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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

2				How To Write Linux PCI Drivers
4			by Martin Mares <mj@ucw.cz> on 07-Feb-2000
5		updated by Grant Grundler <grundler@parisc-linux.org> on 23-Dec-2006
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.
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:
19		http://lwn.net/Kernel/LDD3/
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.
24	Please send questions/comments/patches about Linux PCI API to the
25	"Linux PCI" <linux-pci@atrey.karlin.mff.cuni.cz> mailing list.
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.
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.
42	Once the driver knows about a PCI device and takes ownership, the
43	driver generally needs to perform the following initialization:
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
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
64	Most of these topics are covered in the following sections.
65	For the rest look at LDD3 or <linux/pci.h> .
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.
74	1. pci_register_driver() call
75	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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):
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,...).
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.
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.
107		suspend		Put device into low power state.
108		suspend_late	Put device into low power state.
110		resume_early	Wake device from low power state.
111		resume		Wake device from low power state.
113			(Please see Documentation/power/pci.txt for descriptions
114			of PCI Power Management and the related functions.)
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.
122		err_handler	See Documentation/PCI/pci-error-recovery.txt
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:
129		vendor,device	Vendor and device ID to match (or PCI_ANY_ID)
131		subvendor,	Subsystem vendor and device ID to match (or PCI_ANY_ID)
132		subdevice,
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.
140		class_mask	limit which sub-fields of the class field are compared.
141				See drivers/scsi/sym53c8xx_2/ for example of usage.
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.
150	Most drivers only need PCI_DEVICE() or PCI_DEVICE_CLASS() to set up
151	a pci_device_id table.
153	New PCI IDs may be added to a device driver pci_ids table at runtime
154	as shown below:
156	echo "vendor device subvendor subdevice class class_mask driver_data" > \
157	/sys/bus/pci/drivers/{driver}/new_id
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.
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.
170	Once added, the driver probe routine will be invoked for any unclaimed
171	PCI devices listed in its (newly updated) pci_ids list.
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.
177	1.1 "Attributes" for driver functions/data
179	Please mark the initialization and cleanup functions where appropriate
180	(the corresponding macros are defined in <linux/init.h>):
182		__init		Initialization code. Thrown away after the driver
183				initializes.
184		__exit		Exit code. Ignored for non-modular drivers.
186	Tips on when/where to use the above attributes:
187		o The module_init()/module_exit() functions (and all
188		  initialization functions called _only_ from these)
189		  should be marked __init/__exit.
191		o Do not mark the struct pci_driver.
193		o Do NOT mark a function if you are not sure which mark to use.
194		  Better to not mark the function than mark the function wrong.
198	2. How to find PCI devices manually
199	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
201	PCI drivers should have a really good reason for not using the
202	pci_register_driver() interface to search for PCI devices.
203	The main reason PCI devices are controlled by multiple drivers
204	is because one PCI device implements several different HW services.
205	E.g. combined serial/parallel port/floppy controller.
207	A manual search may be performed using the following constructs:
209	Searching by vendor and device ID:
211		struct pci_dev *dev = NULL;
212		while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
213			configure_device(dev);
215	Searching by class ID (iterate in a similar way):
217		pci_get_class(CLASS_ID, dev)
219	Searching by both vendor/device and subsystem vendor/device ID:
223	You can use the constant PCI_ANY_ID as a wildcard replacement for
224	VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
225	specific vendor, for example.
227	These functions are hotplug-safe. They increment the reference count on
228	the pci_dev that they return. You must eventually (possibly at module unload)
229	decrement the reference count on these devices by calling pci_dev_put().
233	3. Device Initialization Steps
234	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
236	As noted in the introduction, most PCI drivers need the following steps
237	for device initialization:
239		Enable the device
240		Request MMIO/IOP resources
241		Set the DMA mask size (for both coherent and streaming DMA)
242		Allocate and initialize shared control data (pci_allocate_coherent())
243		Access device configuration space (if needed)
244		Register IRQ handler (request_irq())
245		Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
246		Enable DMA/processing engines.
248	The driver can access PCI config space registers at any time.
249	(Well, almost. When running BIST, config space can go away...but
250	that will just result in a PCI Bus Master Abort and config reads
251	will return garbage).
254	3.1 Enable the PCI device
255	~~~~~~~~~~~~~~~~~~~~~~~~~
256	Before touching any device registers, the driver needs to enable
257	the PCI device by calling pci_enable_device(). This will:
258		o wake up the device if it was in suspended state,
259		o allocate I/O and memory regions of the device (if BIOS did not),
260		o allocate an IRQ (if BIOS did not).
262	NOTE: pci_enable_device() can fail! Check the return value.
264	[ OS BUG: we don't check resource allocations before enabling those
265	  resources. The sequence would make more sense if we called
266	  pci_request_resources() before calling pci_enable_device().
