Re: [PATCH] drm/amd/display: use a more accurate check in dm_helpers_dp_read_dpcd()

[Ville Syrjälä]

On Fri, Mar 10, 2023 at 07:48:04PM +0200, Ville Syrjälä wrote:
> On Thu, Mar 09, 2023 at 04:30:27PM -0500, Hamza Mahfooz wrote:
> > We should be checking if drm_dp_dpcd_read() returns the size that we are
> > asking it to read instead of just checking if it is greater than zero.
> > Also, we should WARN_ON() here since this condition is only ever met, if
> > there is an issue worth investigating. So, compare the return value of
> > drm_dp_dpcd_read() to size and WARN_ON() if they aren't equal.
> > 
> > Signed-off-by: Hamza Mahfooz <hamza.mahfooz@xxxxxxx>
> > ---
> >  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> > index 8d598b322e5b..ed2ed7b1d869 100644
> > --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> > +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> > @@ -511,8 +511,8 @@ bool dm_helpers_dp_read_dpcd(
> >  		return false;
> >  	}
> >  
> > -	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> > -			data, size) > 0;
> > +	return !WARN_ON(drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> > +					 data, size) != size);
> 
> Just FYI there are devices out there that violate the DP spec and reads
> from specific DPCD registers simply fail instead of returning the
> expected 0.

And of course anyone can yank the cable anytime, so in
fact pretty much any DPCD read can fail.

-- 
Ville Syrjälä
Intel