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

[Jani Nikula <jani.nikula@intel.com>]

On Wed, 14 Feb 2024, Doug Anderson <dianders@chromium.org> wrote:
> Hi,
>
> On Tue, Feb 13, 2024 at 10:25 PM Hsin-Yi Wang <hsinyi@chromium.org> wrote:
>>
>> On Wed, Feb 14, 2024 at 2:23 PM Douglas Anderson <dianders@chromium.org> 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@chromium.org>
>> > ---
>>
>> Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
>
> 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?


BR,
Jani.


-- 
Jani Nikula, Intel