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


Raphaël G-P

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