On 3/10/23 12:51, Ville Syrjälä wrote: > 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. > Thanks for making this very important point. It seems like drm_dp_dpcd_access checks for that, though, and returns -EPROTO if !(ret == size). So I don't expect this patch to change any behavior. Harry
