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2012年01月29日  

2012-01-29 10:28:36|  分类: linux开发 |  标签: |举报 |字号 订阅

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How To Write Linux PCI Drivers
3
4 by Martin Mares <mj@ucw.cz> on 07-Feb-2000
5 updated by Grant Grundler <grundler@parisc-linux.org> on 23-Dec-2006
6
7 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8 The world of PCI is vast and full of (mostly unpleasant) surprises.
9 Since each CPU architecture implements different chip-sets and PCI devices
10 have different requirements (erm, "features"), the result is the PCI support
11 in the Linux kernel is not as trivial as one would wish. This short paper
12 tries to introduce all potential driver authors to Linux APIs for
13 PCI device drivers.
14
15 A more complete resource is the third edition of "Linux Device Drivers"
16 by Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman.
17 LDD3 is available for free (under Creative Commons License) from:
18
19 http://lwn.net/Kernel/LDD3/
20
21 However, keep in mind that all documents are subject to "bit rot".
22 Refer to the source code if things are not working as described here.
23
24 Please send questions/comments/patches about Linux PCI API to the
25 "Linux PCI" <linux-pci@atrey.karlin.mff.cuni.cz> mailing list.
26
27
28
29 0. Structure of PCI drivers
30 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
31 PCI drivers "discover" PCI devices in a system via pci_register_driver().
32 Actually, it's the other way around. When the PCI generic code discovers
33 a new device, the driver with a matching "description" will be notified.
34 Details on this below.
35
36 pci_register_driver() leaves most of the probing for devices to
37 the PCI layer and supports online insertion/removal of devices [thus
38 supporting hot-pluggable PCI, CardBus, and Express-Card in a single driver].
39 pci_register_driver() call requires passing in a table of function
40 pointers and thus dictates the high level structure of a driver.
41
42 Once the driver knows about a PCI device and takes ownership, the
43 driver generally needs to perform the following initialization:
44
45 Enable the device
46 Request MMIO/IOP resources
47 Set the DMA mask size (for both coherent and streaming DMA)
48 Allocate and initialize shared control data (pci_allocate_coherent())
49 Access device configuration space (if needed)
50 Register IRQ handler (request_irq())
51 Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
52 Enable DMA/processing engines
53
54 When done using the device, and perhaps the module needs to be unloaded,
55 the driver needs to take the follow steps:
56 Disable the device from generating IRQs
57 Release the IRQ (free_irq())
58 Stop all DMA activity
59 Release DMA buffers (both streaming and coherent)
60 Unregister from other subsystems (e.g. scsi or netdev)
61 Release MMIO/IOP resources
62 Disable the device
63
64 Most of these topics are covered in the following sections.
65 For the rest look at LDD3 or <linux/pci.h> .
66
67 If the PCI subsystem is not configured (CONFIG_PCI is not set), most of
68 the PCI functions described below are defined as inline functions either
69 completely empty or just returning an appropriate error codes to avoid
70 lots of ifdefs in the drivers.
71
72
73
74 1. pci_register_driver() call
75 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
76
77 PCI device drivers call pci_register_driver() during their
78 initialization with a pointer to a structure describing the driver
79 (struct pci_driver):
80
81 field name Description
82 ---------- ------------------------------------------------------
83 id_table Pointer to table of device ID's the driver is
84 interested in.  Most drivers should export this
85 table using MODULE_DEVICE_TABLE(pci,...).
86
87 probe This probing function gets called (during execution
88 of pci_register_driver() for already existing
89 devices or later if a new device gets inserted) for
90 all PCI devices which match the ID table and are not
91 "owned" by the other drivers yet. This function gets
92 passed a "struct pci_dev *" for each device whose
93 entry in the ID table matches the device. The probe
94 function returns zero when the driver chooses to
95 take "ownership" of the device or an error code
96 (negative number) otherwise.
97 The probe function always gets called from process
98 context, so it can sleep.
99
100 remove The remove() function gets called whenever a device
101 being handled by this driver is removed (either during
102 deregistration of the driver or when it's manually
103 pulled out of a hot-pluggable slot).
104 The remove function always gets called from process
105 context, so it can sleep.
106
107 suspend Put device into low power state.
108 suspend_late Put device into low power state.
109
110 resume_early Wake device from low power state.