267	  Currently, the device drivers can't detect the bug when when two
268	  devices have been allocated the same range. This is not a common
269	  problem and unlikely to get fixed soon.
271	  This has been discussed before but not changed as of 2.6.19:
272		http://lkml.org/lkml/2006/3/2/194
273	]
275	pci_set_master() will enable DMA by setting the bus master bit
276	in the PCI_COMMAND register. It also fixes the latency timer value if
277	it's set to something bogus by the BIOS.  pci_clear_master() will
278	disable DMA by clearing the bus master bit.
280	If the PCI device can use the PCI Memory-Write-Invalidate transaction,
281	call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
282	and also ensures that the cache line size register is set correctly.
283	Check the return value of pci_set_mwi() as not all architectures
284	or chip-sets may support Memory-Write-Invalidate.  Alternatively,
285	if Mem-Wr-Inval would be nice to have but is not required, call
286	pci_try_set_mwi() to have the system do its best effort at enabling
287	Mem-Wr-Inval.
290	3.2 Request MMIO/IOP resources
291	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
292	Memory (MMIO), and I/O port addresses should NOT be read directly
293	from the PCI device config space. Use the values in the pci_dev structure
294	as the PCI "bus address" might have been remapped to a "host physical"
295	address by the arch/chip-set specific kernel support.
297	See Documentation/io-mapping.txt for how to access device registers
298	or device memory.
300	The device driver needs to call pci_request_region() to verify
301	no other device is already using the same address resource.
302	Conversely, drivers should call pci_release_region() AFTER
303	calling pci_disable_device().
304	The idea is to prevent two devices colliding on the same address range.
306	[ See OS BUG comment above. Currently (2.6.19), The driver can only
307	  determine MMIO and IO Port resource availability _after_ calling
308	  pci_enable_device(). ]
310	Generic flavors of pci_request_region() are request_mem_region()
311	(for MMIO ranges) and request_region() (for IO Port ranges).
312	Use these for address resources that are not described by "normal" PCI
313	BARs.
315	Also see pci_request_selected_regions() below.
318	3.3 Set the DMA mask size
319	~~~~~~~~~~~~~~~~~~~~~~~~~
320	[ If anything below doesn't make sense, please refer to
321	  Documentation/DMA-API.txt. This section is just a reminder that
322	  drivers need to indicate DMA capabilities of the device and is not
323	  an authoritative source for DMA interfaces. ]
325	While all drivers should explicitly indicate the DMA capability
326	(e.g. 32 or 64 bit) of the PCI bus master, devices with more than
327	32-bit bus master capability for streaming data need the driver
328	to "register" this capability by calling pci_set_dma_mask() with
329	appropriate parameters.  In general this allows more efficient DMA
330	on systems where System RAM exists above 4G _physical_ address.
332	Drivers for all PCI-X and PCIe compliant devices must call
333	pci_set_dma_mask() as they are 64-bit DMA devices.
335	Similarly, drivers must also "register" this capability if the device
336	can directly address "consistent memory" in System RAM above 4G physical
337	address by calling pci_set_consistent_dma_mask().
338	Again, this includes drivers for all PCI-X and PCIe compliant devices.
339	Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
340	64-bit DMA capable for payload ("streaming") data but not control
341	("consistent") data.
344	3.4 Setup shared control data
345	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
346	Once the DMA masks are set, the driver can allocate "consistent" (a.k.a. shared)
347	memory.  See Documentation/DMA-API.txt for a full description of
348	the DMA APIs. This section is just a reminder that it needs to be done
349	before enabling DMA on the device.
352	3.5 Initialize device registers
353	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
354	Some drivers will need specific "capability" fields programmed
355	or other "vendor specific" register initialized or reset.
356	E.g. clearing pending interrupts.
359	3.6 Register IRQ handler
360	~~~~~~~~~~~~~~~~~~~~~~~~
361	While calling request_irq() is the last step described here,
362	this is often just another intermediate step to initialize a device.
363	This step can often be deferred until the device is opened for use.
365	All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
366	and use the devid to map IRQs to devices (remember that all PCI IRQ lines
367	can be shared).
369	request_irq() will associate an interrupt handler and device handle
370	with an interrupt number. Historically interrupt numbers represent
371	IRQ lines which run from the PCI device to the Interrupt controller.
372	With MSI and MSI-X (more below) the interrupt number is a CPU "vector".
374	request_irq() also enables the interrupt. Make sure the device is
375	quiesced and does not have any interrupts pending before registering
376	the interrupt handler.
378	MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
379	which deliver interrupts to the CPU via a DMA write to a Local APIC.
380	The fundamental difference between MSI and MSI-X is how multiple
381	"vectors" get allocated. MSI requires contiguous blocks of vectors
382	while MSI-X can allocate several individual ones.
