,
    prelude::*,
    sync::aref::ARef,
    time::Delta,
    transmute::{
        AsBytes,
        FromBytes, //
    },
};

use crate::{
    driver::Bar0,
    gsp::{
        fw::{
            GspMsgElement,
            MsgFunction,
            MsgqRxHeader,
            MsgqTxHeader, //
        },
        PteArray,
        GSP_PAGE_SHIFT,
        GSP_PAGE_SIZE, //
    },
    num,
    regs,
    sbuffer::SBufferIter, //
};

/// Trait implemented by types representing a command to send to the GSP.
///
/// The main purpose of this trait is to provide [`Cmdq::send_command`] with the information it
/// needs to send a given command.
///
/// [`CommandToGsp::init`] in particular is responsible for initializing the command directly
/// into the space reserved for it in the command queue buffer.
///
/// Some commands may be followed by a variable-length payload. For these, the
/// [`CommandToGsp::variable_payload_len`] and [`CommandToGsp::init_variable_payload`] need to be
/// defined as well.
pub(crate) trait CommandToGsp {
    /// Function identifying this command to the GSP.
    const FUNCTION: MsgFunction;

    /// Type generated by [`CommandToGsp::init`], to be written into the command queue buffer.
    type Command: FromBytes + AsBytes;

    /// Error type returned by [`CommandToGsp::init`].
    type InitError;

    /// In-place command initializer responsible for filling the command in the command queue
    /// buffer.
    fn init(&self) -> impl Init<Self::Command, Self::InitError>;

    /// Size of the variable-length payload following the command structure generated by
    /// [`CommandToGsp::init`].
    ///
    /// Most commands don't have a variable-length payload, so this is zero by default.
    fn variable_payload_len(&self) -> usize {
        0
    }

    /// Method initializing the variable-length payload.
    ///
    /// The command buffer is circular, which means that we may need to jump back to its beginning
    /// while in the middle of a command. For this reason, the variable-length payload is
    /// initialized using a [`SBufferIter`].
    ///
    /// This method will receive a buffer of the length returned by
    /// [`CommandToGsp::variable_payload_len`], and must write every single byte of it. Leaving
    /// unwritten space will lead to an error.
    ///
    /// Most commands don't have a variable-length payload, so this does nothing by default.
    fn init_variable_payload(
        &self,
        _dst: &mut SBufferIter<core::array::IntoIter<&mut [u8], 2>>,
    ) -> Result {
        Ok(())
    }
}

/// Trait representing messages received from the GSP.
///
/// This trait tells [`Cmdq::receive_msg`] how it can receive a given type of message.
pub(crate) trait MessageFromGsp: Sized {
    /// Function identifying this message from the GSP.
    const FUNCTION: MsgFunction;

    /// Error type returned by [`MessageFromGsp::read`].
    type InitError;

    /// Type containing the raw message to be read from the message queue.
    type Message: FromBytes;

    /// Method reading the message from the message queue and returning it.
    ///
    /// From a `Self::Message` and a [`SBufferIter`], constructs an instance of `Self` and returns
    /// it.
    fn read(
        msg: &Self::Message,
        sbuffer: &mut SBufferIter<core::array::IntoIter<&[u8], 2>>,
    ) -> Result<Self, Self::InitError>;
}

/// Number of GSP pages making the [`Msgq`].
pub(crate) const MSGQ_NUM_PAGES: u32 = 0x3f;

/// Circular buffer of a [`Msgq`].
///
/// This area of memory is to be shared between the driver and the GSP to exchange commands or
/// messages.
#[repr(C, align(0x1000))]
#[derive(Debug)]
struct MsgqData {
    data: [[u8; GSP_PAGE_SIZE]; num::u32_as_usize(MSGQ_NUM_PAGES)],
}

// Annoyingly we are forced to use a literal to specify the alignment of
// `MsgqData`, so check that it corresponds to the actual GSP page size here.
static_assert!(align_of::<MsgqData>() == GSP_PAGE_SIZE);

