[PATCH 1/2] drm/dp_mst: Revise broadcast msg lct & lcr

[Lin, Wayne]

[AMD Public Use]

> -----Original Message-----
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Sent: Tuesday, February 23, 2021 1:09 AM
> To: Lin, Wayne <Wayne.Lin at amd.com>
> Cc: Brol, Eryk <Eryk.Brol at amd.com>; Zhuo, Qingqing <Qingqing.Zhuo at amd.com>; stable at vger.kernel.org; Zuo, Jerry
> <Jerry.Zuo at amd.com>; dri-devel at lists.freedesktop.org; Kazlauskas, Nicholas <Nicholas.Kazlauskas at amd.com>
> Subject: Re: [PATCH 1/2] drm/dp_mst: Revise broadcast msg lct & lcr
>
> On Mon, Feb 22, 2021 at 07:02:03PM +0200, Ville Syrjälä wrote:
> > On Mon, Feb 22, 2021 at 12:00:26PM +0800, Wayne Lin wrote:
> > > [Why & How]
> > > According to DP spec, broadcast message LCT equals to 1 and LCR
> > > equals to 6. Current implementation is incorrect. Fix it.
> > >
> > > Signed-off-by: Wayne Lin <Wayne.Lin at amd.com>
> > > Cc: stable at vger.kernel.org
> > > ---
> > >  drivers/gpu/drm/drm_dp_mst_topology.c | 10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > index 17dbed0a9800..713ef3b42054 100644
> > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > @@ -2727,8 +2727,14 @@ static int set_hdr_from_dst_qlock(struct drm_dp_sideband_msg_hdr *hdr,
> > >  else
> > >  hdr->broadcast = 0;
> > >  hdr->path_msg = txmsg->path_msg;
> > > -hdr->lct = mstb->lct;
> > > -hdr->lcr = mstb->lct - 1;
> > > +if (hdr->broadcast) {
> > > +hdr->lct = 1;
> > > +hdr->lcr = 6;
> > > +} else {
> > > +hdr->lct = mstb->lct;
> > > +hdr->lcr = mstb->lct - 1;
> > > +}
> > > +
> > >  if (mstb->lct > 1)
> > >  memcpy(hdr->rad, mstb->rad, mstb->lct / 2);
> >
> > We should also do something about RAD no?
>
> Just skip the RAD stuff by s/mstb->lct/hdr->lct/ here I guess?
Thanks Ville!
Since LCT=1, broadcast message doesn't have a RAD and this is taken
care while we're constructing the header in drm_dp_encode_sideband_msg_hdr().
In drm_dp_encode_sideband_msg_hdr(), we skip stuffing RAD if LCT=1.
>
> --
> Ville Syrjälä
> Intel
Regards,
Wayne Lin