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

From: "Sankeerth Billakanti (QUIC)" <quic_sbillaka@quicinc.com>
To: Doug Anderson <dianders@chromium.org>,
	"Sankeerth Billakanti (QUIC)" <quic_sbillaka@quicinc.com>
Cc: quic_kalyant <quic_kalyant@quicinc.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"bjorn.andersson@linaro.org" <bjorn.andersson@linaro.org>,
	"Abhinav Kumar \(QUIC\)" <quic_abhinavk@quicinc.com>,
	quic_vproddut <quic_vproddut@quicinc.com>,
	"airlied@linux.ie" <airlied@linux.ie>,
	"linux-arm-msm@vger.kernel.org" <linux-arm-msm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"swboyd@chromium.org" <swboyd@chromium.org>,
	"sean@poorly.run" <sean@poorly.run>,
	"seanpaul@chromium.org" <seanpaul@chromium.org>,
	"dmitry.baryshkov@linaro.org" <dmitry.baryshkov@linaro.org>,
	"Aravind Venkateswaran \(QUIC\)" <quic_aravindh@quicinc.com>,
	"Kuogee Hsieh \(QUIC\)" <quic_khsieh@quicinc.com>,
	"freedreno@lists.freedesktop.org"
	<freedreno@lists.freedesktop.org>
Subject: RE: [PATCH v6 8/8] drm/msm/dp: Handle eDP mode_valid differently from dp
Date: Mon, 4 Apr 2022 18:20:55 +0000	[thread overview]
Message-ID: <MW4PR02MB71865B8E17F3D194B05013EDE1E59@MW4PR02MB7186.namprd02.prod.outlook.com> (raw)
In-Reply-To: <CAD=FV=VK0D_GzYBv+u+o6-ks-UAsw97__0mWsSn9OycX72LJFg@mail.gmail.com>

Hi Doug,

> On Wed, Mar 30, 2022 at 11:02 PM Sankeerth Billakanti (QUIC)
> <quic_sbillaka@quicinc.com> wrote:
> >
> > Hi Dmitry,
> >
> > > On Wed, 30 Mar 2022 at 19:04, Sankeerth Billakanti
> > > <quic_sbillaka@quicinc.com> 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@quicinc.com>
> > >
> > > 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.

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

> 
> -Doug