From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jarkko Nikula Subject: Re: [PATCH 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code Date: Thu, 18 Oct 2018 11:01:37 +0300 Message-ID: <3093be44-3dc0-4e47-8d4d-b4f69818a0d3@linux.intel.com> References: <20180923144510.4564-1-hdegoede@redhat.com> <20180923144510.4564-2-hdegoede@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Andy Shevchenko , "Rafael J. Wysocki" Cc: Hans de Goede , Andy Shevchenko , Mika Westerberg , Wolfram Sang , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , ACPI Devel Maling List , linux-i2c , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Linux Kernel Mailing List List-Id: linux-i2c@vger.kernel.org On 10/18/2018 10:36 AM, Andy Shevchenko wrote: > On Thu, Oct 18, 2018 at 10:33 AM Rafael J. Wysocki wrote: >> >> On Sun, Sep 23, 2018 at 4:45 PM 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 >> >> 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