/// Unidirectional message queue.
///
/// Contains the data for a message queue, that either the driver or GSP writes to.
///
/// Note that while the write pointer of `tx` corresponds to the `msgq` of the same instance, the
/// read pointer of `rx` actually refers to the `Msgq` owned by the other side.
/// This design ensures that only the driver or GSP ever writes to a given instance of this struct.
#[repr(C)]
// There is no struct defined for this in the open-gpu-kernel-source headers.
// Instead it is defined by code in `GspMsgQueuesInit()`.
struct Msgq {
    /// Header for sending messages, including the write pointer.
    tx: MsgqTxHeader,
    /// Header for receiving messages, including the read pointer.
    rx: MsgqRxHeader,
    /// The message queue proper.
    msgq: MsgqData,
}

/// Structure shared between the driver and the GSP and containing the command and message queues.
#[repr(C)]
struct GspMem {
    /// Self-mapping page table entries.
    ptes: PteArray<{ GSP_PAGE_SIZE / size_of::<u64>() }>,
    /// CPU queue: the driver writes commands here, and the GSP reads them. It also contains the
    /// write and read pointers that the CPU updates.
    ///
    /// This member is read-only for the GSP.
    cpuq: Msgq,
    /// GSP queue: the GSP writes messages here, and the driver reads them. It also contains the
    /// write and read pointers that the GSP updates.
    ///
    /// This member is read-only for the driver.
    gspq: Msgq,
}

// SAFETY: These structs don't meet the no-padding requirements of AsBytes but
// that is not a problem because they are not used outside the kernel.
unsafe impl AsBytes for GspMem {}

// SAFETY: These structs don't meet the no-padding requirements of FromBytes but
// that is not a problem because they are not used outside the kernel.
unsafe impl FromBytes for GspMem {}

/// Wrapper around [`GspMem`] to share it with the GPU using a [`CoherentAllocation`].
///
/// This provides the low-level functionality to communicate with the GSP, including allocation of
/// queue space to write messages to and management of read/write pointers.
///
/// This is shared with the GSP, with clear ownership rules regarding the command queues:
///
/// * The driver owns (i.e. can write to) the part of the CPU message queue between the CPU write
///   pointer and the GSP read pointer. This region is returned by [`Self::driver_write_area`].
/// * The driver owns (i.e. can read from) the part of the GSP message queue between the CPU read
///   pointer and the GSP write pointer. This region is returned by [`Self::driver_read_area`].
struct DmaGspMem(CoherentAllocation<GspMem>);

impl DmaGspMem {
    /// Allocate a new instance and map it for `dev`.
    fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
        const MSGQ_SIZE: u32 = num::usize_into_u32::<{ size_of::<Msgq>() }>();
        const RX_HDR_OFF: u32 = num::usize_into_u32::<{ mem::offset_of!(Msgq, rx) }>();

        let gsp_mem =
            CoherentAllocation::<GspMem>::alloc_coherent(dev, 1, GFP_KERNEL | __GFP_ZERO)?;
        dma_write!(gsp_mem, [0]?.ptes, PteArray::new(gsp_mem.dma_handle())?);
        dma_write!(
            gsp_mem,
            [0]?.cpuq.tx,
            MsgqTxHeader::new(MSGQ_SIZE, RX_HDR_OFF, MSGQ_NUM_PAGES)
        );
        dma_write!(gsp_mem, [0]?.cpuq.rx, MsgqRxHeader::new());

        Ok(Self(gsp_mem))
    }

    /// Returns the region of the CPU message queue that the driver is currently allowed to write
    /// to.
    ///
    /// As the message queue is a circular buffer, the region may be discontiguous in memory. In
    /// that case the second slice will have a non-zero length.
    fn driver_write_area(&mut self) -> (&mut [[u8; GSP_PAGE_SIZE]], &mut [[u8; GSP_PAGE_SIZE]]) {
        let tx = self.cpu_write_ptr() as usize;
        let rx = self.gsp_read_ptr() as usize;

        // SAFETY:
        // - The `CoherentAllocation` contains exactly one object.
        // - We will only access the driver-owned part of the shared memory.
        // - Per the safety statement of the function, no concurrent access will be performed.
        let gsp_mem = &mut unsafe { self.0.as_slice_mut(0, 1) }.unwrap()[0];
        // PANIC: per the invariant of `cpu_write_ptr`, `tx` is `<= MSGQ_NUM_PAGES`.
        let (before_tx, after_tx) = gsp_mem.cpuq.msgq.data.split_at_mut(tx);

