mirror of https://gitee.com/openkylin/linux.git
202 lines
8.1 KiB
ReStructuredText
202 lines
8.1 KiB
ReStructuredText
.. Copyright 2001 Matthew Wilcox
|
|
..
|
|
.. This documentation is free software; you can redistribute
|
|
.. it and/or modify it under the terms of the GNU General Public
|
|
.. License as published by the Free Software Foundation; either
|
|
.. version 2 of the License, or (at your option) any later
|
|
.. version.
|
|
|
|
===============================
|
|
Bus-Independent Device Accesses
|
|
===============================
|
|
|
|
:Author: Matthew Wilcox
|
|
:Author: Alan Cox
|
|
|
|
Introduction
|
|
============
|
|
|
|
Linux provides an API which abstracts performing IO across all busses
|
|
and devices, allowing device drivers to be written independently of bus
|
|
type.
|
|
|
|
Memory Mapped IO
|
|
================
|
|
|
|
Getting Access to the Device
|
|
----------------------------
|
|
|
|
The most widely supported form of IO is memory mapped IO. That is, a
|
|
part of the CPU's address space is interpreted not as accesses to
|
|
memory, but as accesses to a device. Some architectures define devices
|
|
to be at a fixed address, but most have some method of discovering
|
|
devices. The PCI bus walk is a good example of such a scheme. This
|
|
document does not cover how to receive such an address, but assumes you
|
|
are starting with one. Physical addresses are of type unsigned long.
|
|
|
|
This address should not be used directly. Instead, to get an address
|
|
suitable for passing to the accessor functions described below, you
|
|
should call :c:func:`ioremap()`. An address suitable for accessing
|
|
the device will be returned to you.
|
|
|
|
After you've finished using the device (say, in your module's exit
|
|
routine), call :c:func:`iounmap()` in order to return the address
|
|
space to the kernel. Most architectures allocate new address space each
|
|
time you call :c:func:`ioremap()`, and they can run out unless you
|
|
call :c:func:`iounmap()`.
|
|
|
|
Accessing the device
|
|
--------------------
|
|
|
|
The part of the interface most used by drivers is reading and writing
|
|
memory-mapped registers on the device. Linux provides interfaces to read
|
|
and write 8-bit, 16-bit, 32-bit and 64-bit quantities. Due to a
|
|
historical accident, these are named byte, word, long and quad accesses.
|
|
Both read and write accesses are supported; there is no prefetch support
|
|
at this time.
|
|
|
|
The functions are named readb(), readw(), readl(), readq(),
|
|
readb_relaxed(), readw_relaxed(), readl_relaxed(), readq_relaxed(),
|
|
writeb(), writew(), writel() and writeq().
|
|
|
|
Some devices (such as framebuffers) would like to use larger transfers than
|
|
8 bytes at a time. For these devices, the :c:func:`memcpy_toio()`,
|
|
:c:func:`memcpy_fromio()` and :c:func:`memset_io()` functions are
|
|
provided. Do not use memset or memcpy on IO addresses; they are not
|
|
guaranteed to copy data in order.
|
|
|
|
The read and write functions are defined to be ordered. That is the
|
|
compiler is not permitted to reorder the I/O sequence. When the ordering
|
|
can be compiler optimised, you can use __readb() and friends to
|
|
indicate the relaxed ordering. Use this with care.
|
|
|
|
While the basic functions are defined to be synchronous with respect to
|
|
each other and ordered with respect to each other the busses the devices
|
|
sit on may themselves have asynchronicity. In particular many authors
|
|
are burned by the fact that PCI bus writes are posted asynchronously. A
|
|
driver author must issue a read from the same device to ensure that
|
|
writes have occurred in the specific cases the author cares. This kind
|
|
of property cannot be hidden from driver writers in the API. In some
|
|
cases, the read used to flush the device may be expected to fail (if the
|
|
card is resetting, for example). In that case, the read should be done
|
|
from config space, which is guaranteed to soft-fail if the card doesn't
|
|
respond.
|
|
|
|
The following is an example of flushing a write to a device when the
|
|
driver would like to ensure the write's effects are visible prior to
|
|
continuing execution::
|
|
|
|
static inline void
|
|
qla1280_disable_intrs(struct scsi_qla_host *ha)
|
|
{
|
|
struct device_reg *reg;
|
|
|
|
reg = ha->iobase;
|
|
/* disable risc and host interrupts */
|
|
WRT_REG_WORD(®->ictrl, 0);
|
|
/*
|
|
* The following read will ensure that the above write
|
|
* has been received by the device before we return from this
|
|
* function.
