linux-kernel - Re: [PATCH v2] drm/bridge: analogix_dp: Grab runtime PM reference for DP-AUX

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Message-ID: <CAD=FV=XivCxy-7Wszv9pnHELi=gRCMeVasEweza17XTtEoiT4A@mail.gmail.com>
Date:   Wed, 16 Feb 2022 10:10:16 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Brian Norris <briannorris@...omium.org>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Sean Paul <sean@...rly.run>, Jonas Karlman <jonas@...boo.se>,
        LKML <linux-kernel@...r.kernel.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "# 4.0+" <stable@...r.kernel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>
Subject: Re: [PATCH v2] drm/bridge: analogix_dp: Grab runtime PM reference for DP-AUX

Hi,

On Tue, Feb 15, 2022 at 4:41 PM Brian Norris <briannorris@...omium.org> wrote:
>
> On Tue, Feb 15, 2022 at 3:46 PM Doug Anderson <dianders@...omium.org> wrote:
> > On Tue, Feb 15, 2022 at 2:52 PM Brian Norris <briannorris@...omium.org> wrote:
> > > It still makes me wonder what the point
> > > of the /dev/drm_dp_aux<N> interface is though, because it seems like
> > > you're pretty much destined to not have reliable operation through
> > > that means.
> >
> > I can't say I have tons of history for those files. I seem to recall
> > maybe someone using them to have userspace tweak the embedded
> > backlight on some external DP connected panels? I think we also might
> > use it in Chrome OS to update the firmware of panels (dunno if
> > internal or external) in some cases too? I suspect that it works OK
> > for certain situations but it's really not going to work in all
> > cases...
>
> Yes, I believe I'm only submitting patches like this because fwupd
> apparently likes to indiscriminately whack at dpaux devices:
> https://github.com/fwupd/fwupd/tree/main/plugins/synaptics-mst#kernel-dp-aux-interface
>
> That seems like a bad idea.
>
> (We've already disabled that plugin on these systems, but it seems
> wise not to leave the stumbling block here for the next time.)

Yeah, it doesn't seem great. I guess it's _slightly_ less bad since
it's for an external device. As I understand it, they never really
turn off in the same way. It still feels like letting userspace
indiscriminately whack at DP AUX registers isn't a great idea, though.