384	MSI capability can be enabled by calling pci_enable_msi() or
385	pci_enable_msix() before calling request_irq(). This causes
386	the PCI support to program CPU vector data into the PCI device
387	capability registers.
389	If your PCI device supports both, try to enable MSI-X first.
390	Only one can be enabled at a time.  Many architectures, chip-sets,
391	or BIOSes do NOT support MSI or MSI-X and the call to pci_enable_msi/msix
392	will fail. This is important to note since many drivers have
393	two (or more) interrupt handlers: one for MSI/MSI-X and another for IRQs.
394	They choose which handler to register with request_irq() based on the
395	return value from pci_enable_msi/msix().
397	There are (at least) two really good reasons for using MSI:
398	1) MSI is an exclusive interrupt vector by definition.
399	   This means the interrupt handler doesn't have to verify
400	   its device caused the interrupt.
402	2) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
403	   to be visible to the host CPU(s) when the MSI is delivered. This
404	   is important for both data coherency and avoiding stale control data.
405	   This guarantee allows the driver to omit MMIO reads to flush
406	   the DMA stream.
408	See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
409	of MSI/MSI-X usage.
413	4. PCI device shutdown
414	~~~~~~~~~~~~~~~~~~~~~~~
416	When a PCI device driver is being unloaded, most of the following
417	steps need to be performed:
419		Disable the device from generating IRQs
420		Release the IRQ (free_irq())
421		Stop all DMA activity
422		Release DMA buffers (both streaming and consistent)
423		Unregister from other subsystems (e.g. scsi or netdev)
424		Disable device from responding to MMIO/IO Port addresses
425		Release MMIO/IO Port resource(s)
428	4.1 Stop IRQs on the device
429	~~~~~~~~~~~~~~~~~~~~~~~~~~~
430	How to do this is chip/device specific. If it's not done, it opens
431	the possibility of a "screaming interrupt" if (and only if)
432	the IRQ is shared with another device.
434	When the shared IRQ handler is "unhooked", the remaining devices
435	using the same IRQ line will still need the IRQ enabled. Thus if the
436	"unhooked" device asserts IRQ line, the system will respond assuming
437	it was one of the remaining devices asserted the IRQ line. Since none
438	of the other devices will handle the IRQ, the system will "hang" until
439	it decides the IRQ isn't going to get handled and masks the IRQ (100,000
440	iterations later). Once the shared IRQ is masked, the remaining devices
441	will stop functioning properly. Not a nice situation.
443	This is another reason to use MSI or MSI-X if it's available.
444	MSI and MSI-X are defined to be exclusive interrupts and thus
445	are not susceptible to the "screaming interrupt" problem.
448	4.2 Release the IRQ
449	~~~~~~~~~~~~~~~~~~~
450	Once the device is quiesced (no more IRQs), one can call free_irq().
451	This function will return control once any pending IRQs are handled,
452	"unhook" the drivers IRQ handler from that IRQ, and finally release
453	the IRQ if no one else is using it.
456	4.3 Stop all DMA activity
457	~~~~~~~~~~~~~~~~~~~~~~~~~
458	It's extremely important to stop all DMA operations BEFORE attempting
459	to deallocate DMA control data. Failure to do so can result in memory
460	corruption, hangs, and on some chip-sets a hard crash.
462	Stopping DMA after stopping the IRQs can avoid races where the
463	IRQ handler might restart DMA engines.
465	While this step sounds obvious and trivial, several "mature" drivers
466	didn't get this step right in the past.
469	4.4 Release DMA buffers
470	~~~~~~~~~~~~~~~~~~~~~~~
471	Once DMA is stopped, clean up streaming DMA first.
472	I.e. unmap data buffers and return buffers to "upstream"
473	owners if there is one.
475	Then clean up "consistent" buffers which contain the control data.
477	See Documentation/DMA-API.txt for details on unmapping interfaces.
480	4.5 Unregister from other subsystems
481	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
482	Most low level PCI device drivers support some other subsystem
483	like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
484	driver isn't losing resources from that other subsystem.
485	If this happens, typically the symptom is an Oops (panic) when
486	the subsystem attempts to call into a driver that has been unloaded.
489	4.6 Disable Device from responding to MMIO/IO Port addresses
490	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
491	io_unmap() MMIO or IO Port resources and then call pci_disable_device().
492	This is the symmetric opposite of pci_enable_device().
493	Do not access device registers after calling pci_disable_device().
496	4.7 Release MMIO/IO Port Resource(s)
497	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
498	Call pci_release_region() to mark the MMIO or IO Port range as available.
499	Failure to do so usually results in the inability to reload the driver.
503	5. How to access PCI config space
504	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
506	You can use pci_(read|write)_config_(byte|word|dword) to access the config
507	space of a device represented by struct pci_dev *. All these functions return 0
508	when successful or an error code (PCIBIOS_...) which can be translated to a text
509	string by pcibios_strerror. Most drivers expect that accesses to valid PCI
510	devices don't fail.
