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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 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.  Definitions with static const are generally preferred.
127	Use of the deprecated macro DEFINE_PCI_DEVICE_TABLE should be avoided.
129	Each entry consists of:
131		vendor,device	Vendor and device ID to match (or PCI_ANY_ID)
133		subvendor,	Subsystem vendor and device ID to match (or PCI_ANY_ID)
134		subdevice,
136		class		Device class, subclass, and "interface" to match.
137				See Appendix D of the PCI Local Bus Spec or
138				include/linux/pci_ids.h for a full list of classes.
139				Most drivers do not need to specify class/class_mask
140				as vendor/device is normally sufficient.
142		class_mask	limit which sub-fields of the class field are compared.
143				See drivers/scsi/sym53c8xx_2/ for example of usage.
145		driver_data	Data private to the driver.
146				Most drivers don't need to use driver_data field.
147				Best practice is to use driver_data as an index
148				into a static list of equivalent device types,
149				instead of using it as a pointer.
152	Most drivers only need PCI_DEVICE() or PCI_DEVICE_CLASS() to set up
153	a pci_device_id table.
155	New PCI IDs may be added to a device driver pci_ids table at runtime
156	as shown below:
158	echo "vendor device subvendor subdevice class class_mask driver_data" > \
159	/sys/bus/pci/drivers/{driver}/new_id
161	All fields are passed in as hexadecimal values (no leading 0x).
162	The vendor and device fields are mandatory, the others are optional. Users
163	need pass only as many optional fields as necessary:
164		o subvendor and subdevice fields default to PCI_ANY_ID (FFFFFFFF)
165		o class and classmask fields default to 0
166		o driver_data defaults to 0UL.
168	Note that driver_data must match the value used by any of the pci_device_id
169	entries defined in the driver. This makes the driver_data field mandatory
170	if all the pci_device_id entries have a non-zero driver_data value.
172	Once added, the driver probe routine will be invoked for any unclaimed
173	PCI devices listed in its (newly updated) pci_ids list.
175	When the driver exits, it just calls pci_unregister_driver() and the PCI layer
176	automatically calls the remove hook for all devices handled by the driver.
179	1.1 "Attributes" for driver functions/data
181	Please mark the initialization and cleanup functions where appropriate
182	(the corresponding macros are defined in <linux/init.h>):
184		__init		Initialization code. Thrown away after the driver
185				initializes.
186		__exit		Exit code. Ignored for non-modular drivers.
188	Tips on when/where to use the above attributes:
189		o The module_init()/module_exit() functions (and all
190		  initialization functions called _only_ from these)
191		  should be marked __init/__exit.
193		o Do not mark the struct pci_driver.
195		o Do NOT mark a function if you are not sure which mark to use.
196		  Better to not mark the function than mark the function wrong.
200	2. How to find PCI devices manually
201	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
203	PCI drivers should have a really good reason for not using the
204	pci_register_driver() interface to search for PCI devices.
205	The main reason PCI devices are controlled by multiple drivers
206	is because one PCI device implements several different HW services.
207	E.g. combined serial/parallel port/floppy controller.
209	A manual search may be performed using the following constructs:
211	Searching by vendor and device ID:
213		struct pci_dev *dev = NULL;
214		while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
215			configure_device(dev);
217	Searching by class ID (iterate in a similar way):
219		pci_get_class(CLASS_ID, dev)
221	Searching by both vendor/device and subsystem vendor/device ID:
225	You can use the constant PCI_ANY_ID as a wildcard replacement for
226	VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
227	specific vendor, for example.
229	These functions are hotplug-safe. They increment the reference count on
230	the pci_dev that they return. You must eventually (possibly at module unload)
231	decrement the reference count on these devices by calling pci_dev_put().