        if rx <= tx {
            // The area from `tx` up to the end of the ring, and from the beginning of the ring up
            // to `rx`, minus one unit, belongs to the driver.
            if rx == 0 {
                let last = after_tx.len() - 1;
                (&mut after_tx[..last], &mut before_tx[0..0])
            } else {
                (after_tx, &mut before_tx[..rx])
            }
        } else {
            // The area from `tx` to `rx`, minus one unit, belongs to the driver.
            //
            // PANIC: per the invariants of `cpu_write_ptr` and `gsp_read_ptr`, `rx` and `tx` are
            // `<= MSGQ_NUM_PAGES`, and the test above ensured that `rx > tx`.
            (after_tx.split_at_mut(rx - tx).0, &mut before_tx[0..0])
        }
    }

    /// Returns the region of the GSP message queue that the driver is currently allowed to read
    /// from.
    ///
    /// As the message queue is a circular buffer, the region may be discontiguous in memory. In
    /// that case the second slice will have a non-zero length.
    fn driver_read_area(&self) -> (&[[u8; GSP_PAGE_SIZE]], &[[u8; GSP_PAGE_SIZE]]) {
        let tx = self.gsp_write_ptr() as usize;
        let rx = self.cpu_read_ptr() as usize;

        // SAFETY:
        // - The `CoherentAllocation` contains exactly one object.
        // - We will only access the driver-owned part of the shared memory.
        // - Per the safety statement of the function, no concurrent access will be performed.
        let gsp_mem = &unsafe { self.0.as_slice(0, 1) }.unwrap()[0];
        // PANIC: per the invariant of `cpu_read_ptr`, `xx` is `<= MSGQ_NUM_PAGES`.
        let (before_rx, after_rx) = gsp_mem.gspq.msgq.data.split_at(rx);

        match tx.cmp(&rx) {
            cmp::Ordering::Equal => (&after_rx[0..0], &after_rx[0..0]),
            cmp::Ordering::Greater => (&after_rx[..tx], &before_rx[0..0]),
            cmp::Ordering::Less => (after_rx, &before_rx[..tx]),
        }
    }

    /// Allocates a region on the command queue that is large enough to send a command of `size`
    /// bytes.
    ///
    /// This returns a [`GspCommand`] ready to be written to by the caller.
    ///
    /// # Errors
    ///
    /// - `EAGAIN` if the driver area is too small to hold the requested command.
    /// - `EIO` if the command header is not properly aligned.
    fn allocate_command(&mut self, size: usize) -> Result<GspCommand<'_>> {
        // Get the current writable area as an array of bytes.
        let (slice_1, slice_2) = {
            let (slice_1, slice_2) = self.driver_write_area();

            #[allow(clippy::incompatible_msrv)]
            (slice_1.as_flattened_mut(), slice_2.as_flattened_mut())
        };

        // If the GSP is still processing previous messages the shared region
        // may be full in which case we will have to retry once the GSP has
        // processed the existing commands.
        if size_of::<GspMsgElement>() + size > slice_1.len() + slice_2.len() {
            return Err(EAGAIN);
        }

        // Extract area for the `GspMsgElement`.
        let (header, slice_1) = GspMsgElement::from_bytes_mut_prefix(slice_1).ok_or(EIO)?;

        // Create the contents area.
        let (slice_1, slice_2) = if slice_1.len() > size {
            // Contents fits entirely in `slice_1`.
            (&mut slice_1[..size], &mut slice_2[0..0])
        } else {
            // Need all of `slice_1` and some of `slice_2`.
            let slice_2_len = size - slice_1.len();
            (slice_1, &mut slice_2[..slice_2_len])
        };

        Ok(GspCommand {
            header,
            contents: (slice_1, slice_2),
        })
    }

    // Returns the index of the memory page the GSP will write the next message to.
    //
    // # Invariants
    //
    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
    fn gsp_write_ptr(&self) -> u32 {
        let gsp_mem = self.0.start_ptr();

        // SAFETY:
        //  - The 'CoherentAllocation' contains at least one object.
        //  - By the invariants of `CoherentAllocation` the pointer is valid.
        (unsafe { (*gsp_mem).gspq.tx.write_ptr() } % MSGQ_NUM_PAGES)
    }

    // Returns the index of the memory page the GSP will read the next command from.
    //
    // # Invariants
    //
    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
    fn gsp_read_ptr(&self) -> u32 {
        let gsp_mem = self.0.start_ptr();

