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

[Neil Armstrong]

Hi Doug,

On 15/02/2024 16:08, Doug Anderson wrote:
> Hi,
> 
> On Thu, Feb 15, 2024 at 2:24 AM Jani Nikula <jani.nikula at intel.com> wrote:
>>
>> On Wed, 14 Feb 2024, Doug Anderson <dianders at chromium.org> wrote:
>>> Hi,
>>>
>>> On Tue, Feb 13, 2024 at 10:25 PM Hsin-Yi Wang <hsinyi at chromium.org> wrote:
>>>>
>>>> On Wed, Feb 14, 2024 at 2:23 PM Douglas Anderson <dianders at 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 at chromium.org>
>>>>> ---
>>>>
>>>> Reviewed-by: Hsin-Yi Wang <hsinyi at 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?
> 
> 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.

I'll like some test reports to be sure it doesn't break anything,
then I'll be happy to give my ack !

Thanks,
Neil

> 
> -Doug