111 resume Wake device from low power state.
112
113 (Please see Documentation/power/pci.txt for descriptions
114 of PCI Power Management and the related functions.)
115
116 shutdown Hook into reboot_notifier_list (kernel/sys.c).
117 Intended to stop any idling DMA operations.
118 Useful for enabling wake-on-lan (NIC) or changing
119 the power state of a device before reboot.
120 e.g. drivers/net/e100.c.
121
122 err_handler See Documentation/PCI/pci-error-recovery.txt
123
124
125 The ID table is an array of struct pci_device_id entries ending with an
126 all-zero entry; use of the macro DEFINE_PCI_DEVICE_TABLE is the preferred
127 method of declaring the table.  Each entry consists of:
128
129 vendor,device Vendor and device ID to match (or PCI_ANY_ID)
130
131 subvendor, Subsystem vendor and device ID to match (or PCI_ANY_ID)
132 subdevice,
133
134 class Device class, subclass, and "interface" to match.
135 See Appendix D of the PCI Local Bus Spec or
136 include/linux/pci_ids.h for a full list of classes.
137 Most drivers do not need to specify class/class_mask
138 as vendor/device is normally sufficient.
139
140 class_mask limit which sub-fields of the class field are compared.
141 See drivers/scsi/sym53c8xx_2/ for example of usage.
142
143 driver_data Data private to the driver.
144 Most drivers don't need to use driver_data field.
145 Best practice is to use driver_data as an index
146 into a static list of equivalent device types,
147 instead of using it as a pointer.
148
149
150 Most drivers only need PCI_DEVICE() or PCI_DEVICE_CLASS() to set up
151 a pci_device_id table.
152
153 New PCI IDs may be added to a device driver pci_ids table at runtime
154 as shown below:
155
156 echo "vendor device subvendor subdevice class class_mask driver_data" > \
157 /sys/bus/pci/drivers/{driver}/new_id
158
159 All fields are passed in as hexadecimal values (no leading 0x).
160 The vendor and device fields are mandatory, the others are optional. Users
161 need pass only as many optional fields as necessary:
162 o subvendor and subdevice fields default to PCI_ANY_ID (FFFFFFFF)
163 o class and classmask fields default to 0
164 o driver_data defaults to 0UL.
165
166 Note that driver_data must match the value used by any of the pci_device_id
167 entries defined in the driver. This makes the driver_data field mandatory
168 if all the pci_device_id entries have a non-zero driver_data value.
169
170 Once added, the driver probe routine will be invoked for any unclaimed
171 PCI devices listed in its (newly updated) pci_ids list.
172
173 When the driver exits, it just calls pci_unregister_driver() and the PCI layer
174 automatically calls the remove hook for all devices handled by the driver.
175
176
177 1.1 "Attributes" for driver functions/data
178
179 Please mark the initialization and cleanup functions where appropriate
180 (the corresponding macros are defined in <linux/init.h>):
181
182 __init Initialization code. Thrown away after the driver
183 initializes.
184 __exit Exit code. Ignored for non-modular drivers.
185
186
187 __devinit Device initialization code.
188 Identical to __init if the kernel is not compiled
189 with CONFIG_HOTPLUG, normal function otherwise.
190 __devexit The same for __exit.
191
192 Tips on when/where to use the above attributes:
193 o The module_init()/module_exit() functions (and all
194   initialization functions called _only_ from these)
195   should be marked __init/__exit.
196
197 o Do not mark the struct pci_driver.
198
199 o The ID table array should be marked __devinitconst; this is done
200   automatically if the table is declared with DEFINE_PCI_DEVICE_TABLE().
201
202 o The probe() and remove() functions should be marked __devinit
203   and __devexit respectively.  All initialization functions
204   exclusively called by the probe() routine, can be marked __devinit.
205   Ditto for remove() and __devexit.
206
207 o If mydriver_remove() is marked with __devexit(), then all address
208   references to mydriver_remove must use __devexit_p(mydriver_remove)
209   (in the struct pci_driver declaration for example).
210   __devexit_p() will generate the function name _or_ NULL if the
211   function will be discarded.  For an example, see drivers/net/tg3.c.
212
213 o Do NOT mark a function if you are not sure which mark to use.
