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

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Date:   Fri, 8 Apr 2022 15:13:38 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Doug Anderson <dianders@...omium.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

On Fri, 8 Apr 2022 at 03:28, Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Thu, Apr 7, 2022 at 4:36 PM Dmitry Baryshkov
> <dmitry.baryshkov@...aro.org> wrote:
> >
> > The ps8640 driver looks 'working by coincidence'. It calls
> > dp_aux_populate, then immediately after the function returns it checks
> > for the panel. If panel-edp is built as a module, the probe might fail
> > easily.
> > The anx7625 driver has the same kind of issue. The DP AUX bus is
> > populated from the probe() and after some additional work the panel is
> > being checked.
> > This design is fragile and from my quick glance it can break (or be
> > broken) too easy. It reminds me of our drm msm 'probe' loops
> > preventing the device to boot completely if the dsi bridge/panel could
> > not be probed in time.
>
> I did spend some time thinking about this, at least for ps8640. I
> believe that as long as the panel's probe isn't asynchronous.

By panel probe (as a probe of any device) is potentially asynchronous.
As in your example, the PWM might not be present, the regulator probe
might have been delayed, the panel-edp might be built as a module,
which is not present for some reason.

> Basically if the panel isn't ready then ps8640 should return and we'll
> retry later. I do remember the probe loops that we used to have with
> msm and I don't _think_ this would trigger it.

I don't have proof here. But as I wrote, this thing might break at some point.

> That being said, if we need to separate out the AUX bus into a
> sub-device like we did in sn65dsi86 we certainly could.

Ideally we should separate the "bridge" subdevice, like it was done in
sn65dsi86.
So that the aux host probes, registers the EP device, then the bridge
device can not be probed (and thus the drm_bridge is not created)
until the next  bridge becomes available.

-- 
With best wishes
Dmitry