Messages in this thread | | | Subject | Re: [PATCH v2] i2c: Allow ACPI_I2C_OPREGION if I2C is built as a module | From | Hans de Goede <> | Date | Wed, 24 Jan 2018 15:59:13 +0100 |
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Hi,
On 24-01-18 15:44, Andy Shevchenko wrote: > On Wed, Jan 24, 2018 at 3:29 PM, Sinan Kaya <okaya@codeaurora.org> wrote: >> +linux-acpi > > +Cc: Hans
Thank you.
>> On 1/24/2018 1:27 AM, Mika Westerberg wrote: >>> On Wed, Jan 24, 2018 at 06:56:16AM +0100, Wolfram Sang wrote: >>>> On Wed, Jan 03, 2018 at 03:02:31PM +0100, Eric Auger wrote: >>>>> If I2C is built as a module, ACPI_I2C_OPREGION cannot be set >>>>> and any ACPI opregion calls targeting I2C fail with no opregion found. >>>>> >>>>> This patch allows ACPI_I2C_OPREGION to be enabled both if I2C is >>>>> built into the kernel or built as a module. >>>>> >>>>> Signed-off-by: Eric Auger <eric.auger@redhat.com> >>>> >>>> I recall that we had some discussion until ending up with the current >>>> solution. And I finally found it again: >>>> >>>> http://www.serverphorums.com/read.php?12,1001402 >>>> >>>> In any case, I surely want Mika's ack on any change to ACPI related >>>> Kconfig symbols. Adding him to CC... >>> >>> So the problem is/was that what happens if you are in a middle of BIOS >>> AML code touching the opregion and someone unloads the opregion handler? >>> If you can quarantee nothing bad happens, then I'm fine with the patch :) >>> >> >> Rafael to correct me if I got this right. >> >> The behavior of the operating system is well defined in the ACPI specification. >> >> Here is what I tested recently: >> >> ACPI defines _REG method to inform firmware of presence/removal of an operating >> region. >> >> When driver gets loaded, ACPI calls the _REG method with 1 argument. When driver >> gets unloaded, ACPI call the _REG method with 0 argument. >> >> Firmware can use this notification to its advantage to determine when an I2C >> related functionality should be accessed or not. >> >> If firmware doesn't use the _REG method, ACPI defines that AML statements >> accessing the operating region are ignored. >> >> You'll also see a warning from ACPICA saying the OperatingRegion 9 is no longer >> accessible and AML code execution failed. >>
There is the ACPI specification, and then there is reality. In reality many DSTDs do not use the typically named AVB? counter-part of _REG to check before accessing OpRegions.
Also OpRegion availability vs probe ordering is a nightmare.
Lets pretend that all DSTDs are perfect and that some device described in ACPI has a _PS0 method which uses an opregion to turn on some regulator powering the device through i2c, but only if the _REG method for that opregion has been called. So now lets say that the driver for this device loads and tries to bind before the i2c-module has loaded. Before the driver's probe method gets called the driver-core will call _PS0 to power-up the device, which is a nop (*). Then the drivers probe function tries to talk to the device, but fails as the device is not powered, so it returns with -ENODEV.
And then later the i2c-module loads, which is to late to get things working, unless the user rmmods and modprobes the driver for the device manually.
TL;DR: I have to NAK this, I'm sorry but with the current state of ACPI we must simply have some stuff builtin to help with probe-ordering issues. Now if the ACPI code where ever to honor the _DEP method everywhere instead of only for battery devices this might change, but even then things will still be tricky.
Regards,
Hans
*) Typically while logging errors about unavailable OpRegions, but as said lets pretend DSTDs actually check the flags set by _REG everywhere.
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