235	3. Device Initialization Steps
236	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
238	As noted in the introduction, most PCI drivers need the following steps
239	for device initialization:
241		Enable the device
242		Request MMIO/IOP resources
243		Set the DMA mask size (for both coherent and streaming DMA)
244		Allocate and initialize shared control data (pci_allocate_coherent())
245		Access device configuration space (if needed)
246		Register IRQ handler (request_irq())
247		Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
248		Enable DMA/processing engines.
250	The driver can access PCI config space registers at any time.
251	(Well, almost. When running BIST, config space can go away...but
252	that will just result in a PCI Bus Master Abort and config reads
253	will return garbage).
256	3.1 Enable the PCI device
257	~~~~~~~~~~~~~~~~~~~~~~~~~
258	Before touching any device registers, the driver needs to enable
259	the PCI device by calling pci_enable_device(). This will:
260		o wake up the device if it was in suspended state,
261		o allocate I/O and memory regions of the device (if BIOS did not),
262		o allocate an IRQ (if BIOS did not).
264	NOTE: pci_enable_device() can fail! Check the return value.
266	[ OS BUG: we don't check resource allocations before enabling those
267	  resources. The sequence would make more sense if we called
268	  pci_request_resources() before calling pci_enable_device().
269	  Currently, the device drivers can't detect the bug when when two
270	  devices have been allocated the same range. This is not a common
271	  problem and unlikely to get fixed soon.
273	  This has been discussed before but not changed as of 2.6.19:
274		http://lkml.org/lkml/2006/3/2/194
275	]
277	pci_set_master() will enable DMA by setting the bus master bit
278	in the PCI_COMMAND register. It also fixes the latency timer value if
279	it's set to something bogus by the BIOS.  pci_clear_master() will
280	disable DMA by clearing the bus master bit.
282	If the PCI device can use the PCI Memory-Write-Invalidate transaction,
283	call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
284	and also ensures that the cache line size register is set correctly.
285	Check the return value of pci_set_mwi() as not all architectures
286	or chip-sets may support Memory-Write-Invalidate.  Alternatively,
287	if Mem-Wr-Inval would be nice to have but is not required, call
288	pci_try_set_mwi() to have the system do its best effort at enabling
289	Mem-Wr-Inval.
292	3.2 Request MMIO/IOP resources
293	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
294	Memory (MMIO), and I/O port addresses should NOT be read directly
295	from the PCI device config space. Use the values in the pci_dev structure
296	as the PCI "bus address" might have been remapped to a "host physical"
297	address by the arch/chip-set specific kernel support.
299	See Documentation/io-mapping.txt for how to access device registers
300	or device memory.
302	The device driver needs to call pci_request_region() to verify
303	no other device is already using the same address resource.
304	Conversely, drivers should call pci_release_region() AFTER
305	calling pci_disable_device().
306	The idea is to prevent two devices colliding on the same address range.
308	[ See OS BUG comment above. Currently (2.6.19), The driver can only
309	  determine MMIO and IO Port resource availability _after_ calling
310	  pci_enable_device(). ]
312	Generic flavors of pci_request_region() are request_mem_region()
313	(for MMIO ranges) and request_region() (for IO Port ranges).
314	Use these for address resources that are not described by "normal" PCI
315	BARs.
317	Also see pci_request_selected_regions() below.
320	3.3 Set the DMA mask size
321	~~~~~~~~~~~~~~~~~~~~~~~~~
322	[ If anything below doesn't make sense, please refer to
323	  Documentation/DMA-API.txt. This section is just a reminder that
324	  drivers need to indicate DMA capabilities of the device and is not
325	  an authoritative source for DMA interfaces. ]
327	While all drivers should explicitly indicate the DMA capability
328	(e.g. 32 or 64 bit) of the PCI bus master, devices with more than
329	32-bit bus master capability for streaming data need the driver
330	to "register" this capability by calling pci_set_dma_mask() with
331	appropriate parameters.  In general this allows more efficient DMA
332	on systems where System RAM exists above 4G _physical_ address.
