Re: [PATCH] drm/dp: For MST hub, Get max_link_rate&max_lane from extended rx capability field if EXTENDED_RECEIVER_CAPABILITY_FILED_PRESENT is set.

[Lyude Paul <lyude@redhat.com>]

We just added a new helper for DPCD retrieval to drm_dp_helper.c (which also
handles grabbing the extended receiver caps), we should probably make use of
it here 

On Wed, 2020-09-09 at 14:31 +0800, Koba Ko wrote:
> On Thu, Aug 27, 2020 at 1:30 PM Koba Ko <koba.ko@canonical.com> wrote:
> > Currently, DRM get the capability of the mst hub only from DP_DPCD_REV and
> > get the slower speed even the mst hub can run in the faster speed.
> > 
> > As per DP-1.3, First check DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT.
> > If DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT is 1, read the DP_DP13_DPCD_REV
> > to
> > get the faster capability.
> > If DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT is 0, read DP_DPCD_REV.
> > 
> > Signed-off-by: Koba Ko <koba.ko@canonical.com>
> > ---
> >  drivers/gpu/drm/drm_dp_mst_topology.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > index 67dd72ea200e..3b84c6801281 100644
> > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > @@ -3497,6 +3497,8 @@ static int drm_dp_get_vc_payload_bw(u8 dp_link_bw,
> > u8  dp_link_count)
> >  int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr,
> > bool mst_state)
> >  {
> >         int ret = 0;
> > +       u8 dpcd_ext = 0;
> > +       unsigned int dpcd_offset = 0;
> >         struct drm_dp_mst_branch *mstb = NULL;
> > 
> >         mutex_lock(&mgr->payload_lock);
> > @@ -3510,9 +3512,15 @@ int drm_dp_mst_topology_mgr_set_mst(struct
> > drm_dp_mst_topology_mgr *mgr, bool ms
> >                 struct drm_dp_payload reset_pay;
> > 
> >                 WARN_ON(mgr->mst_primary);
> > +               drm_dp_dpcd_read(mgr->aux,
> > +                                DP_TRAINING_AUX_RD_INTERVAL,
> > +                                &dpcd_ext, sizeof(dpcd_ext));
> > +
> > +               dpcd_offset =
> > +                       ((dpcd_ext &
> > DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT) ?  DP_DP13_DPCD_REV :
> > DP_DPCD_REV);
> > 
> >                 /* get dpcd info */
> > -               ret = drm_dp_dpcd_read(mgr->aux, DP_DPCD_REV, mgr->dpcd,
> > DP_RECEIVER_CAP_SIZE);
> > +               ret = drm_dp_dpcd_read(mgr->aux, dpcd_offset, mgr->dpcd,
> > DP_RECEIVER_CAP_SIZE);
> >                 if (ret != DP_RECEIVER_CAP_SIZE) {
> >                         DRM_DEBUG_KMS("failed to read DPCD\n");
> >                         goto out_unlock;
> > --
> > 2.25.1
> > 
> Add Lyude Paul
> 
-- 
Cheers,
	Lyude Paul (she/her)
	Software Engineer at Red Hat