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

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Message-ID: <3bb823e4-4724-7072-fe9f-7b8a355c8e50@foss.st.com>
Date:   Fri, 2 Jul 2021 11:23:21 +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

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