Re: [PATCH v2] i2c: Allow ACPI_I2C_OPREGION if I2C is built as a module

[Mika Westerberg]

On Wed, Jan 24, 2018 at 08:29:44AM -0500, Sinan Kaya wrote:
> +linux-acpi
> 
> 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@xxxxxxxxxx>
> >>
> >> 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.
> 
> Also note that someone can always unbind an I2C driver from ACPI even with built-in
> module.

Yes, that's right.

However, the I2C core parts are not removed when you unbind a driver and
the imporant piece is i2c_acpi_space_handler() which then stays in the
memory even if any of the drivers get removed. After this patch you can
also remove the i2c_acpi_space_handler() which might cause issues if
there is AML code running at the same time using the opregion.

Commit da3c6647ee08 ("I2C/ACPI: Clean up I2C ACPI code and Add
CONFIG_I2C_ACPI config") says following:

    Current there is a race between removing I2C ACPI operation region
    and ACPI AML code accessing. So make i2c core built-in if
    CONFIG_I2C_ACPI is set.

But I don't remember all the details unfortunately so adding Rafael and
Tianyu if they can shed some more light into this ;-)