Re: [PATCH] drm/bridge: Avoid uninitialized variable warning

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 28 Jun 2022 10:45:19 +0300
Dan Carpenter <dan.carpenter@oracle.com> wrote:

> On Tue, Jun 28, 2022 at 09:26:09AM +0200, Boris Brezillon wrote:
> > On Tue, 28 Jun 2022 09:59:51 +0300
> > Dan Carpenter <dan.carpenter@oracle.com> wrote:
> >   
> > > This works, but technically it uses "num_in_bus_fmts" before it has been
> > > initialized so it leads to static checker warnings and probably KMEMsan
> > > warnings at run time.  Reverse the checks so it checks for failure first
> > > and then check for unsupported formats next.
> > > 
> > > Fixes: f32df58acc68 ("drm/bridge: Add the necessary bits to support bus format negotiation")
> > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > > ---
> > >  drivers/gpu/drm/drm_bridge.c | 12 ++++++------
> > >  1 file changed, 6 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> > > index e275b4ca344b..00cbde654472 100644
> > > --- a/drivers/gpu/drm/drm_bridge.c
> > > +++ b/drivers/gpu/drm/drm_bridge.c
> > > @@ -897,10 +897,10 @@ static int 	(struct drm_bridge *first_bridge,
> > >  							conn_state,
> > >  							out_bus_fmt,
> > >  							&num_in_bus_fmts);
> > > -	if (!num_in_bus_fmts)
> > > -		return -ENOTSUPP;
> > > -	else if (!in_bus_fmts)
> > > +	if (!in_bus_fmts)
> > >  		return -ENOMEM;
> > > +	else if (!num_in_bus_fmts)
> > > +		return -ENOTSUPP;  
> > 
> > Well, it changes the error we return when num_in_bus_fmts = 0
> > && in_bus_fmts == NULL which is not an ENOMEM situation, so I'd rather
> > initialize num_{in,out}_bus_fmts to 0 here.
> >   
> 
> I can do that but there is no real consistency in how
> ->atomic_get_input_bus_fmts() functions are implemented.  Some set  
> *num_input_fmts = 0; before the kmalloc() and then reset it to
> *num_input_fmts = 1; if the allocation succeeds.  Some just set it to
> *num_input_fmts = 1 at the start.
> 
> This bug only affects the imx code like:
> imx8qm_ldb_bridge_atomic_get_input_bus_fmts()
> imx8qxp_pixel_link_bridge_atomic_get_input_bus_fmts
> 

I'd say imx8qm_ldb_bridge_atomic_get_input_bus_fmts() and
imx8qxp_pixel_link_bridge_atomic_get_input_bus_fmts() should be patched
to set *num_input_fmts = 0 when they return NULL on purpose, as
documented here [1].

[1]https://elixir.bootlin.com/linux/latest/source/include/drm/drm_bridge.h#L453