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

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

[Lyude Paul]

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@redhat.com>
Tested-by: Jérôme de Bretagne <jerome.debretagne@gmail.com>
Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416
Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()")
Cc: <stable@vger.kernel.org> # 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;
+
 	if (dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE)
 		len *= 4;
 
-- 
2.30.2

[PATCH 2/2] drm/dp: Drop open-coded drm_dp_is_branch() in drm_dp_read_downstream_info()

[Lyude Paul]

Noticed this while fixing another issue in drm_dp_read_downstream_info(),
the open coded DP_DOWNSTREAMPORT_PRESENT check here just duplicates what we
already do in drm_dp_is_branch(), so just get rid of it.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 27c8c5bdf7d9..0f84df8798ab 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -677,9 +677,7 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
 	memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
 
 	/* No downstream info to read */
-	if (!drm_dp_is_branch(dpcd) ||
-	    dpcd[DP_DPCD_REV] < DP_DPCD_REV_10 ||
-	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
+	if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] < DP_DPCD_REV_10)
 		return 0;
 
 	/* Some branches advertise having 0 downstream ports, despite also advertising they have a
-- 
2.30.2

Re: [Intel-gfx] [PATCH 2/2] drm/dp: Drop open-coded drm_dp_is_branch() in drm_dp_read_downstream_info()

[Ville Syrjälä]

On Fri, Apr 30, 2021 at 06:34:28PM -0400, Lyude Paul wrote:
> Noticed this while fixing another issue in drm_dp_read_downstream_info(),
> the open coded DP_DOWNSTREAMPORT_PRESENT check here just duplicates what we
> already do in drm_dp_is_branch(), so just get rid of it.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 27c8c5bdf7d9..0f84df8798ab 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -677,9 +677,7 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
>  	memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
>  
>  	/* No downstream info to read */
> -	if (!drm_dp_is_branch(dpcd) ||
> -	    dpcd[DP_DPCD_REV] < DP_DPCD_REV_10 ||
> -	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
> +	if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] < DP_DPCD_REV_10)

BTW that DPCD_REV check looks rather wrong.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

>  		return 0;
>  
>  	/* Some branches advertise having 0 downstream ports, despite also advertising they have a
> -- 
> 2.30.2
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Ville Syrjälä
Intel

Re: [Intel-gfx] [PATCH 2/2] drm/dp: Drop open-coded drm_dp_is_branch() in drm_dp_read_downstream_info()

[Lyude Paul]

On Mon, 2021-05-03 at 06:06 +0300, Ville Syrjälä wrote:
> On Fri, Apr 30, 2021 at 06:34:28PM -0400, Lyude Paul wrote:
> > Noticed this while fixing another issue in drm_dp_read_downstream_info(),
> > the open coded DP_DOWNSTREAMPORT_PRESENT check here just duplicates what
> > we
> > already do in drm_dp_is_branch(), so just get rid of it.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > ---
> >  drivers/gpu/drm/drm_dp_helper.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c
> > b/drivers/gpu/drm/drm_dp_helper.c
> > index 27c8c5bdf7d9..0f84df8798ab 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -677,9 +677,7 @@ int drm_dp_read_downstream_info(struct drm_dp_aux
> > *aux,
> >         memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
> >  
> >         /* No downstream info to read */
> > -       if (!drm_dp_is_branch(dpcd) ||
> > -           dpcd[DP_DPCD_REV] < DP_DPCD_REV_10 ||
> > -           !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
> > +       if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] < DP_DPCD_REV_10)
> 
> BTW that DPCD_REV check looks rather wrong.
> 
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

I'll send out a separate fix for this in just a moment, thanks for pointing it
out!

> 
> >                 return 0;
> >  
> >         /* Some branches advertise having 0 downstream ports, despite also
> > advertising they have a
> > -- 
> > 2.30.2
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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@redhat.com>
> Tested-by: Jérôme de Bretagne <jerome.debretagne@gmail.com>
> Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416
> Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()")
> Cc: <stable@vger.kernel.org> # 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@linux.intel.com>

>  	if (dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE)
>  		len *= 4;
>  
> -- 
> 2.30.2
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Ville Syrjälä
Intel