linux-kernel - Re: [PATCH v2] drm/vc4: Fix pm_runtime_get

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Message-ID: <20220420132814.3sy2ojw3dxnd5tcz@houat>
Date:   Wed, 20 Apr 2022 15:28:14 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Miaoqian Lin <linmq006@...il.com>
Cc:     Emma Anholt <emma@...olt.net>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Eric Anholt <eric@...olt.net>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] drm/vc4: Fix pm_runtime_get_sync() usage

On Wed, Apr 20, 2022 at 04:05:35PM +0800, Miaoqian Lin wrote:
> 
> On 2022/4/20 15:51, Maxime Ripard wrote:
> > On Wed, Apr 20, 2022 at 12:49:48AM +0000, Miaoqian Lin wrote:
> >> If the device is already in a runtime PM enabled state
> >> pm_runtime_get_sync() will return 1, so a test for negative
> >> value should be used to check for errors.
> >>
> >> Also, we need to call pm_runtime_put_noidle() when pm_runtime_get_sync()
> >> fails, so use pm_runtime_resume_and_get() instead. this function
> >> will handle this.
> >>
> >> Fixes: 4078f5757144 ("drm/vc4: Add DSI driver")
> >> Signed-off-by: Miaoqian Lin <linmq006@...il.com>
> >> ---
> >> change in v2:
> >> - switch to pm_runtime_resume_and_get() to fix refcount leak.
> >> ---
> >>  drivers/gpu/drm/vc4/vc4_dsi.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
> >> index 752f921735c6..9d7ffaf6bc70 100644
> >> --- a/drivers/gpu/drm/vc4/vc4_dsi.c
> >> +++ b/drivers/gpu/drm/vc4/vc4_dsi.c
> >> @@ -846,8 +846,8 @@ static void vc4_dsi_encoder_enable(struct drm_encoder *encoder)
> >>  	unsigned long phy_clock;
> >>  	int ret;
> >>  
> >> -	ret = pm_runtime_get_sync(dev);
> >> -	if (ret) {
> >> +	ret = pm_runtime_resume_and_get(dev);
> >> +	if (ret < 0) {
> > pm_runtime_resume_and_get will return 0 on success, so the previous check was correct
> 
> previous check is for pm_runtime_get_sync() not for pm_runtime_resume_and_get (),
> 
> I switch to pm_runtime_resume_and_get() to fix the refcount leak bug at the same time.
> 
> Sure it's ok to use check if(ret) to check the retval, I just follow a more common way
> 
> for usage of pm_runtime_resume_and_get() in the codebase—— check ret<0
> 
> Since pm_runtime_resume_and_get() return negative error code.

If it ain't broke, don't fix it. The previous condition was working
perfectly fine to catch the errors from pm_runtime_resume_and_get,
there's no reason to change it.

Maxime

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