|
|
*/
|
|
RD_REG_WORD(®->ictrl);
|
|
ha->flags.ints_enabled = 0;
|
|
}
|
|
|
|
In addition to write posting, on some large multiprocessing systems
|
|
(e.g. SGI Challenge, Origin and Altix machines) posted writes won't be
|
|
strongly ordered coming from different CPUs. Thus it's important to
|
|
properly protect parts of your driver that do memory-mapped writes with
|
|
locks and use the :c:func:`mmiowb()` to make sure they arrive in the
|
|
order intended. Issuing a regular readX() will also ensure write ordering,
|
|
but should only be used when the
|
|
driver has to be sure that the write has actually arrived at the device
|
|
(not that it's simply ordered with respect to other writes), since a
|
|
full readX() is a relatively expensive operation.
|
|
|
|
Generally, one should use :c:func:`mmiowb()` prior to releasing a spinlock
|
|
that protects regions using :c:func:`writeb()` or similar functions that
|
|
aren't surrounded by readb() calls, which will ensure ordering
|
|
and flushing. The following pseudocode illustrates what might occur if
|
|
write ordering isn't guaranteed via :c:func:`mmiowb()` or one of the
|
|
readX() functions::
|
|
|
|
CPU A: spin_lock_irqsave(&dev_lock, flags)
|
|
CPU A: ...
|
|
CPU A: writel(newval, ring_ptr);
|
|
CPU A: spin_unlock_irqrestore(&dev_lock, flags)
|
|
...
|
|
CPU B: spin_lock_irqsave(&dev_lock, flags)
|
|
CPU B: writel(newval2, ring_ptr);
|
|
CPU B: ...
|
|
CPU B: spin_unlock_irqrestore(&dev_lock, flags)
|
|
|
|
In the case above, newval2 could be written to ring_ptr before newval.
|
|
Fixing it is easy though::
|
|
|
|
CPU A: spin_lock_irqsave(&dev_lock, flags)
|
|
CPU A: ...
|
|
CPU A: writel(newval, ring_ptr);
|
|
CPU A: mmiowb(); /* ensure no other writes beat us to the device */
|
|
CPU A: spin_unlock_irqrestore(&dev_lock, flags)
|
|
...
|
|
CPU B: spin_lock_irqsave(&dev_lock, flags)
|
|
CPU B: writel(newval2, ring_ptr);
|
|
CPU B: ...
|
|
CPU B: mmiowb();
|
|
CPU B: spin_unlock_irqrestore(&dev_lock, flags)
|
|
|
|
See tg3.c for a real world example of how to use :c:func:`mmiowb()`
|
|
|
|
PCI ordering rules also guarantee that PIO read responses arrive after any
|
|
outstanding DMA writes from that bus, since for some devices the result of
|
|
a readb() call may signal to the driver that a DMA transaction is
|
|
complete. In many cases, however, the driver may want to indicate that the
|
|
next readb() call has no relation to any previous DMA writes
|
|
performed by the device. The driver can use readb_relaxed() for
|
|
these cases, although only some platforms will honor the relaxed
|
|
semantics. Using the relaxed read functions will provide significant
|
|
performance benefits on platforms that support it. The qla2xxx driver
|
|
provides examples of how to use readX_relaxed(). In many cases, a majority
|
|
of the driver's readX() calls can safely be converted to readX_relaxed()
|
|
calls, since only a few will indicate or depend on DMA completion.
|
|
|
|
Port Space Accesses
|
|
===================
|
|
|
|
Port Space Explained
|
|
--------------------
|
|
|
|
Another form of IO commonly supported is Port Space. This is a range of
|
|
addresses separate to the normal memory address space. Access to these
|
|
addresses is generally not as fast as accesses to the memory mapped
|
|
addresses, and it also has a potentially smaller address space.
|
|
|
|
Unlike memory mapped IO, no preparation is required to access port
|
|
space.
|
|
|
|
Accessing Port Space
|
|
--------------------
|
|
|
|
Accesses to this space are provided through a set of functions which
|
|
allow 8-bit, 16-bit and 32-bit accesses; also known as byte, word and
|
|
long. These functions are :c:func:`inb()`, :c:func:`inw()`,
|
|
:c:func:`inl()`, :c:func:`outb()`, :c:func:`outw()` and
|
|
:c:func:`outl()`.
|
|
|
|
Some variants are provided for these functions. Some devices require
|
|
that accesses to their ports are slowed down. This functionality is
|
|
provided by appending a ``_p`` to the end of the function.
|
|
There are also equivalents to memcpy. The :c:func:`ins()` and
|
|
:c:func:`outs()` functions copy bytes, words or longs to the given
|
|
port.
|
|
|
|
Public Functions Provided
|
|
=========================
|
|
|
|
.. kernel-doc:: arch/x86/include/asm/io.h
|
|
:internal:
|
|
|
|
.. kernel-doc:: lib/pci_iomap.c
|
|
:export:
|