512	If you don't have a struct pci_dev available, you can call
513	pci_bus_(read|write)_config_(byte|word|dword) to access a given device
514	and function on that bus.
516	If you access fields in the standard portion of the config header, please
517	use symbolic names of locations and bits declared in <linux/pci.h>.
519	If you need to access Extended PCI Capability registers, just call
520	pci_find_capability() for the particular capability and it will find the
521	corresponding register block for you.
525	6. Other interesting functions
526	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
528	pci_get_domain_bus_and_slot()	Find pci_dev corresponding to given domain,
529					bus and slot and number. If the device is
530					found, its reference count is increased.
531	pci_set_power_state()		Set PCI Power Management state (0=D0 ... 3=D3)
532	pci_find_capability()		Find specified capability in device's capability
533					list.
534	pci_resource_start()		Returns bus start address for a given PCI region
535	pci_resource_end()		Returns bus end address for a given PCI region
536	pci_resource_len()		Returns the byte length of a PCI region
537	pci_set_drvdata()		Set private driver data pointer for a pci_dev
538	pci_get_drvdata()		Return private driver data pointer for a pci_dev
539	pci_set_mwi()			Enable Memory-Write-Invalidate transactions.
540	pci_clear_mwi()			Disable Memory-Write-Invalidate transactions.
544	7. Miscellaneous hints
545	~~~~~~~~~~~~~~~~~~~~~~
547	When displaying PCI device names to the user (for example when a driver wants
548	to tell the user what card has it found), please use pci_name(pci_dev).
550	Always refer to the PCI devices by a pointer to the pci_dev structure.
551	All PCI layer functions use this identification and it's the only
552	reasonable one. Don't use bus/slot/function numbers except for very
553	special purposes -- on systems with multiple primary buses their semantics
554	can be pretty complex.
556	Don't try to turn on Fast Back to Back writes in your driver.  All devices
557	on the bus need to be capable of doing it, so this is something which needs
558	to be handled by platform and generic code, not individual drivers.
562	8. Vendor and device identifications
563	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
565	One is not required to add new device ids to include/linux/pci_ids.h.
566	Please add PCI_VENDOR_ID_xxx for vendors and a hex constant for device ids.
568	PCI_VENDOR_ID_xxx constants are re-used. The device ids are arbitrary
569	hex numbers (vendor controlled) and normally used only in a single
570	location, the pci_device_id table.
572	Please DO submit new vendor/device ids to pciids.sourceforge.net project.
576	9. Obsolete functions
577	~~~~~~~~~~~~~~~~~~~~~
579	There are several functions which you might come across when trying to
580	port an old driver to the new PCI interface.  They are no longer present
581	in the kernel as they aren't compatible with hotplug or PCI domains or
582	having sane locking.
584	pci_find_device()	Superseded by pci_get_device()
585	pci_find_subsys()	Superseded by pci_get_subsys()
586	pci_find_slot()		Superseded by pci_get_domain_bus_and_slot()
587	pci_get_slot()		Superseded by pci_get_domain_bus_and_slot()
590	The alternative is the traditional PCI device driver that walks PCI
591	device lists. This is still possible but discouraged.
595	10. MMIO Space and "Write Posting"
596	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
598	Converting a driver from using I/O Port space to using MMIO space
599	often requires some additional changes. Specifically, "write posting"
600	needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
601	already do this. I/O Port space guarantees write transactions reach the PCI
602	device before the CPU can continue. Writes to MMIO space allow the CPU
603	to continue before the transaction reaches the PCI device. HW weenies
604	call this "Write Posting" because the write completion is "posted" to
605	the CPU before the transaction has reached its destination.
607	Thus, timing sensitive code should add readl() where the CPU is
608	expected to wait before doing other work.  The classic "bit banging"
609	sequence works fine for I/O Port space:
611	       for (i = 8; --i; val >>= 1) {
612	               outb(val & 1, ioport_reg);      /* write bit */
613	               udelay(10);
614	       }
616	The same sequence for MMIO space should be:
618	       for (i = 8; --i; val >>= 1) {
619	               writeb(val & 1, mmio_reg);      /* write bit */
620	               readb(safe_mmio_reg);           /* flush posted write */
621	               udelay(10);
622	       }
624	It is important that "safe_mmio_reg" not have any side effects that
625	interferes with the correct operation of the device.
627	Another case to watch out for is when resetting a PCI device. Use PCI
628	Configuration space reads to flush the writel(). This will gracefully
629	handle the PCI master abort on all platforms if the PCI device is
630	expected to not respond to a readl().  Most x86 platforms will allow
631	MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
632	(e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").
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