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 ;-)