214   Better to not mark the function than mark the function wrong.
215
216
217
218 2. How to find PCI devices manually
219 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
220
221 PCI drivers should have a really good reason for not using the
222 pci_register_driver() interface to search for PCI devices.
223 The main reason PCI devices are controlled by multiple drivers
224 is because one PCI device implements several different HW services.
225 E.g. combined serial/parallel port/floppy controller.
226
227 A manual search may be performed using the following constructs:
228
229 Searching by vendor and device ID:
230
231 struct pci_dev *dev = NULL;
232 while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
233 configure_device(dev);
234
235 Searching by class ID (iterate in a similar way):
236
237 pci_get_class(CLASS_ID, dev)
238
239 Searching by both vendor/device and subsystem vendor/device ID:
240
241 pci_get_subsys(VENDOR_ID,DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).
242
243 You can use the constant PCI_ANY_ID as a wildcard replacement for
244 VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
245 specific vendor, for example.
246
247 These functions are hotplug-safe. They increment the reference count on
248 the pci_dev that they return. You must eventually (possibly at module unload)
249 decrement the reference count on these devices by calling pci_dev_put().
250
251
252
253 3. Device Initialization Steps
254 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
255
256 As noted in the introduction, most PCI drivers need the following steps
257 for device initialization:
258
259 Enable the device
260 Request MMIO/IOP resources
261 Set the DMA mask size (for both coherent and streaming DMA)
262 Allocate and initialize shared control data (pci_allocate_coherent())
263 Access device configuration space (if needed)
264 Register IRQ handler (request_irq())
265 Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
266 Enable DMA/processing engines.
267
268 The driver can access PCI config space registers at any time.
269 (Well, almost. When running BIST, config space can go away...but
270 that will just result in a PCI Bus Master Abort and config reads
271 will return garbage).
272
273
274 3.1 Enable the PCI device
275 ~~~~~~~~~~~~~~~~~~~~~~~~~
276 Before touching any device registers, the driver needs to enable
277 the PCI device by calling pci_enable_device(). This will:
278 o wake up the device if it was in suspended state,
279 o allocate I/O and memory regions of the device (if BIOS did not),
280 o allocate an IRQ (if BIOS did not).
281
282 NOTE: pci_enable_device() can fail! Check the return value.
283
284 [ OS BUG: we don't check resource allocations before enabling those
285   resources. The sequence would make more sense if we called
286   pci_request_resources() before calling pci_enable_device().
287   Currently, the device drivers can't detect the bug when when two
288   devices have been allocated the same range. This is not a common
289   problem and unlikely to get fixed soon.
290
291   This has been discussed before but not changed as of 2.6.19:
292 http://lkml.org/lkml/2006/3/2/194
293 ]
294
295 pci_set_master() will enable DMA by setting the bus master bit
296 in the PCI_COMMAND register. It also fixes the latency timer value if
297 it's set to something bogus by the BIOS.  pci_clear_master() will
298 disable DMA by clearing the bus master bit.
299
300 If the PCI device can use the PCI Memory-Write-Invalidate transaction,
301 call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
302 and also ensures that the cache line size register is set correctly.
303 Check the return value of pci_set_mwi() as not all architectures
304 or chip-sets may support Memory-Write-Invalidate.  Alternatively,
305 if Mem-Wr-Inval would be nice to have but is not required, call
306 pci_try_set_mwi() to have the system do its best effort at enabling
307 Mem-Wr-Inval.
308
309
310 3.2 Request MMIO/IOP resources
311 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
312 Memory (MMIO), and I/O port addresses should NOT be read directly
313 from the PCI device config space. Use the values in the pci_dev structure
314 as the PCI "bus address" might have been remapped to a "host physical"
315 address by the arch/chip-set specific kernel support.
316
317 See Documentation/io-mapping.txt for how to access device registers
318 or device memory.
319
320 The device driver needs to call pci_request_region() to verify
321 no other device is already using the same address resource.
322 Conversely, drivers should call pci_release_region() AFTER
323 calling pci_disable_device().
324 The idea is to prevent two devices colliding on the same address range.
325
326 [ See OS BUG comment above. Currently (2.6.19), The driver can only
327   determine MMIO and IO Port resource availability _after_ calling
328   pci_enable_device(). ]
329
330 Generic flavors of pci_request_region() are request_mem_region()
331 (for MMIO ranges) and request_region() (for IO Port ranges).
