linux-kernel - Re: [PATCH v9 3/4] drm/msm/dp: wait for hpd high before aux transaction

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Date:   Fri, 22 Apr 2022 18:38:46 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     robdclark@...il.com, seanpaul@...omium.org,
        quic_kalyant@...cinc.com, quic_abhinavk@...cinc.com,
        dianders@...omium.org, quic_khsieh@...cinc.com,
        bjorn.andersson@...aro.org, sean@...rly.run, airlied@...ux.ie,
        daniel@...ll.ch, dmitry.baryshkov@...aro.org,
        quic_vproddut@...cinc.com, quic_aravindh@...cinc.com,
        steev@...i.org
Subject: Re: [PATCH v9 3/4] drm/msm/dp: wait for hpd high before aux transaction

Quoting Sankeerth Billakanti (2022-04-22 02:11:05)
> The source device should ensure the sink is ready before proceeding to
> read the sink capability or perform any aux transactions. The sink
> will indicate its readiness by asserting the HPD line. The controller
> driver needs to wait for the hpd line to be asserted by the sink before
> it performs any aux transactions.
>
> The eDP sink is assumed to be always connected. It needs power from the
> source and its HPD line will be asserted only after the panel is powered
> on. The panel power will be enabled from the panel-edp driver and only
> after that, the hpd line will be asserted.
>
> Whereas for DP, the sink can be hotplugged and unplugged anytime. The hpd
> line gets asserted to indicate the sink is connected and ready. Hence
> there is no need to wait for the hpd line to be asserted for a DP sink.
>
> Signed-off-by: Sankeerth Billakanti <quic_sbillaka@...cinc.com>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>
> ---

Reviewed-by: Stephen Boyd <swboyd@...omium.org>