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| -rw-r--r-- | Documentation/driver-api/device-io.rst | 201 | ||||
| -rw-r--r-- | Documentation/driver-api/index.rst | 1 |
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diff --git a/Documentation/DocBook/deviceiobook.tmpl b/Documentation/DocBook/deviceiobook.tmpl deleted file mode 100644 index 54199a0dcf9a..000000000000 --- a/Documentation/DocBook/deviceiobook.tmpl +++ /dev/null @@ -1,323 +0,0 @@ -<?xml version="1.0" encoding="UTF-8"?> -<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" - "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []> - -<book id="DoingIO"> - <bookinfo> - <title>Bus-Independent Device Accesses</title> - - <authorgroup> - <author> - <firstname>Matthew</firstname> - <surname>Wilcox</surname> - <affiliation> - <address> - <email>matthew@wil.cx</email> - </address> - </affiliation> - </author> - </authorgroup> - - <authorgroup> - <author> - <firstname>Alan</firstname> - <surname>Cox</surname> - <affiliation> - <address> - <email>alan@lxorguk.ukuu.org.uk</email> - </address> - </affiliation> - </author> - </authorgroup> - - <copyright> - <year>2001</year> - <holder>Matthew Wilcox</holder> - </copyright> - - <legalnotice> - <para> - 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. - </para> - - <para> - This program is distributed in the hope that it will be - useful, but WITHOUT ANY WARRANTY; without even the implied - warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. - See the GNU General Public License for more details. - </para> - - <para> - You should have received a copy of the GNU General Public - License along with this program; if not, write to the Free - Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, - MA 02111-1307 USA - </para> - - <para> - For more details see the file COPYING in the source - distribution of Linux. - </para> - </legalnotice> - </bookinfo> - -<toc></toc> - - <chapter id="intro"> - <title>Introduction</title> - <para> - Linux provides an API which abstracts performing IO across all busses - and devices, allowing device drivers to be written independently of - bus type. - </para> - </chapter> - - <chapter id="bugs"> - <title>Known Bugs And Assumptions</title> - <para> - None. - </para> - </chapter> - - <chapter id="mmio"> - <title>Memory Mapped IO</title> - <sect1 id="getting_access_to_the_device"> - <title>Getting Access to the Device</title> - <para> - 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. - </para> - - <para> - This address should not be used directly. Instead, to get an - address suitable for passing to the accessor functions described - below, you should call <function>ioremap</function>. - An address suitable for accessing the device will be returned to you. - </para> - - <para> - After you've finished using the device (say, in your module's - exit routine), call <function>iounmap</function> in order to return - the address space to the kernel. Most architectures allocate new - address space each time you call <function>ioremap</function>, and - they can run out unless you call <function>iounmap</function>. - </para> - </sect1> - - <sect1 id="accessing_the_device"> - <title>Accessing the device</title> - <para> - 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. - </para> - - <para> - The functions are named <function>readb</function>, - <function>readw</function>, <function>readl</function>, - <function>readq</function>, <function>readb_relaxed</function>, - <function>readw_relaxed</function>, <function>readl_relaxed</function>, - <function>readq_relaxed</function>, <function>writeb</function>, - <function>writew</function>, <function>writel</function> and - <function>writeq</function>. - </para> - - <para> - Some devices (such as framebuffers) would like to use larger - transfers than 8 bytes at a time. For these devices, the - <function>memcpy_toio</function>, <function>memcpy_fromio</function> - and <function>memset_io</function> functions are provided. - Do not use memset or memcpy on IO addresses; they - are not guaranteed to copy data in order. - </para> - - <para> - 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 <function> - __readb</function> and friends to indicate the relaxed ordering. Use - this with care. - </para> - - <para> - 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. - </para> - - <para> - 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. - </para> - -<programlisting> -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(&reg->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(&reg->ictrl); - ha->flags.ints_enabled = 0; -} -</programlisting> - - <para> - 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 <function>mmiowb</function> to make sure they - arrive in the order intended. Issuing a regular <function>readX - </function> 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 <function>readX</function> is a relatively - expensive operation. - </para> - - <para> - Generally, one should use <function>mmiowb</function> prior to - releasing a spinlock that protects regions using <function>writeb - </function> or similar functions that aren't surrounded by <function> - readb</function> calls, which will ensure ordering and flushing. The - following pseudocode illustrates what might occur if write ordering - isn't guaranteed via <function>mmiowb</function> or one of the - <function>readX</function> functions. - </para> - -<programlisting> -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) -</programlisting> - - <para> - In the case above, newval2 could be written to ring_ptr before - newval. Fixing it is easy though: - </para> - -<programlisting> -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) -</programlisting> - - <para> - See tg3.c for a real world example of how to use <function>mmiowb - </function> - </para> - - <para> - 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 <function>readb</function> 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 - <function>readb</function> call has no relation to any previous DMA - writes performed by the device. The driver can use - <function>readb_relaxed</function> 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 <function>readX_relaxed</function>. In many cases, - a majority of the driver's <function>readX</function> calls can - safely be converted to <function>readX_relaxed</function> calls, since - only a few will indicate or depend on DMA completion. - </para> - </sect1> - - </chapter> - - <chapter id="port_space_accesses"> - <title>Port Space Accesses</title> - <sect1 id="port_space_explained"> - <title>Port Space Explained</title> - - <para> - 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. - </para> - - <para> - Unlike memory mapped IO, no preparation is required - to access port space. - </para> - - </sect1> - <sect1 id="accessing_port_space"> - <title>Accessing Port Space</title> - <para> - 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 - <function>inb</function>, <function>inw</function>, - <function>inl</function>, <function>outb</function>, - <function>outw</function> and <function>outl</function>. - </para> - - <para> - 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 <function>_p</function> - to the end of the function. There are also equivalents to memcpy. - The <function>ins</function> and <function>outs</function> - functions copy bytes, words or longs to the given port. - </para> - </sect1> - - </chapter> - - <chapter id="pubfunctions"> - <title>Public Functions Provided</title> -!Iarch/x86/include/asm/io.h -!Elib/pci_iomap.c - </chapter> - -</book> diff --git a/Documentation/driver-api/device-io.rst b/Documentation/driver-api/device-io.rst new file mode 100644 index 000000000000..b00b23903078 --- /dev/null +++ b/Documentation/driver-api/device-io.rst @@ -0,0 +1,201 @@ +.. 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: diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst index a2e5db07756c..365ce64abd7c 100644 --- a/Documentation/driver-api/index.rst +++ b/Documentation/driver-api/index.rst @@ -16,6 +16,7 @@ available subsections can be seen below. basics infrastructure + device-io dma-buf device_link message-based |