1.. Copyright 2001 Matthew Wilcox 2.. 3.. This documentation is free software; you can redistribute 4.. it and/or modify it under the terms of the GNU General Public 5.. License as published by the Free Software Foundation; either 6.. version 2 of the License, or (at your option) any later 7.. version. 8 9=============================== 10Bus-Independent Device Accesses 11=============================== 12 13:Author: Matthew Wilcox 14:Author: Alan Cox 15 16Introduction 17============ 18 19Linux provides an API which abstracts performing IO across all busses 20and devices, allowing device drivers to be written independently of bus 21type. 22 23Memory Mapped IO 24================ 25 26Getting Access to the Device 27---------------------------- 28 29The most widely supported form of IO is memory mapped IO. That is, a 30part of the CPU's address space is interpreted not as accesses to 31memory, but as accesses to a device. Some architectures define devices 32to be at a fixed address, but most have some method of discovering 33devices. The PCI bus walk is a good example of such a scheme. This 34document does not cover how to receive such an address, but assumes you 35are starting with one. Physical addresses are of type unsigned long. 36 37This address should not be used directly. Instead, to get an address 38suitable for passing to the accessor functions described below, you 39should call :c:func:`ioremap()`. An address suitable for accessing 40the device will be returned to you. 41 42After you've finished using the device (say, in your module's exit 43routine), call :c:func:`iounmap()` in order to return the address 44space to the kernel. Most architectures allocate new address space each 45time you call :c:func:`ioremap()`, and they can run out unless you 46call :c:func:`iounmap()`. 47 48Accessing the device 49-------------------- 50 51The part of the interface most used by drivers is reading and writing 52memory-mapped registers on the device. Linux provides interfaces to read 53and write 8-bit, 16-bit, 32-bit and 64-bit quantities. Due to a 54historical accident, these are named byte, word, long and quad accesses. 55Both read and write accesses are supported; there is no prefetch support 56at this time. 57 58The functions are named readb(), readw(), readl(), readq(), 59readb_relaxed(), readw_relaxed(), readl_relaxed(), readq_relaxed(), 60writeb(), writew(), writel() and writeq(). 61 62Some devices (such as framebuffers) would like to use larger transfers than 638 bytes at a time. For these devices, the :c:func:`memcpy_toio()`, 64:c:func:`memcpy_fromio()` and :c:func:`memset_io()` functions are 65provided. Do not use memset or memcpy on IO addresses; they are not 66guaranteed to copy data in order. 67 68The read and write functions are defined to be ordered. That is the 69compiler is not permitted to reorder the I/O sequence. When the ordering 70can be compiler optimised, you can use __readb() and friends to 71indicate the relaxed ordering. Use this with care. 72 73While the basic functions are defined to be synchronous with respect to 74each other and ordered with respect to each other the busses the devices 75sit on may themselves have asynchronicity. In particular many authors 76are burned by the fact that PCI bus writes are posted asynchronously. A 77driver author must issue a read from the same device to ensure that 78writes have occurred in the specific cases the author cares. This kind 79of property cannot be hidden from driver writers in the API. In some 80cases, the read used to flush the device may be expected to fail (if the 81card is resetting, for example). In that case, the read should be done 82from config space, which is guaranteed to soft-fail if the card doesn't 83respond. 84 85The following is an example of flushing a write to a device when the 86driver would like to ensure the write's effects are visible prior to 87continuing execution:: 88 89 static inline void 90 qla1280_disable_intrs(struct scsi_qla_host *ha) 91 { 92 struct device_reg *reg; 93 94 reg = ha->iobase; 95 /* disable risc and host interrupts */ 96 WRT_REG_WORD(®->ictrl, 0); 97 /* 98 * The following read will ensure that the above write 99 * has been received by the device before we return from this 100 * function. 