[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 9:26 PM
> 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 Tue, Feb 23, 2021 at 05:32:32AM +0000, Lin, Wayne wrote:
> > [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.
>
> Ugh. How many levels of these do we really need...
> Either way I'd prefer the code be consistent so you don't have to sacrifice so many brain cells to understand what should be trivial
> details.
Hi Ville,
Ya I know.. Currently it goes few levels to encapsulate the final mst packet.
>From my understanding, this function is trying to prepare needed data and the actual mst packet header is constructed in drm_dp_encode_sideband_msg_hdr().
However, I will push another version by your suggestion.
Thanks for your time!
>
> --
> Ville Syrjälä
> Intel
Regards,
Wayne Lin