Root/
1 | |
2 | 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 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"). |
651 | |
652 |
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