Date:   Thu, 18 Oct 2018 10:47:44 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit
 PMIC bus semaphore code

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@...hat.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@...hat.com>
>>>
>>> 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

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