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

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Date:   Fri, 8 Apr 2022 17:58:28 +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 16:43, Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Fri, Apr 8, 2022 at 5:20 AM Dmitry Baryshkov
> <dmitry.baryshkov@...aro.org> wrote:
> >
> > > 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.
> >
> > I think panel-edp would already handle the delay, so we do not need to
> > have this logic in the DP driver.
>
> There's a whole discussion about this between Stephen and me in patch
> #5 ("drm/msm/dp: wait for hpd high before any sink interaction").
> Basically:
>
> * If panel HPD is hooked up to the dedicated HPD pin on the eDP
> controller then the panel driver doesn't have a way to read it.

I refreshed that dialog. I must admit, I have missed the fact that the
HPD pin might not be visible as the GPIO pin.

> * We can't leverage the existing "HPD" query functions in DRM because
> those indicate whether a panel is _physically_ connected. For eDP, it
> always is.

Yes, I was thinking about (mis)using the
drm_bridge_connector_hpd_notify() for generic HPD-related
notifications (to tell eDP that it should check the current state). I
have abandoned that idea.

> For now the rule is that the AUX transfer function is in charge of
> waiting for HPD for eDP if the dedicated HPD pin is used. If we want
> to re-invent this we could, but that system works, isn't _too_ ugly,
> and we're already making big enough changes in this series.

The is_hpd_asserted() looks like a good callback for the aux bus.
It will allow the panel driver to check if the panel is powered up (in
the absence of the GPIO pin).

> > > 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?
> >
> > Yes, please. I still suppose that we'd need a separate case to
> > power_on eDP's PHY during the probe time. Maybe I'm mistaken here.
>
> I think the ideal way is to do it like Kieran's proposal for sn65dsi86:
>
> https://lore.kernel.org/r/20220317131250.1481275-4-kieran.bingham+renesas@ideasonboard.com/
>
> * When enabling HPD (physical hot plug detect) in the hpd_enable()
> callback you do a pm_runtime_get(). You do the
> pm_runtime_put_autosuspend() when disabling. This is only used for DP
> since we only provide DRM_BRIDGE_OP_HPD for DP, not for eDP.
>
> * We do a pm_runtime_get() / pm_runtime_put_autosuspend() in the AUX
> transfer routine. While holding the pm_runtime reference we check HPD.
> For DP we return immediately if HPD isn't asserted. For eDP, we delay.
>
> * We do the pm_runtime_get() in pre_enable and the pm_runtime_put() in
> post_disable. For DP this will add a 2nd refcount (since we probably
> were holding the reference for HPD). For eDP this will cause us to
> power on.
>
> * If there's any other time we need to read HW registers, and we
> aren't guaranteed to already have a pm_runtime reference (like during
> probe), we can do a temporary pm_runtime_get() /
> pm_runtime_put_autosuspend().

This looks good. I'd be more than welcome to review such series.

Note: I think this would require using
drm_bridge_connector_enable_hpd() in the DP code.
Hopefully at some point we would be able to move all
drm_bridge_connector calls to the core msm layer.
--
With best wishes
Dmitry