Date:   Fri, 10 Mar 2023 19:51:13 +0200
From:   Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>
To:     Hamza Mahfooz <hamza.mahfooz@amd.com>
Cc:     amd-gfx@lists.freedesktop.org, Ian Chen <ian.chen@amd.com>,
        Leo Li <sunpeng.li@amd.com>,
        Qingqing Zhuo <qingqing.zhuo@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Wayne Lin <Wayne.Lin@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        Sung Joon Kim <sungjoon.kim@amd.com>
Subject: Re: [PATCH] drm/amd/display: use a more accurate check in
 dm_helpers_dp_read_dpcd()
Message-ID: <ZAtuET8c8kSgyX4Y@intel.com>
References: <20230309213027.256243-1-hamza.mahfooz@amd.com>
 <ZAttVGN/VdrfkH3y@intel.com>
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On Fri, Mar 10, 2023 at 07:48:04PM +0200, Ville Syrj?l? 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@amd.com>
> > ---
> >  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 Syrj?l?
Intel