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

[Hans de Goede]

HI,

On 18-10-18 10:38, Rafael J. Wysocki wrote:
> On Thursday, October 18, 2018 10:34:57 AM CEST Hans de Goede wrote:
> > Hi,
> >
> > On 18-10-18 09:29, Rafael J. Wysocki 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.
> >
> > In that case let me send out a v4, with the following chunk added to the
> > 2nd patch:
> >
> > --- a/drivers/acpi/Kconfig
> > +++ b/drivers/acpi/Kconfig
> > @@ -515,7 +515,7 @@ config CRC_PMIC_OPREGION
> >
> >    config XPOWER_PMIC_OPREGION
> >           bool "ACPI operation region support for XPower AXP288 PMIC"
> > -       depends on MFD_AXP20X_I2C
> > +       depends on MFD_AXP20X_I2C && IOSF_MBI
> >           help
> >             This config adds ACPI operation region support for XPower AXP288 PMIC.
> >
> > This is necessary to avoid compilation issues on non x86 systems (where the
> > asm/iosf_mbi.h header is not available) and on x86 systems in case
> > IOSF_MBI support is not enabled there.  Note that the AXP288 PMIC is
> > connected through the LPSS i2c controller, so either we have IOSF_MBI support
> > selected through the X86_INTEL_LPSS option, or we have a kernel where the
> > opregion will never work anyways.
>
> I'd prefer to get an incremental patch for that at this point.

Ok, then I will prepare and send out an incremental patch for that.

Regards,

Hans