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

[Jarkko Nikula]

On 10/18/2018 10:36 AM, Andy Shevchenko wrote:
> On Thu, Oct 18, 2018 at 10:33 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> > On Sun, Sep 23, 2018 at 4:45 PM Hans de Goede <hdegoede@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > 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