332 Use these for address resources that are not described by "normal" PCI
333 BARs.
334
335 Also see pci_request_selected_regions() below.
336
337
338 3.3 Set the DMA mask size
339 ~~~~~~~~~~~~~~~~~~~~~~~~~
340 [ If anything below doesn't make sense, please refer to
341   Documentation/DMA-API.txt. This section is just a reminder that
342   drivers need to indicate DMA capabilities of the device and is not
343   an authoritative source for DMA interfaces. ]
344
345 While all drivers should explicitly indicate the DMA capability
346 (e.g. 32 or 64 bit) of the PCI bus master, devices with more than
347 32-bit bus master capability for streaming data need the driver
348 to "register" this capability by calling pci_set_dma_mask() with
349 appropriate parameters.  In general this allows more efficient DMA
350 on systems where System RAM exists above 4G _physical_ address.
351
352 Drivers for all PCI-X and PCIe compliant devices must call
353 pci_set_dma_mask() as they are 64-bit DMA devices.
354
355 Similarly, drivers must also "register" this capability if the device
356 can directly address "consistent memory" in System RAM above 4G physical
357 address by calling pci_set_consistent_dma_mask().
358 Again, this includes drivers for all PCI-X and PCIe compliant devices.
359 Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
360 64-bit DMA capable for payload ("streaming") data but not control
361 ("consistent") data.
362
363
364 3.4 Setup shared control data
365 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
366 Once the DMA masks are set, the driver can allocate "consistent" (a.k.a. shared)
367 memory.  See Documentation/DMA-API.txt for a full description of
368 the DMA APIs. This section is just a reminder that it needs to be done
369 before enabling DMA on the device.
370
371
372 3.5 Initialize device registers
373 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
374 Some drivers will need specific "capability" fields programmed
375 or other "vendor specific" register initialized or reset.
376 E.g. clearing pending interrupts.
377
378
379 3.6 Register IRQ handler
380 ~~~~~~~~~~~~~~~~~~~~~~~~
381 While calling request_irq() is the last step described here,
382 this is often just another intermediate step to initialize a device.
383 This step can often be deferred until the device is opened for use.
384
385 All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
386 and use the devid to map IRQs to devices (remember that all PCI IRQ lines
387 can be shared).
388
389 request_irq() will associate an interrupt handler and device handle
390 with an interrupt number. Historically interrupt numbers represent
391 IRQ lines which run from the PCI device to the Interrupt controller.
392 With MSI and MSI-X (more below) the interrupt number is a CPU "vector".
393
394 request_irq() also enables the interrupt. Make sure the device is
395 quiesced and does not have any interrupts pending before registering
396 the interrupt handler.
397
398 MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
399 which deliver interrupts to the CPU via a DMA write to a Local APIC.
400 The fundamental difference between MSI and MSI-X is how multiple
401 "vectors" get allocated. MSI requires contiguous blocks of vectors
402 while MSI-X can allocate several individual ones.
403
404 MSI capability can be enabled by calling pci_enable_msi() or
405 pci_enable_msix() before calling request_irq(). This causes
406 the PCI support to program CPU vector data into the PCI device
407 capability registers.
408
409 If your PCI device supports both, try to enable MSI-X first.
410 Only one can be enabled at a time.  Many architectures, chip-sets,
411 or BIOSes do NOT support MSI or MSI-X and the call to pci_enable_msi/msix
412 will fail. This is important to note since many drivers have
413 two (or more) interrupt handlers: one for MSI/MSI-X and another for IRQs.
414 They choose which handler to register with request_irq() based on the
415 return value from pci_enable_msi/msix().
416
417 There are (at least) two really good reasons for using MSI:
418 1) MSI is an exclusive interrupt vector by definition.
419    This means the interrupt handler doesn't have to verify
420    its device caused the interrupt.
421
422 2) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
423    to be visible to the host CPU(s) when the MSI is delivered. This
424    is important for both data coherency and avoiding stale control data.
425    This guarantee allows the driver to omit MMIO reads to flush
426    the DMA stream.
427
428 See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
429 of MSI/MSI-X usage.
