linux-kernel - Re: [PATCH] drm/omap: Fix runtime PM imbalance in dsi_runtime

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Message-ID: <20200822104852.GA5966@pendragon.ideasonboard.com>
Date:   Sat, 22 Aug 2020 13:48:52 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     Dinghao Liu <dinghao.liu@....edu.cn>, kjlu@....edu,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Sebastian Reichel <sebastian.reichel@...labora.com>,
        Tony Lindgren <tony@...mide.com>,
        zhengbin <zhengbin13@...wei.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/omap: Fix runtime PM imbalance in dsi_runtime_get

Hi Tomi,

On Fri, Aug 21, 2020 at 03:06:59PM +0300, Tomi Valkeinen wrote:
> On 21/08/2020 10:45, Dinghao Liu wrote:
> > pm_runtime_get_sync() increments the runtime PM usage counter
> > even when it returns an error code. However, users of
> > dsi_runtime_get(), a direct wrapper of pm_runtime_get_sync(),
> > assume that PM usage counter will not change on error. Thus a
> > pairing decrement is needed on the error handling path to keep
> > the counter balanced.
> > 
> > Fixes: 4fbafaf371be7 ("OMAP: DSS2: Use PM runtime & HWMOD support")
> > Signed-off-by: Dinghao Liu <dinghao.liu@....edu.cn>
> > ---
> >  drivers/gpu/drm/omapdrm/dss/dsi.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/omapdrm/dss/dsi.c b/drivers/gpu/drm/omapdrm/dss/dsi.c
> > index eeccf40bae41..973bfa14a104 100644
> > --- a/drivers/gpu/drm/omapdrm/dss/dsi.c
> > +++ b/drivers/gpu/drm/omapdrm/dss/dsi.c
> > @@ -1112,8 +1112,11 @@ static int dsi_runtime_get(struct dsi_data *dsi)
> >  	DSSDBG("dsi_runtime_get\n");
> >  
> >  	r = pm_runtime_get_sync(dsi->dev);
> > -	WARN_ON(r < 0);
> > -	return r < 0 ? r : 0;
> > +	if (WARN_ON(r < 0)) {
> > +		pm_runtime_put_noidle(dsi->dev);
> > +		return r;
> > +	}
> > +	return 0;
> >  }
> 
> Thanks! Good catch. I think this is broken in all the other modules in omapdrm too (e.g. dispc.c,
> venc.c, etc).
> 
> Would you like to update the patch to cover the whole omapdrm?

Just for yoru information, there has been quite a few similar patches
submitted all across the kernel. I believe this is an issue of the
pm_runtime_get_sync() API, which really shouldn't require a put() when
it fails. For drivers that really don't expect pm_runtime_get_sync() to
fail (no I2C access to a regulator for instance, only SoC-internal
operations) I've instead decided to ignore the error completely. I don't
think poluting the whole kernel code base with this kind of "fixes" is a
good idea.

-- 
Regards,

Laurent Pinchart