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

[Sankeerth Billakanti (QUIC)]

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.
> 

For DP, the HPD handler mainly initiates link training and gets the EDID.

Before adding support for a separate eDP panel, we had discussed about
this internally and decided to emulate eDP HPD during enable(). Main reason
being, eDP power is guaranteed to be on only after bridge_enable().
So, eDP link training can happen and sustain only after bridge_enable().

Emulating HPD before/during get_modes will not have any effect because:

1. get_modes() will go to panel's get_modes() function to power on read EDID

2. panel power will be turned off after get_modes(). Panel power off will
reset every write transaction in DPCD. anyway invalidating link training

3. mode_valid will land in dp driver but panel will not be powered on at that
time and we cannot do aux transfers or DPCD read writes.

> > 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?
> 

get_modes() currently does not land in DP driver. It gets executed in panel
bridge. But the mode_valid() comes to DP driver to check the controller
compatibility.

> --
> With best wishes
> Dmitry

Thank you,
Sankeerth