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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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