Re: [PATCH v3 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code

[Jarkko Nikula]

On 10/14/2018 04:17 PM, Wolfram Sang wrote:
> On Thu, Oct 11, 2018 at 04:29:09PM +0200, Hans de Goede 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>
>
> For the record: once the designware maintainers are okay with this
> change, I am also okay with it going via the x86 platform tree.
Acked-by: Jarkko Nikula <jarkko.nikula@xxxxxxxxxxxxxxx>
Tested-by: Jarkko Nikula <jarkko.nikula@xxxxxxxxxxxxxxx>