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

[Doug Anderson]

Hi,

On Tue, Feb 15, 2022 at 4:41 PM Brian Norris <briannorris@xxxxxxxxxxxx> wrote:
>
> On Tue, Feb 15, 2022 at 3:46 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> > On Tue, Feb 15, 2022 at 2:52 PM Brian Norris <briannorris@xxxxxxxxxxxx> 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.