Messages in this thread | | | From | Guenter Roeck <> | Subject | Re: [PATCH] i2c: piix4: Replace piix4_smbus driver's cd6h/cd7h port io accesses with mmio accesses | Date | Thu, 6 Jan 2022 10:18:21 -0800 |
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On 1/4/22 11:34 AM, Terry Bowman wrote: > Hi Jean and Guenter, > > This is a gentle reminder to review my previous response when possible. Thanks! > > Regards, > Terry > > On 12/13/21 11:48 AM, Terry Bowman wrote: >> Hi Jean and Guenter, >> >> Jean, Thanks for your responses. I added comments below. >> >> I added Guenter to this email because his input is needed for adding the same >> changes to the sp5100_tco driver. The sp5100_tco and piix4_smbus driver >> must use the same synchronization logic for the shared register. >> >> On 11/5/21 11:05, Jean Delvare wrote: >>> On Tue, 7 Sep 2021 18:37:20 +0200, Jean Delvare wrote: >>>> More generally, I am worried about the overall design. The driver >>>> originally used per-access I/O port requesting because keeping the I/O >>>> ports busy all the time would prevent other drivers from working. Do we >>>> still need to do the same with the new code? If it is possible and safe >>>> to have a permanent mapping to the memory ports, that would be a lot >>>> faster. >>>> >> >> Permanent mapping would likely improve performance but will not provide the >> needed synchronization. As you mentioned below the sp5100 driver only uses >> the DECODEEN register during initialization but the access must be >> synchronized or an i2c transaction or sp5100_tco timer enable access may be >> lost. I considered alternatives but most lead to driver coupling or considerable >> complexity. >> >>>> On the other hand, the read-modify-write cycle in >>>> piix4_setup_sb800_smba() is unsafe if 2 drivers can actually call >>>> request_mem_region() on the same memory area successfully. >>>> >>>> I'm not opposed to taking your patch with minimal changes (as long as >>>> the code is safe) and working on performance improvements later. >>> >> >> I confirmed through testing the request_mem_region() and request_muxed_region() >> macros provide exclusive locking. One difference between the 2 macros is the >> flag parameter, IORESOURCE_MUXED. request_muxed_region() uses the >> IORESOURCE_MUXED flag to retry the region lock if it's already locked. >> request_mem_region() does not use the IORESOURCE_MUXED and as a result will >> return -EBUSY immediately if the region is already locked. >> >> I must clarify: the piix4_smbus v1 patch uses request_mem_region() which is not >> correct because it doesn't retry locking an already locked region. The driver >> must support retrying the lock or piix4_smbus and sp5100_tco drivers may >> potentially fail loading. I added proposed piix4_smbus v2 changes below to solve. >> >> I propose reusing the existing request_*() framework from include/linux/ioport.h >> and kernel/resource.c. A new helper macro will be required to provide an >> interface to the "muxed" iomem locking functionality already present in >> kernel/resource.c. The new macro will be similar to request_muxed_region() >> but will instead operate on iomem. This should provide the same performance >> while using the existing framework. >> >> My plan is to add the following to include/linux/ioport.h in v2. This macro >> will add the interface for using "muxed" iomem support: >> #define request_mem_muxed_region(start,n,name) __request_region(&iomem_resource, (start), (n), (name), IORESOURCE_MUXED) >> >> The proposed changes will need review from more than one subsystem maintainer. >> The macro addition in include/linux/ioport.h would reside in a >> different maintainer's tree than this driver. The change to use the >> request_mem_muxed_region() macro will also be made to the sp5100_tco driver. >> The v2 review will include maintainers from subsystems owning piix4_smbus >> driver, sp5100_tco driver, and include/linux/ioport.h. >> >> The details provided above are described in a piix4_smbus context but would also be >> applied to the sp5100_tco driver for synchronizing the shared register. >> >> Jean and Guenter, do you have concerns or changes you prefer to the proposal I >> described above? >>
I think you'll need approval from someone with authority to accept the suggested change in include/linux/ioport.h. No idea who that would be.
Guenter
>>> I looked some more at the code. I was thinking that maybe if the >>> registers accessed by the two drivers (i2c-piix4 and sp5100_tco) were >>> disjoint, then each driver could simply request subsets of the mapped >>> memory. >>> >>> Unfortunately, while most registers are indeed exclusively used by one >>> of the drivers, there's one register (0x00 = IsaDecode) which is used >>> by both. So this simple approach isn't possible. >>> >>> That being said, the register in question is only accessed at device >>> initialization time (on the sp5100_tco side, that's in function >>> sp5100_tco_setupdevice) and only for some devices (Embedded FCH). So >>> one approach which may work is to let the i2c-piix4 driver instantiate >>> the watchdog platform device in that case, instead of having sp5100_tco >>> instantiate its own device as is currently the case. That way, the >>> i2c-piix4 driver would request the "shared" memory area, perform the >>> initialization steps for both functions (SMBus and watchdog) and then >>> instantiate the watchdog device so that sp5100_tco gets loaded and goes >>> on with the runtime management of the watchdog device. >>> >>> If I'm not mistaken, this is what the i2c-i801 driver is already doing >>> for the watchdog device in all recent Intel chipsets. So maybe the same >>> approach can work for the i2c-piix4 driver for the AMD chipsets. >>> However I must confess that I did not try to do it nor am I familiar >>> with the sp5100_tco driver details, so maybe it's not possible for some >>> reason. >>> >>> If it's not possible then the only safe approach would be to migrate >>> i2c-piix4 and sp5100_tco to a true MFD setup with 3 separate drivers: >>> one new MFD PCI driver binding to the PCI device, providing access to >>> the registers with proper locking, and instantiating the platform >>> device, one driver for SMBus (basically i2c-piix4 converted to a >>> platform driver and relying on the MFD driver for register access) and >>> one driver for the watchdog (basically sp5100_tco converted to a >>> platform driver and relying on the MFD driver for register access). >>> That's a much larger change though, so I suppose we'd try avoid it if >>> at all possible. >>>
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