        // SAFETY:
        //  - The 'CoherentAllocation' contains at least one object.
        //  - By the invariants of `CoherentAllocation` the pointer is valid.
        (unsafe { (*gsp_mem).gspq.rx.read_ptr() } % MSGQ_NUM_PAGES)
    }

    // Returns the index of the memory page the CPU can read the next message from.
    //
    // # Invariants
    //
    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
    fn cpu_read_ptr(&self) -> u32 {
        let gsp_mem = self.0.start_ptr();

        // SAFETY:
        //  - The ['CoherentAllocation'] contains at least one object.
        //  - By the invariants of CoherentAllocation the pointer is valid.
        (unsafe { (*gsp_mem).cpuq.rx.read_ptr() } % MSGQ_NUM_PAGES)
    }

    // Informs the GSP that it can send `elem_count` new pages into the message queue.
    fn advance_cpu_read_ptr(&mut self, elem_count: u32) {
        let rptr = self.cpu_read_ptr().wrapping_add(elem_count) % MSGQ_NUM_PAGES;

        // Ensure read pointer is properly ordered.
        fence(Ordering::SeqCst);

        let gsp_mem = self.0.start_ptr_mut();

        // SAFETY:
        //  - The 'CoherentAllocation' contains at least one object.
        //  - By the invariants of `CoherentAllocation` the pointer is valid.
        unsafe { (*gsp_mem).cpuq.rx.set_read_ptr(rptr) };
    }

    // Returns the index of the memory page the CPU can write the next command to.
    //
    // # Invariants
    //
    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
    fn cpu_write_ptr(&self) -> u32 {
        let gsp_mem = self.0.start_ptr();

        // SAFETY:
        //  - The 'CoherentAllocation' contains at least one object.
        //  - By the invariants of `CoherentAllocation` the pointer is valid.
        (unsafe { (*gsp_mem).cpuq.tx.write_ptr() } % MSGQ_NUM_PAGES)
    }

    // Informs the GSP that it can process `elem_count` new pages from the command queue.
    fn advance_cpu_write_ptr(&mut self, elem_count: u32) {
        let wptr = self.cpu_write_ptr().wrapping_add(elem_count) & MSGQ_NUM_PAGES;
        let gsp_mem = self.0.start_ptr_mut();

        // SAFETY:
        //  - The 'CoherentAllocation' contains at least one object.
        //  - By the invariants of `CoherentAllocation` the pointer is valid.
        unsafe { (*gsp_mem).cpuq.tx.set_write_ptr(wptr) };

        // Ensure all command data is visible before triggering the GSP read.
        fence(Ordering::SeqCst);
    }
}

/// A command ready to be sent on the command queue.
///
/// This is the type returned by [`DmaGspMem::allocate_command`].
struct GspCommand<'a> {
    // Writable reference to the header of the command.
    header: &'a mut GspMsgElement,
    // Writable slices to the contents of the command. The second slice is zero unless the command
    // loops over the command queue.
    contents: (&'a mut [u8], &'a mut [u8]),
}

/// A message ready to be processed from the message queue.
///
/// This is the type returned by [`Cmdq::wait_for_msg`].
struct GspMessage<'a> {
    // Reference to the header of the message.
    header: &'a GspMsgElement,
    // Slices to the contents of the message. The second slice is zero unless the message loops
    // over the message queue.
    contents: (&'a [u8], &'a [u8]),
}

/// GSP command queue.
///
/// Provides the ability to send commands and receive messages from the GSP using a shared memory
/// area.
pub(crate) struct Cmdq {
    /// Device this command queue belongs to.
    dev: ARef<device::Device>,
    /// Current command sequence number.
    seq: u32,
    /// Memory area shared with the GSP for communicating commands and messages.
    gsp_mem: DmaGspMem,
}

impl Cmdq {
    /// Offset of the data after the PTEs.
    const POST_PTE_OFFSET: usize = core::mem::offset_of!(GspMem, cpuq);

    /// Offset of command queue ring buffer.
    pub(crate) const CMDQ_OFFSET: usize = core::mem::offset_of!(GspMem, cpuq)
        + core::mem::offset_of!(Msgq, msgq)
        - Self::POST_PTE_OFFSET;

