linux-kernel - Re: [PATCH v6 1/8] drm/msm/dp: Add eDP support via aux

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Date:   Thu, 7 Apr 2022 17:21:06 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        "Sankeerth Billakanti (QUIC)" <quic_sbillaka@...cinc.com>,
        quic_kalyant <quic_kalyant@...cinc.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        quic_vproddut <quic_vproddut@...cinc.com>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@...cinc.com>,
        freedreno <freedreno@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
        Sean Paul <seanpaul@...omium.org>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Sean Paul <sean@...rly.run>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/8] drm/msm/dp: Add eDP support via aux_bus

Hi,

On Thu, Apr 7, 2022 at 4:46 PM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> > The way I'm arguing it should work is that:
> >
> > 1. A whole bunch of the DP init code should move to the DP driver's
> > probe function. This includes parsing the DT, acquiring clocks,
> > getting a handle to our PHY, and IO mapping registers. As far as I
> > know, there's no reason to wait on all the components being probed in
> > order to do this stuff.
>
> Yes. And that's one of the reasons I tried to stay away from the DP
> driver. Each time I open the source code, my hands itch to start
> refactoring the code.
>
> >
> > 2. Once we have done the above things, it should be possible to do AUX
> > transfers, correct? ...and then we can populate the AUX bus from the
> > probe function too.
>
> No. In the DP case the AUX bus is inaccessible until the dongle is
> plugged (see all the HPD handling, phy_init()/phy_power_on() is hidden
> somewhere in that path)

I guess my thought was that in DP you could still create the AUX bus
at probe time. Then for DP you just return an instant "transfer
failed" from the AUX bus if HPD isn't asserted. For eDP (as discussed
elsewhere) when we try to do an AUX transfer then we delay until HPD
is there.

So we can still acquire resources (clocks, PHY, io maps, etc) at probe
time for DP and create the AUX bus, right? It will just return
"-ENODEV" if HPD isn't asserted and you're DP?

-Doug