101 */ 102 RD_REG_WORD(®->ictrl); 103 ha->flags.ints_enabled = 0; 104 } 105 106In addition to write posting, on some large multiprocessing systems 107(e.g. SGI Challenge, Origin and Altix machines) posted writes won't be 108strongly ordered coming from different CPUs. Thus it's important to 109properly protect parts of your driver that do memory-mapped writes with 110locks and use the :c:func:`mmiowb()` to make sure they arrive in the 111order intended. Issuing a regular readX() will also ensure write ordering, 112but should only be used when the 113driver has to be sure that the write has actually arrived at the device 114(not that it's simply ordered with respect to other writes), since a 115full readX() is a relatively expensive operation. 116 117Generally, one should use :c:func:`mmiowb()` prior to releasing a spinlock 118that protects regions using :c:func:`writeb()` or similar functions that 119aren't surrounded by readb() calls, which will ensure ordering 120and flushing. The following pseudocode illustrates what might occur if 121write ordering isn't guaranteed via :c:func:`mmiowb()` or one of the 122readX() functions:: 123 124 CPU A: spin_lock_irqsave(&dev_lock, flags) 125 CPU A: ... 126 CPU A: writel(newval, ring_ptr); 127 CPU A: spin_unlock_irqrestore(&dev_lock, flags) 128 ... 129 CPU B: spin_lock_irqsave(&dev_lock, flags) 130 CPU B: writel(newval2, ring_ptr); 131 CPU B: ... 132 CPU B: spin_unlock_irqrestore(&dev_lock, flags) 133 134In the case above, newval2 could be written to ring_ptr before newval. 135Fixing it is easy though:: 136 137 CPU A: spin_lock_irqsave(&dev_lock, flags) 138 CPU A: ... 139 CPU A: writel(newval, ring_ptr); 140 CPU A: mmiowb(); /* ensure no other writes beat us to the device */ 141 CPU A: spin_unlock_irqrestore(&dev_lock, flags) 142 ... 143 CPU B: spin_lock_irqsave(&dev_lock, flags) 144 CPU B: writel(newval2, ring_ptr); 145 CPU B: ... 146 CPU B: mmiowb(); 147 CPU B: spin_unlock_irqrestore(&dev_lock, flags) 148 149See tg3.c for a real world example of how to use :c:func:`mmiowb()` 150 151PCI ordering rules also guarantee that PIO read responses arrive after any 152outstanding DMA writes from that bus, since for some devices the result of 153a readb() call may signal to the driver that a DMA transaction is 154complete. In many cases, however, the driver may want to indicate that the 155next readb() call has no relation to any previous DMA writes 156performed by the device. The driver can use readb_relaxed() for 157these cases, although only some platforms will honor the relaxed 158semantics. Using the relaxed read functions will provide significant 159performance benefits on platforms that support it. The qla2xxx driver 160provides examples of how to use readX_relaxed(). In many cases, a majority 161of the driver's readX() calls can safely be converted to readX_relaxed() 162calls, since only a few will indicate or depend on DMA completion. 163 164Port Space Accesses 165=================== 166 167Port Space Explained 168-------------------- 169 170Another form of IO commonly supported is Port Space. This is a range of 171addresses separate to the normal memory address space. Access to these 172addresses is generally not as fast as accesses to the memory mapped 173addresses, and it also has a potentially smaller address space. 174 175Unlike memory mapped IO, no preparation is required to access port 176space. 177 178Accessing Port Space 179-------------------- 180 181Accesses to this space are provided through a set of functions which 182allow 8-bit, 16-bit and 32-bit accesses; also known as byte, word and 183long. These functions are :c:func:`inb()`, :c:func:`inw()`, 184:c:func:`inl()`, :c:func:`outb()`, :c:func:`outw()` and 185:c:func:`outl()`. 186 187Some variants are provided for these functions. Some devices require 188that accesses to their ports are slowed down. This functionality is 189provided by appending a ``_p`` to the end of the function. 190There are also equivalents to memcpy. The :c:func:`ins()` and 191:c:func:`outs()` functions copy bytes, words or longs to the given 192port. 193 194Public Functions Provided 195========================= 196 197.. kernel-doc:: arch/x86/include/asm/io.h 198 :internal: 199 200.. kernel-doc:: lib/pci_iomap.c 201 :export: 202