    /// Offset of message queue ring buffer.
    pub(crate) const STATQ_OFFSET: usize = core::mem::offset_of!(GspMem, gspq)
        + core::mem::offset_of!(Msgq, msgq)
        - Self::POST_PTE_OFFSET;

    /// Number of page table entries for the GSP shared region.
    pub(crate) const NUM_PTES: usize = size_of::<GspMem>() >> GSP_PAGE_SHIFT;

    /// Creates a new command queue for `dev`.
    pub(crate) fn new(dev: &device::Device<device::Bound>) -> Result<Cmdq> {
        let gsp_mem = DmaGspMem::new(dev)?;

        Ok(Cmdq {
            dev: dev.into(),
            seq: 0,
            gsp_mem,
        })
    }

    /// Computes the checksum for the message pointed to by `it`.
    ///
    /// A message is made of several parts, so `it` is an iterator over byte slices representing
    /// these parts.
    fn calculate_checksum<T: Iterator<Item = u8>>(it: T) -> u32 {
        let sum64 = it
            .enumerate()
            .map(|(idx, byte)| (((idx % 8) * 8) as u32, byte))
            .fold(0, |acc, (rol, byte)| acc ^ u64::from(byte).rotate_left(rol));

        ((sum64 >> 32) as u32) ^ (sum64 as u32)
    }

    /// Notifies the GSP that we have updated the command queue pointers.
    fn notify_gsp(bar: &Bar0) {
        regs::NV_PGSP_QUEUE_HEAD::default()
            .set_address(0)
            .write(bar);
    }

    /// Sends `command` to the GSP.
    ///
    /// # Errors
    ///
    /// - `EAGAIN` if there was not enough space in the command queue to send the command.
    /// - `EIO` if the variable payload requested by the command has not been entirely
    ///   written to by its [`CommandToGsp::init_variable_payload`] method.
    ///
    /// Error codes returned by the command initializers are propagated as-is.
    pub(crate) fn send_command<M>(&mut self, bar: &Bar0, command: M) -> Result
    where
        M: CommandToGsp,
        // This allows all error types, including `Infallible`, to be used for `M::InitError`.
        Error: From<M::InitError>,
    {
        let command_size = size_of::<M::Command>() + command.variable_payload_len();
        let dst = self.gsp_mem.allocate_command(command_size)?;

        // Extract area for the command itself.
        let (cmd, payload_1) = M::Command::from_bytes_mut_prefix(dst.contents.0).ok_or(EIO)?;

        // Fill the header and command in-place.
        let msg_element = GspMsgElement::init(self.seq, command_size, M::FUNCTION);
        // SAFETY: `msg_header` and `cmd` are valid references, and not touched if the initializer
        // fails.
        unsafe {
            msg_element.__init(core::ptr::from_mut(dst.header))?;
            command.init().__init(core::ptr::from_mut(cmd))?;
        }

        // Fill the variable-length payload.
        if command_size > size_of::<M::Command>() {
            let mut sbuffer =
                SBufferIter::new_writer([&mut payload_1[..], &mut dst.contents.1[..]]);
            command.init_variable_payload(&mut sbuffer)?;

            if !sbuffer.is_empty() {
                return Err(EIO);
            }
        }

        // Compute checksum now that the whole message is ready.
        dst.header
            .set_checksum(Cmdq::calculate_checksum(SBufferIter::new_reader([
                dst.header.as_bytes(),
                dst.contents.0,
                dst.contents.1,
            ])));

        dev_dbg!(
            &self.dev,
            "GSP RPC: send: seq# {}, function={}, length=0x{:x}\n",
            self.seq,
            M::FUNCTION,
            dst.header.length(),
        );

        // All set - update the write pointer and inform the GSP of the new command.
        let elem_count = dst.header.element_count();
        self.seq += 1;
        self.gsp_mem.advance_cpu_write_ptr(elem_count);
        Cmdq::notify_gsp(bar);

