Re: [PATCH 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code

[Jarkko Nikula <jarkko.nikula@linux.intel.com>]

On 10/18/2018 10:36 AM, Andy Shevchenko wrote:
> On Thu, Oct 18, 2018 at 10:33 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>>
>> On Sun, Sep 23, 2018 at 4:45 PM Hans de Goede <hdegoede@redhat.com> wrote:
>>>
>>> On some BYT/CHT systems the SoC's P-Unit shares the I2C bus with the
>>> kernel. The P-Unit has a semaphore for the PMIC bus which we can take to
>>> block it from accessing the shared bus while the kernel wants to access it.
>>>
>>> Currently we have the I2C-controller driver acquiring and releasing the
>>> semaphore around each I2C transfer. There are 2 problems with this:
>>>
>>> 1) PMIC accesses often come in the form of a read-modify-write on one of
>>> the PMIC registers, we currently release the P-Unit's PMIC bus semaphore
>>> between the read and the write. If the P-Unit modifies the register during
>>> this window?, then we end up overwriting the P-Unit's changes.
>>> I believe that this is mostly an academic problem, but I'm not sure.
>>>
>>> 2) To safely access the shared I2C bus, we need to do 3 things:
>>> a) Notify the GPU driver that we are starting a window in which it may not
>>> access the P-Unit, since the P-Unit seems to ignore the semaphore for
>>> explicit power-level requests made by the GPU driver
>>> b) Make a pm_qos request to force all CPU cores out of C6/C7 since entering
>>> C6/C7 while we hold the semaphore hangs the SoC
>>> c) Finally take the P-Unit's PMIC bus semaphore
>>> All 3 these steps together are somewhat expensive, so ideally if we have
>>> a bunch of i2c transfers grouped together we only do this once for the
>>> entire group.
>>>
>>> Taking the read-modify-write on a PMIC register as example then ideally we
>>> would only do all 3 steps once at the beginning and undo all 3 steps once
>>> at the end.
>>>
>>> For this we need to be able to take the semaphore from within e.g. the PMIC
>>> opregion driver, yet we do not want to remove the taking of the semaphore
>>> from the I2C-controller driver, as that is still necessary to protect many
>>> other code-paths leading to accessing the shared I2C bus.
>>>
>>> This means that we first have the PMIC driver acquire the semaphore and
>>> then have the I2C controller driver trying to acquire it again.
>>>
>>> To make this possible this commit does the following:
>>>
>>> 1) Move the semaphore code from being private to the I2C controller driver
>>> into the generic iosf_mbi code, which already has other code to deal with
>>> the shared bus so that it can be accessed outside of the I2C bus driver.
>>>
>>> 2) Rework the code so that it can be called multiple times nested, while
>>> still blocking I2C accesses while e.g. the GPU driver has indicated the
>>> P-Unit needs the bus through a iosf_mbi_punit_acquire() call.
>>>
>>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
>>
>> If there are no objections or concerns regarding this patch, I'm
>> inclined to take the entire series including it.
> 
> Please, go ahead, it looks good to me.
> Thanks!
> 
Just in case note: please remember to take patches and tags from v3 of 
the series, not from this v1 thread.

-- 
Jarkko