[PATCH] drm/stm: Enable RPM during fbdev registration

[Marek Vasut]

On 11/5/20 10:39 AM, Daniel Vetter wrote:
> On Wed, Nov 04, 2020 at 01:52:00PM +0100, Marek Vasut wrote:
>> Enable runtime PM before registering the fbdev emulation and disable it
>> afterward, otherwise register access to the LTDC IP during the fbdev
>> emulation registration might hang the system.
>>
>> The problem happens because RPM is activated at the end of ltdc_load(),
>> but the fbdev emulation registration happens only after that, and ends
>> up calling ltdc_crtc_mode_set_nofb(), which checks whether RPM is active
>> and only if it is not active, calls pm_runtime_get_sync() to enable the
>> clock and so on. If the clock are not enabled, any register access in
>> ltdc_crtc_mode_set_nofb() could hang the platform completely.
>>
>> This patch makes sure that ltdc_crtc_mode_set_nofb() is called within
>> pm_runtime_get_sync(), so with clock enabled.

[...]

> This looks like you're papering over a bug in your modeset code. If
> userspace later on does a setpar on the fbdev chardev, the exact same
> thing could happen. You need to fix your modeset code to avoid this, not
> sprinkle temporary rpm_get/put all over some top level entry points,
> because you can't even patch those all.

I have a feeling all those pm_runtime_active() checks in the driver 
might be the root cause of this ? I wonder why the code doesn't use 
pm_runtime_{get,put}_sync() only when accessing registers. Thoughts?