        Ok(())
    }

    /// Wait for a message to become available on the message queue.
    ///
    /// This works purely at the transport layer and does not interpret or validate the message
    /// beyond the advertised length in its [`GspMsgElement`].
    ///
    /// This method returns:
    ///
    /// - A reference to the [`GspMsgElement`] of the message,
    /// - Two byte slices with the contents of the message. The second slice is empty unless the
    ///   message loops across the message queue.
    ///
    /// # Errors
    ///
    /// - `ETIMEDOUT` if `timeout` has elapsed before any message becomes available.
    /// - `EIO` if there was some inconsistency (e.g. message shorter than advertised) on the
    ///   message queue.
    ///
    /// Error codes returned by the message constructor are propagated as-is.
    fn wait_for_msg(&self, timeout: Delta) -> Result<GspMessage<'_>> {
        // Wait for a message to arrive from the GSP.
        let (slice_1, slice_2) = read_poll_timeout(
            || Ok(self.gsp_mem.driver_read_area()),
            |driver_area| !driver_area.0.is_empty(),
            Delta::from_millis(1),
            timeout,
        )
        .map(|(slice_1, slice_2)| {
            #[allow(clippy::incompatible_msrv)]
            (slice_1.as_flattened(), slice_2.as_flattened())
        })?;

        // Extract the `GspMsgElement`.
        let (header, slice_1) = GspMsgElement::from_bytes_prefix(slice_1).ok_or(EIO)?;

        dev_dbg!(
            self.dev,
            "GSP RPC: receive: seq# {}, function={:?}, length=0x{:x}\n",
            header.sequence(),
            header.function(),
            header.length(),
        );

        let payload_length = header.payload_length();

        // Check that the driver read area is large enough for the message.
        if slice_1.len() + slice_2.len() < payload_length {
            return Err(EIO);
        }

        // Cut the message slices down to the actual length of the message.
        let (slice_1, slice_2) = if slice_1.len() > payload_length {
            // PANIC: we checked above that `slice_1` is at least as long as `payload_length`.
            (slice_1.split_at(payload_length).0, &slice_2[0..0])
        } else {
            (
                slice_1,
                // PANIC: we checked above that `slice_1.len() + slice_2.len()` is at least as
                // large as `payload_length`.
                slice_2.split_at(payload_length - slice_1.len()).0,
            )
        };

        // Validate checksum.
        if Cmdq::calculate_checksum(SBufferIter::new_reader([
            header.as_bytes(),
            slice_1,
            slice_2,
        ])) != 0
        {
            dev_err!(
                self.dev,
                "GSP RPC: receive: Call {} - bad checksum\n",
                header.sequence()
            );
            return Err(EIO);
        }

        Ok(GspMessage {
            header,
            contents: (slice_1, slice_2),
        })
    }

    /// Receive a message from the GSP.
    ///
    /// `init` is a closure tasked with processing the message. It receives a reference to the
    /// message in the message queue, and a [`SBufferIter`] pointing to its variable-length
    /// payload, if any.
    ///
    /// The expected message is specified using the `M` generic parameter. If the pending message
    /// is different, `EAGAIN` is returned and the unexpected message is dropped.
    ///
    /// This design is by no means final, but it is simple and will let us go through GSP
    /// initialization.
    ///
    /// # Errors
    ///
    /// - `ETIMEDOUT` if `timeout` has elapsed before any message becomes available.
    /// - `EIO` if there was some inconsistency (e.g. message shorter than advertised) on the
    ///   message queue.
    /// - `EINVAL` if the function of the message was unrecognized.
    pub(crate) fn receive_msg<M: MessageFromGsp>(&mut self, timeout: Delta) -> Result<M>
    where
        // This allows all error types, including `Infallible`, to be used for `M::InitError`.
        Error: From<M::InitError>,
    {
        let message = self.wait_for_msg(timeout)?;
        let function = message.header.function().map_err(|_| EINVAL)?;

        // Extract the message. Store the result as we want to advance the read pointer even in
        // case of failure.
        let result = if function == M::FUNCTION {
            let (cmd, contents_1) = M::Message::from_bytes_prefix(message.contents.0).ok_or(EIO)?;
            let mut sbuffer = SBufferIter::new_reader([contents_1, message.contents.1]);

            M::read(cmd, &mut sbuffer).map_err(|e| e.into())
        } else {
            Err(ERANGE)
        };

        // Advance the read pointer past this message.
        self.gsp_mem.advance_cpu_read_ptr(u32::try_from(
            message.header.length().div_ceil(GSP_PAGE_SIZE),
        )?);

        result
    }

    /// Returns the DMA handle of the command queue's shared memory region.
    pub(crate) fn dma_handle(&self) -> DmaAddress {
        self.gsp_mem.0.dma_handle()
    }
}