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

[Marek Vasut]

On 7/2/21 11:23 AM, Raphael Gallais-Pou wrote:
> Hello Marek,

Hi,

> Sorry for the late answer.

No worries, take your time

> 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.

I just have to wonder, how come other drivers don't need these if 
(!pm_runtime_active()) pm_runtime_get_sync() conditions. I think they 
just get/put the runtime PM within a call itself, not across function 
calls. Maybe that could be the right fix here too ?