Messages in this thread | | | Subject | Re: [PATCH v3 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code | From | Jarkko Nikula <> | Date | Mon, 15 Oct 2018 17:15:40 +0300 |
| |
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@redhat.com> > > 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@linux.intel.com> Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
| |