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

On 7/2/21 8:07 PM, Marek Vasut wrote:

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

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Hello Marek,

Hi,

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Sorry for the late answer.

No worries, take your time

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On 6/30/21 2:35 AM, Marek Vasut wrote:

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On 6/29/21 1:58 PM, Raphael GALLAIS-POU - foss wrote:

[...]

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+++ 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 ?

Hello Marek,


I've run a deeper analysis over this implementation.

If I may take rockchip's "rockchip_drm_vop.c" driver, there is an boolean "is_enabled" set to true when crtc_atomic_enable is called.

The above implementation could save us from adding such field in the ltdc_dev structure.

Another solution could be in order to simply call pm_runtime_get_sync() in ltdc_crtc_mode_set_nofb() and by removing this condition in ltdc_atomic_crtc_disable() the driver behaves just like the first version of this patch.

In this way, it avoids such conditions and seems more to get along with the current implementation.


Regards,

Raphaël


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