Re: [PATCH v2 5/6] drm/vc4: Support the case where the DSI device is disabled

From: Boris Brezillon <boris.brezillon@bootlin.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, Pawel Moll <pawel.moll@arm.com>,
	Ian Campbell <ijc+devicetree@hellion.org.uk>,
	David Airlie <airlied@linux.ie>,
	dri-devel@lists.freedesktop.org, Rob Herring <robh+dt@kernel.org>,
	Kumar Gala <galak@codeaurora.org>
Subject: Re: [PATCH v2 5/6] drm/vc4: Support the case where the DSI device is disabled
Date: Fri, 4 May 2018 14:05:25 +0200	[thread overview]
Message-ID: <20180504140525.152d8d8e@bbrezillon> (raw)
In-Reply-To: <20180504102833.GJ13459@ulmo>

On Fri, 4 May 2018 12:28:33 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Thu, May 03, 2018 at 06:40:08PM +0200, Boris Brezillon wrote:
> > Having a device with a status property != "okay" in the DT is a valid
> > use case, and we should not prevent the registration of the DRM device
> > when the DSI device connected to the DSI controller is disabled.
> > 
> > Consider the ENODEV return code as a valid result and do not expose the
> > DSI encoder/connector when it happens.
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
> > ---
> >  drivers/gpu/drm/vc4/vc4_dsi.c | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
> > index 8aa897835118..db2f137f8b7b 100644
> > --- a/drivers/gpu/drm/vc4/vc4_dsi.c
> > +++ b/drivers/gpu/drm/vc4/vc4_dsi.c
> > @@ -1606,8 +1606,18 @@ static int vc4_dsi_bind(struct device *dev, struct device *master, void *data)
> >  
> >  	ret = drm_of_find_panel_or_bridge(dev->of_node, 0, 0,
> >  					  &panel, &dsi->bridge);
> > -	if (ret)
> > +	if (ret) {
> > +		/* If the bridge or panel pointed by dev->of_node is not
> > +		 * enabled, just return 0 here so that we don't prevent the DRM
> > +		 * dev from being registered. Of course that means the DSI
> > +		 * encoder won't be exposed, but that's not a problem since
> > +		 * nothing is connected to it.
> > +		 */
> > +		if (ret == -ENODEV)
> > +			return 0;
> > +
> >  		return ret;
> > +	}
> >  
> >  	if (panel) {
> >  		dsi->bridge = devm_drm_panel_bridge_add(dev, panel,
> > @@ -1652,7 +1662,8 @@ static void vc4_dsi_unbind(struct device *dev, struct device *master,
> >  	struct vc4_dev *vc4 = to_vc4_dev(drm);
> >  	struct vc4_dsi *dsi = dev_get_drvdata(dev);
> >  
> > -	pm_runtime_disable(dev);
> > +	if (dsi->bridge)
> > +		pm_runtime_disable(dev);  
> 
> Is this safe? This uses component/master, so dsi->bridge is going to
> remain valid until the driver's ->remove() is called. So technically you
> could have a situation where drm_of_find_panel_or_bridge() returned some
> error code that remains stored in dsi->bridge and cause the above
> condition to be incorrectly true.

No, because of_drm_find_bridge() (which is called from
drm_of_find_panel_or_bridge() returns either NULL or a valid bridge
pointer), so dsi->bridge either points to a valid bridge object or is
NULL. Am I missing something?

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