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.

[Ville Syrjälä]

On Wed, Sep 09, 2020 at 12:33:09PM -0400, Lyude Paul wrote:
> 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 

Someone should really rework this whole thing so that the driver can
pass in the link rate+lane count when setting up the link. There's no
reason to assume that the source device capabilities match or exceed
the MST device caps. It would also allow the driver the dynamically
adjust these in response to link failures.

> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > ---
> > >  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
> 
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-- 
Ville Syrjälä
Intel