linux-kernel - Re: [PATCH] drm/stm: ltdc: improve pm

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Date:   Tue, 17 Aug 2021 11:43:13 +0200
From:   Raphael Gallais-Pou <raphael.gallais-pou@...s.st.com>
To:     Marek Vasut <marex@...x.de>
CC:     Yannick FERTRE <yannick.fertre@...com>,
        Philippe CORNU <philippe.cornu@...com>,
        Raphael GALLAIS-POU <raphael.gallais-pou@...com>,
        Yannick FERTRE - foss <yannick.fertre@...s.st.com>,
        Philippe CORNU - foss <philippe.cornu@...s.st.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        David Airlie <airlied@...ux.ie>,
        "Daniel Vetter" <daniel@...ll.ch>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE - foss <alexandre.torgue@...s.st.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stephen Boyd <swboyd@...omium.org>
Subject: 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:
>> 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 ?


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