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

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Date:   Fri, 2 Jul 2021 20:07:34 +0200
From:   Marek Vasut <marex@...x.de>
To:     Raphael Gallais-Pou <raphael.gallais-pou@...s.st.com>
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 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 ?