linux-kernel - Re: [PATCH] drm/bridge: analogix_dp: Fix bailout for devm_drm_bridge

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Message-ID: <20250806114150.57e1ad97@booty>
Date: Wed, 6 Aug 2025 11:41:50 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Liu Ying <victor.liu@....com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 andrzej.hajda@...el.com, neil.armstrong@...aro.org, rfoss@...nel.org,
 Laurent.pinchart@...asonboard.com, jonas@...boo.se,
 jernej.skrabec@...il.com, maarten.lankhorst@...ux.intel.com,
 mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com,
 simona@...ll.ch, dmitry.baryshkov@....qualcomm.com, dianders@...omium.org,
 damon.ding@...k-chips.com, m.szyprowski@...sung.com
Subject: Re: [PATCH] drm/bridge: analogix_dp: Fix bailout for
 devm_drm_bridge_alloc()

On Wed, 6 Aug 2025 17:38:17 +0800
Liu Ying <victor.liu@....com> wrote:

> On 08/06/2025, Luca Ceresoli wrote:
> > Hello Liu,
> > 
> > On Wed,  6 Aug 2025 16:54:24 +0800
> > Liu Ying <victor.liu@....com> wrote:
> >   
> >> devm_drm_bridge_alloc() returns ERR_PTR on failure instead of a
> >> NULL pointer, so use IS_ERR() to check the returned pointer.
> >>
> >> Fixes: 48f05c3b4b70 ("drm/bridge: analogix_dp: Use devm_drm_bridge_alloc() API")
> >> Signed-off-by: Liu Ying <victor.liu@....com>
> >> ---
> >>  drivers/gpu/drm/bridge/analogix/analogix_dp_core.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/bridge/analogix/analogix_dp_core.c b/drivers/gpu/drm/bridge/analogix/analogix_dp_core.c
> >> index ed35e567d117..86cf898a09bb 100644
> >> --- a/drivers/gpu/drm/bridge/analogix/analogix_dp_core.c
> >> +++ b/drivers/gpu/drm/bridge/analogix/analogix_dp_core.c
> >> @@ -1474,7 +1474,7 @@ analogix_dp_probe(struct device *dev, struct analogix_dp_plat_data *plat_data)
> >>  
> >>  	dp = devm_drm_bridge_alloc(dev, struct analogix_dp_device, bridge,
> >>  				   &analogix_dp_bridge_funcs);
> >> -	if (!dp)
> >> +	if (IS_ERR(dp))
> >>  		return ERR_PTR(-ENOMEM);  
> > 
> > Good catch, thanks!
> > 
> > You fix is correct but now I realized one additional fix is needed in
> > the following line:
> > 
> > -		return ERR_PTR(-ENOMEM);
> > +		return ERR_PTR(dp);  
> 
> You mean ERR_CAST(dp)?

Ah, indeed!

> > Can you send a v2 with that fixed?  
> 
> I find devm_drm_bridge_alloc() always returns ERR_PTR(-ENOMEM) in this
> driver context, so it seems fine to keep the existing ERR_PTR(-ENOMEM).
> If you prefer ERR_CAST(dp), I may send v2 to use ERR_CAST(dp).

The documentation says:

 * Returns:                  
 * Pointer to new bridge, or ERR_PTR on failure.

So let's follow that, which is a (relatively) immutable contract, and
not the implementation which might change over time.

This is also what all callers of devm_drm_bridge_alloc() do, unless I
missed something.

Thanks,
Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com