Re: [PATCH 1/2] drm/dp: Handle zeroed port counts in drm_dp_read_downstream_info()

[Ville Syrjälä]

On Fri, Apr 30, 2021 at 06:34:27PM -0400, Lyude Paul wrote:
> While the DP specification isn't entirely clear on if this should be
> allowed or not, some branch devices report having downstream ports present
> while also reporting a downstream port count of 0. So to avoid breaking
> those devices, we need to handle this in drm_dp_read_downstream_info().
> 
> So, to do this we assume there's no downstream port info when the
> downstream port count is 0.
> 
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Tested-by: Jérôme de Bretagne <jerome.debretagne@xxxxxxxxx>
> Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416
> Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()")
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index cb56d74e9d38..27c8c5bdf7d9 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -682,7 +682,14 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
>  	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
>  		return 0;
>  
> +	/* Some branches advertise having 0 downstream ports, despite also advertising they have a
> +	 * downstream port present. The DP spec isn't clear on if this is allowed or not, but since
> +	 * some branches do it we need to handle it regardless.
> +	 */
>  	len = drm_dp_downstream_port_count(dpcd);
> +	if (!len)
> +		return 0;
> +

Seems sane enough.

Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>

>  	if (dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE)
>  		len *= 4;
>  
> -- 
> 2.30.2
> 
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-- 
Ville Syrjälä
Intel