430
431
432
433 4. PCI device shutdown
434 ~~~~~~~~~~~~~~~~~~~~~~~
435
436 When a PCI device driver is being unloaded, most of the following
437 steps need to be performed:
438
439 Disable the device from generating IRQs
440 Release the IRQ (free_irq())
441 Stop all DMA activity
442 Release DMA buffers (both streaming and consistent)
443 Unregister from other subsystems (e.g. scsi or netdev)
444 Disable device from responding to MMIO/IO Port addresses
445 Release MMIO/IO Port resource(s)
446
447
448 4.1 Stop IRQs on the device
449 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
450 How to do this is chip/device specific. If it's not done, it opens
451 the possibility of a "screaming interrupt" if (and only if)
452 the IRQ is shared with another device.
453
454 When the shared IRQ handler is "unhooked", the remaining devices
455 using the same IRQ line will still need the IRQ enabled. Thus if the
456 "unhooked" device asserts IRQ line, the system will respond assuming
457 it was one of the remaining devices asserted the IRQ line. Since none
458 of the other devices will handle the IRQ, the system will "hang" until
459 it decides the IRQ isn't going to get handled and masks the IRQ (100,000
460 iterations later). Once the shared IRQ is masked, the remaining devices
461 will stop functioning properly. Not a nice situation.
462
463 This is another reason to use MSI or MSI-X if it's available.
464 MSI and MSI-X are defined to be exclusive interrupts and thus
465 are not susceptible to the "screaming interrupt" problem.
466
467
468 4.2 Release the IRQ
469 ~~~~~~~~~~~~~~~~~~~
470 Once the device is quiesced (no more IRQs), one can call free_irq().
471 This function will return control once any pending IRQs are handled,
472 "unhook" the drivers IRQ handler from that IRQ, and finally release
473 the IRQ if no one else is using it.
474
475
476 4.3 Stop all DMA activity
477 ~~~~~~~~~~~~~~~~~~~~~~~~~
478 It's extremely important to stop all DMA operations BEFORE attempting
479 to deallocate DMA control data. Failure to do so can result in memory
480 corruption, hangs, and on some chip-sets a hard crash.
481
482 Stopping DMA after stopping the IRQs can avoid races where the
483 IRQ handler might restart DMA engines.
484
485 While this step sounds obvious and trivial, several "mature" drivers
486 didn't get this step right in the past.
487
488
489 4.4 Release DMA buffers
490 ~~~~~~~~~~~~~~~~~~~~~~~
491 Once DMA is stopped, clean up streaming DMA first.
492 I.e. unmap data buffers and return buffers to "upstream"
493 owners if there is one.
494
495 Then clean up "consistent" buffers which contain the control data.
496
497 See Documentation/DMA-API.txt for details on unmapping interfaces.
498
499
500 4.5 Unregister from other subsystems
501 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
502 Most low level PCI device drivers support some other subsystem
503 like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
504 driver isn't losing resources from that other subsystem.
505 If this happens, typically the symptom is an Oops (panic) when
506 the subsystem attempts to call into a driver that has been unloaded.
507
508
509 4.6 Disable Device from responding to MMIO/IO Port addresses
510 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
511 io_unmap() MMIO or IO Port resources and then call pci_disable_device().
512 This is the symmetric opposite of pci_enable_device().
513 Do not access device registers after calling pci_disable_device().
514
515
516 4.7 Release MMIO/IO Port Resource(s)
517 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
518 Call pci_release_region() to mark the MMIO or IO Port range as available.
519 Failure to do so usually results in the inability to reload the driver.
520
521
522
523 5. How to access PCI config space
524 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
525
526 You can use pci_(read|write)_config_(byte|word|dword) to access the config
527 space of a device represented by struct pci_dev *. All these functions return 0
528 when successful or an error code (PCIBIOS_...) which can be translated to a text
529 string by pcibios_strerror. Most drivers expect that accesses to valid PCI
530 devices don't fail.
531
532 If you don't have a struct pci_dev available, you can call
533 pci_bus_(read|write)_config_(byte|word|dword) to access a given device
534 and function on that bus.
535
536 If you access fields in the standard portion of the config header, please
537 use symbolic names of locations and bits declared in <linux/pci.h>.