334	Drivers for all PCI-X and PCIe compliant devices must call
335	pci_set_dma_mask() as they are 64-bit DMA devices.
337	Similarly, drivers must also "register" this capability if the device
338	can directly address "consistent memory" in System RAM above 4G physical
339	address by calling pci_set_consistent_dma_mask().
340	Again, this includes drivers for all PCI-X and PCIe compliant devices.
341	Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
342	64-bit DMA capable for payload ("streaming") data but not control
343	("consistent") data.
346	3.4 Setup shared control data
347	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
348	Once the DMA masks are set, the driver can allocate "consistent" (a.k.a. shared)
349	memory.  See Documentation/DMA-API.txt for a full description of
350	the DMA APIs. This section is just a reminder that it needs to be done
351	before enabling DMA on the device.
354	3.5 Initialize device registers
355	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
356	Some drivers will need specific "capability" fields programmed
357	or other "vendor specific" register initialized or reset.
358	E.g. clearing pending interrupts.
361	3.6 Register IRQ handler
362	~~~~~~~~~~~~~~~~~~~~~~~~
363	While calling request_irq() is the last step described here,
364	this is often just another intermediate step to initialize a device.
365	This step can often be deferred until the device is opened for use.
367	All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
368	and use the devid to map IRQs to devices (remember that all PCI IRQ lines
369	can be shared).
371	request_irq() will associate an interrupt handler and device handle
372	with an interrupt number. Historically interrupt numbers represent
373	IRQ lines which run from the PCI device to the Interrupt controller.
374	With MSI and MSI-X (more below) the interrupt number is a CPU "vector".
376	request_irq() also enables the interrupt. Make sure the device is
377	quiesced and does not have any interrupts pending before registering
378	the interrupt handler.
380	MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
381	which deliver interrupts to the CPU via a DMA write to a Local APIC.
382	The fundamental difference between MSI and MSI-X is how multiple
383	"vectors" get allocated. MSI requires contiguous blocks of vectors
384	while MSI-X can allocate several individual ones.
386	MSI capability can be enabled by calling pci_enable_msi() or
387	pci_enable_msix() before calling request_irq(). This causes
388	the PCI support to program CPU vector data into the PCI device
389	capability registers.
391	If your PCI device supports both, try to enable MSI-X first.
392	Only one can be enabled at a time.  Many architectures, chip-sets,
393	or BIOSes do NOT support MSI or MSI-X and the call to pci_enable_msi/msix
394	will fail. This is important to note since many drivers have
395	two (or more) interrupt handlers: one for MSI/MSI-X and another for IRQs.
396	They choose which handler to register with request_irq() based on the
397	return value from pci_enable_msi/msix().
399	There are (at least) two really good reasons for using MSI:
400	1) MSI is an exclusive interrupt vector by definition.
401	   This means the interrupt handler doesn't have to verify
402	   its device caused the interrupt.
404	2) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
405	   to be visible to the host CPU(s) when the MSI is delivered. This
406	   is important for both data coherency and avoiding stale control data.
407	   This guarantee allows the driver to omit MMIO reads to flush
408	   the DMA stream.
410	See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
411	of MSI/MSI-X usage.
415	4. PCI device shutdown
416	~~~~~~~~~~~~~~~~~~~~~~~
418	When a PCI device driver is being unloaded, most of the following
419	steps need to be performed:
421		Disable the device from generating IRQs
422		Release the IRQ (free_irq())
423		Stop all DMA activity
424		Release DMA buffers (both streaming and consistent)
425		Unregister from other subsystems (e.g. scsi or netdev)
426		Disable device from responding to MMIO/IO Port addresses
427		Release MMIO/IO Port resource(s)
430	4.1 Stop IRQs on the device
431	~~~~~~~~~~~~~~~~~~~~~~~~~~~
432	How to do this is chip/device specific. If it's not done, it opens
433	the possibility of a "screaming interrupt" if (and only if)
434	the IRQ is shared with another device.
