linux-kernel - Re: [PATCH v7 4/4] Support the eDP modes given by panel

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Date:   Thu, 14 Apr 2022 09:40:04 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sankeerth Billakanti <quic_sbillaka@...cinc.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Clark <robdclark@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        quic_kalyant <quic_kalyant@...cinc.com>,
        "Abhinav Kumar (QUIC)" <quic_abhinavk@...cinc.com>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@...cinc.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        quic_vproddut <quic_vproddut@...cinc.com>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        Steev Klimaszewski <steev@...i.org>
Subject: Re: [PATCH v7 4/4] Support the eDP modes given by panel

Hi,

On Thu, Apr 14, 2022 at 5:20 AM Sankeerth Billakanti
<quic_sbillaka@...cinc.com> wrote:
>
> The eDP controller does not have a reliable way keep panel
> powered on to read the sink capabilities. So, the controller
> driver cannot validate if a mode can be supported by the
> source. We will rely on the panel driver to populate only
> the supported modes for now.
>
> Signed-off-by: Sankeerth Billakanti <quic_sbillaka@...cinc.com>
> ---
>  drivers/gpu/drm/msm/dp/dp_display.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index c7277f0..0f18a16 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -998,6 +998,14 @@ enum drm_mode_status dp_bridge_mode_valid(struct drm_bridge *bridge,
>                 return -EINVAL;
>         }
>
> +       /*
> +        * The eDP controller currently does not have a reliable way of
> +        * enabling panel power to read sink capabilities. So, we rely
> +        * on the panel driver to populate only supported modes for now.
> +        */
> +       if (dp->is_edp)
> +               return MODE_OK;

As discussed out-of-band, I agree that this is the right thing for now
and making this assumption won't break anything. In general the set of
eDP panels is known ahead of time it's fairly unlikely someone would
set things up so that a panel couldn't use the mode it was reporting.

Longer term we should figure out a way to solve this but it doesn't
have to be today. To properly implement mode_valid() we've got to
combine knowledge from the panel (mostly rates supported and number of
lanes supported) with the controller (rates supported, number of lanes
supported/hooked up on this board).

In any case:

Reviewed-by: Douglas Anderson <dianders@...omium.org>