538
539 If you need to access Extended PCI Capability registers, just call
540 pci_find_capability() for the particular capability and it will find the
541 corresponding register block for you.
542
543
544
545 6. Other interesting functions
546 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
547
548 pci_find_slot() Find pci_dev corresponding to given bus and
549 slot numbers.
550 pci_set_power_state() Set PCI Power Management state (0=D0 ... 3=D3)
551 pci_find_capability() Find specified capability in device's capability
552 list.
553 pci_resource_start() Returns bus start address for a given PCI region
554 pci_resource_end() Returns bus end address for a given PCI region
555 pci_resource_len() Returns the byte length of a PCI region
556 pci_set_drvdata() Set private driver data pointer for a pci_dev
557 pci_get_drvdata() Return private driver data pointer for a pci_dev
558 pci_set_mwi() Enable Memory-Write-Invalidate transactions.
559 pci_clear_mwi() Disable Memory-Write-Invalidate transactions.
560
561
562
563 7. Miscellaneous hints
564 ~~~~~~~~~~~~~~~~~~~~~~
565
566 When displaying PCI device names to the user (for example when a driver wants
567 to tell the user what card has it found), please use pci_name(pci_dev).
568
569 Always refer to the PCI devices by a pointer to the pci_dev structure.
570 All PCI layer functions use this identification and it's the only
571 reasonable one. Don't use bus/slot/function numbers except for very
572 special purposes -- on systems with multiple primary buses their semantics
573 can be pretty complex.
574
575 Don't try to turn on Fast Back to Back writes in your driver.  All devices
576 on the bus need to be capable of doing it, so this is something which needs
577 to be handled by platform and generic code, not individual drivers.
578
579
580
581 8. Vendor and device identifications
582 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
583
584 One is not required to add new device ids to include/linux/pci_ids.h.
585 Please add PCI_VENDOR_ID_xxx for vendors and a hex constant for device ids.
586
587 PCI_VENDOR_ID_xxx constants are re-used. The device ids are arbitrary
588 hex numbers (vendor controlled) and normally used only in a single
589 location, the pci_device_id table.
590
591 Please DO submit new vendor/device ids to pciids.sourceforge.net project.
592
593
594
595 9. Obsolete functions
596 ~~~~~~~~~~~~~~~~~~~~~
597
598 There are several functions which you might come across when trying to
599 port an old driver to the new PCI interface.  They are no longer present
600 in the kernel as they aren't compatible with hotplug or PCI domains or
601 having sane locking.
602
603 pci_find_device() Superseded by pci_get_device()
604 pci_find_subsys() Superseded by pci_get_subsys()
605 pci_find_slot() Superseded by pci_get_slot()
606
607
608 The alternative is the traditional PCI device driver that walks PCI
609 device lists. This is still possible but discouraged.
610
611
612
613 10. MMIO Space and "Write Posting"
614 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
615
616 Converting a driver from using I/O Port space to using MMIO space
617 often requires some additional changes. Specifically, "write posting"
618 needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
619 already do this. I/O Port space guarantees write transactions reach the PCI
620 device before the CPU can continue. Writes to MMIO space allow the CPU
621 to continue before the transaction reaches the PCI device. HW weenies
622 call this "Write Posting" because the write completion is "posted" to
623 the CPU before the transaction has reached its destination.
624
625 Thus, timing sensitive code should add readl() where the CPU is
626 expected to wait before doing other work.  The classic "bit banging"
627 sequence works fine for I/O Port space:
628
629        for (i = 8; --i; val >>= 1) {
630                outb(val & 1, ioport_reg);      /* write bit */
631                udelay(10);
632        }
633
634 The same sequence for MMIO space should be:
635
636        for (i = 8; --i; val >>= 1) {
637                writeb(val & 1, mmio_reg);      /* write bit */
638                readb(safe_mmio_reg);           /* flush posted write */
639                udelay(10);
640        }
641
642 It is important that "safe_mmio_reg" not have any side effects that
643 interferes with the correct operation of the device.
644
645 Another case to watch out for is when resetting a PCI device. Use PCI
646 Configuration space reads to flush the writel(). This will gracefully
647 handle the PCI master abort on all platforms if the PCI device is
648 expected to not respond to a readl().  Most x86 platforms will allow
649 MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
650 (e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").
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