436	When the shared IRQ handler is "unhooked", the remaining devices
437	using the same IRQ line will still need the IRQ enabled. Thus if the
438	"unhooked" device asserts IRQ line, the system will respond assuming
439	it was one of the remaining devices asserted the IRQ line. Since none
440	of the other devices will handle the IRQ, the system will "hang" until
441	it decides the IRQ isn't going to get handled and masks the IRQ (100,000
442	iterations later). Once the shared IRQ is masked, the remaining devices
443	will stop functioning properly. Not a nice situation.
445	This is another reason to use MSI or MSI-X if it's available.
446	MSI and MSI-X are defined to be exclusive interrupts and thus
447	are not susceptible to the "screaming interrupt" problem.
450	4.2 Release the IRQ
451	~~~~~~~~~~~~~~~~~~~
452	Once the device is quiesced (no more IRQs), one can call free_irq().
453	This function will return control once any pending IRQs are handled,
454	"unhook" the drivers IRQ handler from that IRQ, and finally release
455	the IRQ if no one else is using it.
458	4.3 Stop all DMA activity
459	~~~~~~~~~~~~~~~~~~~~~~~~~
460	It's extremely important to stop all DMA operations BEFORE attempting
461	to deallocate DMA control data. Failure to do so can result in memory
462	corruption, hangs, and on some chip-sets a hard crash.
464	Stopping DMA after stopping the IRQs can avoid races where the
465	IRQ handler might restart DMA engines.
467	While this step sounds obvious and trivial, several "mature" drivers
468	didn't get this step right in the past.
471	4.4 Release DMA buffers
472	~~~~~~~~~~~~~~~~~~~~~~~
473	Once DMA is stopped, clean up streaming DMA first.
474	I.e. unmap data buffers and return buffers to "upstream"
475	owners if there is one.
477	Then clean up "consistent" buffers which contain the control data.
479	See Documentation/DMA-API.txt for details on unmapping interfaces.
482	4.5 Unregister from other subsystems
483	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
484	Most low level PCI device drivers support some other subsystem
485	like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
486	driver isn't losing resources from that other subsystem.
487	If this happens, typically the symptom is an Oops (panic) when
488	the subsystem attempts to call into a driver that has been unloaded.
491	4.6 Disable Device from responding to MMIO/IO Port addresses
492	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
493	io_unmap() MMIO or IO Port resources and then call pci_disable_device().
494	This is the symmetric opposite of pci_enable_device().
495	Do not access device registers after calling pci_disable_device().
498	4.7 Release MMIO/IO Port Resource(s)
499	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
500	Call pci_release_region() to mark the MMIO or IO Port range as available.
501	Failure to do so usually results in the inability to reload the driver.
505	5. How to access PCI config space
506	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
508	You can use pci_(read|write)_config_(byte|word|dword) to access the config
509	space of a device represented by struct pci_dev *. All these functions return 0
510	when successful or an error code (PCIBIOS_...) which can be translated to a text
511	string by pcibios_strerror. Most drivers expect that accesses to valid PCI
512	devices don't fail.
514	If you don't have a struct pci_dev available, you can call
515	pci_bus_(read|write)_config_(byte|word|dword) to access a given device
516	and function on that bus.
518	If you access fields in the standard portion of the config header, please
519	use symbolic names of locations and bits declared in <linux/pci.h>.
521	If you need to access Extended PCI Capability registers, just call
522	pci_find_capability() for the particular capability and it will find the
523	corresponding register block for you.
527	6. Other interesting functions
528	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
530	pci_get_domain_bus_and_slot()	Find pci_dev corresponding to given domain,
531					bus and slot and number. If the device is
532					found, its reference count is increased.
533	pci_set_power_state()		Set PCI Power Management state (0=D0 ... 3=D3)
534	pci_find_capability()		Find specified capability in device's capability
535					list.
