Re: [PATCH] drm/stm: ltdc: improve pm_runtime to stop clocks

[Raphael Gallais-Pou]

Hello Marek,


Sorry for the late answer.


On 6/30/21 2:35 AM, Marek Vasut wrote:
> On 6/29/21 1:58 PM, Raphael GALLAIS-POU - foss wrote:
>
> [...]
>
> > +++ b/drivers/gpu/drm/stm/ltdc.c
> > @@ -425,10 +425,17 @@ static void ltdc_crtc_atomic_enable(struct 
> > drm_crtc *crtc,
> >   {
> >       struct ltdc_device *ldev = crtc_to_ltdc(crtc);
> >       struct drm_device *ddev = crtc->dev;
> > +    int ret;
> >         DRM_DEBUG_DRIVER("\n");
> >   -    pm_runtime_get_sync(ddev->dev);
> > +    if (!pm_runtime_active(ddev->dev)) {
> > +        ret = pm_runtime_get_sync(ddev->dev);
>
> All these if (!pm_runtime_active()) then pm_runtime_get_sync() calls 
> look like workaround for some larger issue. Shouldn't the pm_runtime 
> do some refcounting on its own , so this shouldn't be needed ?


This problem purely comes from the driver internals, so I don't think it 
is a workaround.

Because of the "ltdc_crtc_mode_set_nofb" function which does not have 
any "symmetrical" call, such as enable/disable functions, there was two 
calls to pm_runtime_get_sync against one call to pm_runtime_put_sync.

This instability resulted in the LTDC clocks being always enabled, even 
when the peripheral was disabled. This could be seen in the clk_summary 
as explained in the patch summary among other things.

By doing so, we first check if the clocks are not already activated, and 
in that case we call pm_runtime_get_sync.



Regards,

Raphaël G-P