Hi All, Here is a non RFC version of my previous RFC series for coordinating cht i2c-pmic and i915-punit accesses. New in this non RFC version is that forcewake accesses are coodinated now too. The problem: ============ This series fixes an issue where the following messages are shown in dmesg: [drm:fw_domains_get [i915]] *ERROR* render: timed out waiting for forcewake ack request. [drm:fw_domains_get [i915]] *MEDIA* render: timed out waiting for forcewake ack request. i2c_designware 808622C1:06: punit semaphore timed out, resetting i2c_designware 808622C1:06: PUNIT SEM: 2 i2c_designware 808622C1:06: couldn't acquire bus ownership Usually followed by a gfx or system freeze, this is happening both on my cherrytrail tablet as well as being seen by an user commenting on: https://bugzilla.kernel.org/show_bug.cgi?id=155241 This problem is triggered by my patch series to fix the punit i2c bus semaphore code in drivers/i2c/busses/designware-baytrail.c : https://patchwork.ozlabs.org/patch/710067/ https://patchwork.ozlabs.org/patch/710068/ https://patchwork.ozlabs.org/patch/710069/ https://patchwork.ozlabs.org/patch/710070/ https://patchwork.ozlabs.org/patch/710071/ https://patchwork.ozlabs.org/patch/710072/ Which actually allows i2c access to pmic-s wich need bus-access arbritration for the first time on cherrytrail, combined with patches from me to actually make the axp288_fuel_gauge driver load, which leads to regular pmic i2c accesses (when userspace checks the battery level). The fix / workaround ==================== Testing has shown that just taking the punit i2c bus semaphore on i2c accesses to a shared pmic i2c bus is not enough to avoid problem, e.g. besides this series to coordinate i915 access, we also need: https://patchwork.ozlabs.org/patch/710070/ This series introduces 2 mechanisms to coordinate direct pmic accesses vs punit requests which lead to pmic accesses. The first mechanism is a simple mutex, which works well for the direct punit accesses the i915 code does. However testing has shown this is not enough, we also need to avoid doing a forcewake when a driver is directly accessing the pmic i2c bus, otherwise we get the timeout errors quoted above, the fact that after the forcewake errors the designware-baytrail.c code also fails to acquire the lock, suggests that doing forcewakes during the access window causes the i2c bus to get stuck (or the punit to get confused). The deal with the forcewake issue a notifier is introduced which allows drivers to get notified before any pmic i2c transfer begins (and after it ends). This is used in the i915 driver to do a forcewake_get(FORCEWAKE_ALL) before the access begins avoiding needing to it while the access is in progress. Testing has shown that this avoids the problem for both me and the user, where as before it could be reproduced at will. The implementation of the fix ============================= I've decided to put the mutex and the notifier in the iosf_mbi code, as that is somewhat of a common middle ground between the i915 driver and the designware-baytrail.c code, with the latter already depending on the iosf_mbi code. I'm open for moving them if someone has a suggestion for a better place to put them. If iosf_mbi support is not enabled then all the functions used will become static inline NOPs and nothing changes on the i915 side. If the code is enabled and runs on a system which does not need pmic i2c bus arbritration, then the mutex lock / unlock calls added to the i915 code will still be made, but there won't be any contention, so other then the overhead of the lock / unlock there will not be any delays. These calls are not made in any hot paths. The biggest downside IMHO is that taking FORCEWAKE_ALL before each bus access will cause extra GPU wakeups, and thus powerdrain. I plan to make the impact of this minimal by modifying any drivers (axp288 code, with which I'm quite familiar by now) to limit their i2c accesses to a bare minimum. Specifically I plan to e.g. cache the battery level and wait at least 15 seconds (max 4 times a minute) before getting a fresh reading even if userspace asks for it more often, combined with some mechanism to only do 1 access-begin and 1 access-end notification for i2c accesses in quick succession. Alternatives ============ It would be tempting to say that this is not worth the trouble and we should just disallow direct i2c accesses to the pmic bus. Unfortunately that will cause some major issues on affected devices: -No battery monitoring -No "AC" plugged in monitoring -If booted with a normal USB-A -> micro-USB cable, or no cable, plugged in and then the user replaces the cable with an otg USB-host cable / adapter, the id-pin shorting will enable a 5v boost convertor, but we need to disable the pmic's USB-Vbus path otherwise it will start drawing current from the boost convertor, leading to aprox 300mA of extra battery drain, this is done by the axp288_charger driver, which needs direct i2c access to the pmic bus Way forward =========== I realize the workaround this patch-set adds is not the prettiest code ever seen, but I see no otherway to get things like battery monitoring and proper OTG support working on affected devices without breaking drm/i915 support. So I would like to see this merged in some form, I'm happy to make any kind of requested changes; and/or to try alternative fixes. Regards, Hans -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
