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?