linux-kernel - Re: [PATCH v6 2/5] drm/dp: Allow an early call to register DDC i2c bus

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Date:   Mon, 17 May 2021 13:17:51 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Jani Nikula <jani.nikula@...el.com>
Cc:     Lyude Paul <lyude@...hat.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        David Airlie <airlied@...hat.com>,
        Ville Syrjala <ville.syrjala@...el.com>,
        Rob Clark <robdclark@...omium.org>,
        Wolfram Sang <wsa@...nel.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Neil Armstrong <narmstrong@...libre.com>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Jonas Karlman <jonas@...boo.se>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Stanislav Lisovskiy <stanislav.lisovskiy@...el.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Thierry Reding <treding@...dia.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Steev Klimaszewski <steev@...i.org>,
        linux-i2c <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH v6 2/5] drm/dp: Allow an early call to register DDC i2c bus

Hi,

On Fri, May 14, 2021 at 4:16 AM Jani Nikula <jani.nikula@...el.com> wrote:
>
> On Fri, 07 May 2021, Lyude Paul <lyude@...hat.com> wrote:
> > On Fri, 2021-05-07 at 17:00 -0500, Bjorn Andersson wrote:
> >> On Fri 07 May 16:18 CDT 2021, Lyude Paul wrote:
> >>
> >> > Adding ville from Intel to also get their take on this.
> >> >
> >> > In general we've been trying to move DRM to a design where we don't expose
> >> > any
> >> > devices until everything is ready. That's pretty much the main reason that
> >> > we
> >> > register things during bridge attach time. Note though that even before
> >> > the
> >> > DDC bus is registered it should still be usable, just things like
> >> > get_device()
> >> > won't work.
> >> >
> >> > This isn't the first time we've run into a problem like the one you're
> >> > trying
> >> > to solve though, Tegra currently has a similar issue. Something we
> >> > discussed
> >> > as a possible long-term solution for this was splitting i2c_add_adapter()
> >> > into
> >> > a minimal initialization function and a registration function. Linux's
> >> > device
> >> > core already allows for this (device_initialize() and device_add(), which
> >> > are
> >> > called together when device_register() is called). Would this be a
> >> > solution
> >> > that might work for you (and even better, would you possibly be willing to
> >> > write the patches? :)
> >> >
> >>
> >> It's not enough that the adapter is half-baked, because the bridge's
> >> initialization depends on that the panel device is done probing, and the
> >> panel driver will only complete its probe if it can find it's resources.
> >>
> >> So we need a mechanism to fully create the resources exposed by the
> >> bridge chip (i2c bus, gpio chip and (soon) a pwm chip), then allow the
> >> panel to probe and after that initialize the bridge.
> >>
> >> We did discuss possible ways to register these resources and then
> >> "sleep for a while" before resolving the panel, but what we came up with
> >> was definitely suboptimal - and ugly.
> >
> > Sigh, I'm really starting to wonder if we should reconsider the rules on
> > exposing ddc adapters early...
> >
> > Danvet, Jani, and/or airlied: can I get your take on this?
>
> Granted, I did not study this in detail, but it sounds like we'd need to
> be able to add and use an i2c adapter in kernel, before deciding to
> register it with the userspace. But that does not seem to be as trivial
> as making it possible to call the now-static i2c_register_adapter()
> separately.

To close the loop: I think the point is now moot in v7. Now crossing
my fingers that approach can gain momentum. If not, I might come back
here. ;-)

-Doug