Re: [PATCH] drm/msm: Check for powered down HW in the devfreq callbacks

[Eric Anholt]

On Fri, May 1, 2020 at 11:26 AM Jordan Crouse <jcrouse@xxxxxxxxxxxxxx> wrote:
>
> Writing to the devfreq sysfs nodes while the GPU is powered down can
> result in a system crash (on a5xx) or a nasty GMU error (on a6xx):
>
>  $ /sys/class/devfreq/5000000.gpu# echo 500000000 > min_freq
>   [  104.841625] platform 506a000.gmu: [drm:a6xx_gmu_set_oob]
>         *ERROR* Timeout waiting for GMU OOB set GPU_DCVS: 0x0
>
> Despite the fact that we carefully try to suspend the devfreq device when
> the hardware is powered down there are lots of holes in the governors that
> don't check for the suspend state and blindly call into the devfreq
> callbacks that end up triggering hardware reads in the GPU driver.
>
> Check the power state in the gpu_busy() and gpu_set_freq() callbacks for
> a5xx and a6xx to make sure that the hardware is active before trying to
> access it.

Chatted on IRC -- while this avoids the instaboot on db820c when
setting /sys/class/devfreq/devfreq1/min_freq, I think we should be
using pm_runtime_get_if_in_use() to avoid the races while still
avoiding bringing up the GPU.