Re: [PATCH v6 8/8] drm/msm/dp: Handle eDP mode_valid differently from dp

[Dmitry Baryshkov]

On Mon, 4 Apr 2022 at 21:21, Sankeerth Billakanti (QUIC)
<quic_sbillaka@xxxxxxxxxxx> wrote:
>
> Hi Doug,
>
> > On Wed, Mar 30, 2022 at 11:02 PM Sankeerth Billakanti (QUIC)
> > <quic_sbillaka@xxxxxxxxxxx> wrote:
> > >
> > > Hi Dmitry,
> > >
> > > > On Wed, 30 Mar 2022 at 19:04, Sankeerth Billakanti
> > > > <quic_sbillaka@xxxxxxxxxxx> wrote:
> > > > >
> > > > > The panel-edp driver modes needs to be validated differently from
> > > > > DP because the link capabilities are not available for EDP by that time.
> > > > >
> > > > > Signed-off-by: Sankeerth Billakanti <quic_sbillaka@xxxxxxxxxxx>
> > > >
> > > > This should not be necessary after
> > > >
> > https://patchwork.freedesktop.org/patch/479261/?series=101682&rev=1.
> > > > Could you please check?
> > > >
> > >
> > > The check for DP_MAX_PIXEL_CLK_KHZ is not necessary anymore but we
> > > need to return early for eDP because unlike DP, eDP context will not
> > > have the information about the number of lanes and link clock.
> > >
> > > So, I will modify the patch to return after the DP_MAX_PIXEL_CLK_KHZ
> > check if is_eDP is set.
> >
> > I haven't walked through all the relevant code but something you said above
> > sounds strange. You say that for eDP we don't have info about the number
> > of lanes? We _should_.
> >
> > It's certainly possible to have a panel that supports _either_ 1 or 2 lanes but
> > then only physically connect 1 lane to it. ...or you could have a panel that
> > supports 2 or 4 lanes and you only connect 1 lane.
> > See, for instance, ti_sn_bridge_parse_lanes. There we assume 4 lanes but if
> > a "data-lanes" property is present then we can use that to know that fewer
> > lanes are physically connected.
> >
> > It's also possible to connect more lanes to a panel than it supports.
> > You could connect 2 lanes to it but then it only supports 1. This case needs to
> > be handled as well...
> >
>
> I was referring to the checks we do for DP in dp_bridge_mode_valid. We check if the
> Link bandwidth can support the pixel bandwidth. For an external DP connection, the
> Initial DPCD/EDID read after cable connection will return the sink capabilities like link
> rate, lane count and bpp information that are used to we filter out the unsupported
> modes from the list of modes from EDID.
>
> For eDP case, the dp driver performs the first dpcd read during bridge_enable. The
> dp_bridge_mode_valid function is executed before bridge_enable and hence does
> not have the full link or the sink capabilities information like external DP connection,
> by then.

It sounds to me like we should emulate the HPD event for eDP to be
handled earlier than the get_modes()/prepare() calls are attempted.
However this might open another can of worms.

> So, we need to proceed with the reported mode for eDP.

Well... Even if during the first call to get_modes() the DPCD is not
read, during subsequent calls the driver has necessary information, so
it can proceed with all the checks, can't it?

-- 
With best wishes
Dmitry