536	pci_resource_start()		Returns bus start address for a given PCI region
537	pci_resource_end()		Returns bus end address for a given PCI region
538	pci_resource_len()		Returns the byte length of a PCI region
539	pci_set_drvdata()		Set private driver data pointer for a pci_dev
540	pci_get_drvdata()		Return private driver data pointer for a pci_dev
541	pci_set_mwi()			Enable Memory-Write-Invalidate transactions.
542	pci_clear_mwi()			Disable Memory-Write-Invalidate transactions.
546	7. Miscellaneous hints
547	~~~~~~~~~~~~~~~~~~~~~~
549	When displaying PCI device names to the user (for example when a driver wants
550	to tell the user what card has it found), please use pci_name(pci_dev).
552	Always refer to the PCI devices by a pointer to the pci_dev structure.
553	All PCI layer functions use this identification and it's the only
554	reasonable one. Don't use bus/slot/function numbers except for very
555	special purposes -- on systems with multiple primary buses their semantics
556	can be pretty complex.
558	Don't try to turn on Fast Back to Back writes in your driver.  All devices
559	on the bus need to be capable of doing it, so this is something which needs
560	to be handled by platform and generic code, not individual drivers.
564	8. Vendor and device identifications
565	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
567	Do not add new device or vendor IDs to include/linux/pci_ids.h unless they
568	are shared across multiple drivers.  You can add private definitions in
569	your driver if they're helpful, or just use plain hex constants.
571	The device IDs are arbitrary hex numbers (vendor controlled) and normally used
572	only in a single location, the pci_device_id table.
574	Please DO submit new vendor/device IDs to http://pciids.sourceforge.net/.
578	9. Obsolete functions
579	~~~~~~~~~~~~~~~~~~~~~
581	There are several functions which you might come across when trying to
582	port an old driver to the new PCI interface.  They are no longer present
583	in the kernel as they aren't compatible with hotplug or PCI domains or
584	having sane locking.
586	pci_find_device()	Superseded by pci_get_device()
587	pci_find_subsys()	Superseded by pci_get_subsys()
588	pci_find_slot()		Superseded by pci_get_domain_bus_and_slot()
589	pci_get_slot()		Superseded by pci_get_domain_bus_and_slot()
592	The alternative is the traditional PCI device driver that walks PCI
593	device lists. This is still possible but discouraged.
597	10. MMIO Space and "Write Posting"
598	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
600	Converting a driver from using I/O Port space to using MMIO space
601	often requires some additional changes. Specifically, "write posting"
602	needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
603	already do this. I/O Port space guarantees write transactions reach the PCI
604	device before the CPU can continue. Writes to MMIO space allow the CPU
605	to continue before the transaction reaches the PCI device. HW weenies
606	call this "Write Posting" because the write completion is "posted" to
607	the CPU before the transaction has reached its destination.
609	Thus, timing sensitive code should add readl() where the CPU is
610	expected to wait before doing other work.  The classic "bit banging"
611	sequence works fine for I/O Port space:
613	       for (i = 8; --i; val >>= 1) {
614	               outb(val & 1, ioport_reg);      /* write bit */
615	               udelay(10);
616	       }
618	The same sequence for MMIO space should be:
620	       for (i = 8; --i; val >>= 1) {
621	               writeb(val & 1, mmio_reg);      /* write bit */
622	               readb(safe_mmio_reg);           /* flush posted write */
623	               udelay(10);
624	       }
626	It is important that "safe_mmio_reg" not have any side effects that
627	interferes with the correct operation of the device.
629	Another case to watch out for is when resetting a PCI device. Use PCI
630	Configuration space reads to flush the writel(). This will gracefully
631	handle the PCI master abort on all platforms if the PCI device is
632	expected to not respond to a readl().  Most x86 platforms will allow
633	MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
634	(e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").
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