[PATCH 1/2] drm/dp_mst: Fix GUID DPCD write to non-root MST branch devices

[Jani Nikula]

On Tue, 01 Apr 2025, Imre Deak <imre.deak at intel.com> wrote:
> The return value on success of drm_dp_send_dpcd_write() called for
> non-root MST branch devices from drm_dp_check_mstb_guid() is the number
> of bytes transferred. Atm this return value (in case of a complete read)
> will be regarded incorrectly as an error by the caller of
> drm_dp_check_mstb_guid(). Fix this by converting the return value for a
> complete read to the expected success code (0) and for a partial read to
> a failure code (-EPROTO).
>
> Fixes: 2554da0de3e8 ("drm/display: dp-mst-topology: use new DCPD access helpers")
> Cc: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> Cc: Lyude Paul <lyude at redhat.com>
> Signed-off-by: Imre Deak <imre.deak at intel.com>

Reviewed-by: Jani Nikula <jani.nikula at intel.com>

> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index de3fc6090c906..619f461e02f76 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -2200,6 +2200,8 @@ static int drm_dp_check_mstb_guid(struct drm_dp_mst_branch *mstb, guid_t *guid)
>  			ret = drm_dp_send_dpcd_write(mstb->mgr,
>  						     mstb->port_parent,
>  						     DP_GUID, sizeof(buf), buf);
> +			if (ret >= 0)
> +				ret = ret == sizeof(buf) ? 0 : -EPROTO;
>  		} else {
>  			ret = drm_dp_dpcd_write_data(mstb->mgr->aux,
>  						     DP_GUID, buf, sizeof(buf));

-- 
Jani Nikula, Intel