Re: [PATCH] drm/dp: Don't attempt AUX transfers when eDP panels are not powered

[Doug Anderson]

Hi,

On Thu, Feb 15, 2024 at 2:24 AM Jani Nikula <jani.nikula@xxxxxxxxx> wrote:
>
> On Wed, 14 Feb 2024, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> > Hi,
> >
> > On Tue, Feb 13, 2024 at 10:25 PM Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
> >>
> >> On Wed, Feb 14, 2024 at 2:23 PM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
> >> >
> >> > If an eDP panel is not powered on then any attempts to talk to it over
> >> > the DP AUX channel will timeout. Unfortunately these attempts may be
> >> > quite slow. Userspace can initiate these attempts either via a
> >> > /dev/drm_dp_auxN device or via the created i2c device.
> >> >
> >> > Making the DP AUX drivers timeout faster is a difficult proposition.
> >> > In theory we could just poll the panel's HPD line in the AUX transfer
> >> > function and immediately return an error there. However, this is
> >> > easier said than done. For one thing, there's no hard requirement to
> >> > hook the HPD line up for eDP panels and it's OK to just delay a fixed
> >> > amount. For another thing, the HPD line may not be fast to probe. On
> >> > parade-ps8640 we need to wait for the bridge chip's firmware to boot
> >> > before we can get the HPD line and this is a slow process.
> >> >
> >> > The fact that the transfers are taking so long to timeout is causing
> >> > real problems. The open source fwupd daemon sometimes scans DP busses
> >> > looking for devices whose firmware need updating. If it happens to
> >> > scan while a panel is turned off this scan can take a long time. The
> >> > fwupd daemon could try to be smarter and only scan when eDP panels are
> >> > turned on, but we can also improve the behavior in the kernel.
> >> >
> >> > Let's let eDP panels drivers specify that a panel is turned off and
> >> > then modify the common AUX transfer code not to attempt a transfer in
> >> > this case.
> >> >
> >> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> >> > ---
> >>
> >> Reviewed-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
> >
> > Thanks for the review!
> >
> > Given that this touches core DRM code and that I never got
> > confirmation that Jani's concerns were addressed with my previous
> > response, I'm still going to wait a little while before applying. I'm
> > on vacation for most of next week, but if there are no further replies
> > between now and then I'll plan to apply this to "drm-misc-next" the
> > week of Feb 26th. If someone else wants to apply this before I do then
> > I certainly won't object. Jani: if you feel this needs more discussion
> > or otherwise object to this patch landing then please yell. Likewise
> > if anyone else in the community wants to throw in their opinion, feel
> > free.
>
> Sorry for dropping the ball after my initial response. I simply have not
> had the time to look into this.
>
> It would be great to get, say, drm-misc maintainer ack on this before
> merging. It's not fair for me to stall this any longer, I'll trust their
> judgement.
>
> Reasonable?

I'd be more than happy for one of the drm-misc maintainers to Ack.
I'll move Maxime, Thomas, and Maarten to the "To:" line to see if that
helps get through their filters.

-Doug