Re: [PATCH] drm/client: check whether CRTC is active before waiting for vblank

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 22.05.26 um 16:01 schrieb Icenowy Zheng:
> 在 2026-05-22五的 15:43 +0200，Thomas Zimmermann写道：
>> Hi
>>
>> Am 22.05.26 um 15:28 schrieb Ville Syrjälä:
>> [...]
>>>>>> But why does your HW use CRTC 1 in the first place.
>>>>> Could be eg. the enabled outputs can't be driven with CRTC 0.
> Yes, for many embedded display solutions the CRTC-connector map is
> totally fixed.
>
>>>>> I guess what you want to do is pick the first crtc from
>>>>> modesets[]
>>>>> which is enabled. Or perhaps even "pick the Nth enabled crtc
>>>>> from
>>>>> modesets[] based on the ioctl argument".
>>>> The enable-status of each CRTC could change later on, which might
>>>> lead
>>>> to problems.
>>> Sound like a locking issue if someone is changing the configuration
>>> at the same time we're trying to do the vblank wait here.
>> I mean that the connected outputs could change at a later point or we
>> could have multiple CRTCs in use. Today, someone in #intel-gfx
>> reported
>> a problem with panning if multiple CRTCs are in use.
>>
>> Therefore picking a CRTC freely could be a problem. Let's say we
>> configure modes from one CRTC, but later wait/pan/flush with another
>> CRTC. I would not trust this to work correctly.
>>
>> Hence, my suggestion is to select a primary CRTC during the fbdev
>> client's probe and use it for all later operations until the next
>> probe
>> happens.  All other CRTCs would mirror the primary one.
> What will happen if the "primary CRTC" is then disabled because of no
> connected connectors can be driven with it?

This happens during a client hotplug event. We'd re-detect all 
connectors, pick the CRTC/output with the lowest spec as new primary and 
mirror it to all other connected outputs.

Best regards
Thomas

>
> Thanks,
> Icenowy
>
>> Best regards
>> Thomas
>>
>>
>>>> Picking the one CRTC/output with the lowest spec and
>>>> mirroring it to the others might work. This CRTC would then be
>>>> the one
>>>> to wait for.
>>>>
>>>> Best regards
>>>> Thomas
>>>>
>>>> -- 
>>>> --
>>>> Thomas Zimmermann
>>>> Graphics Driver Developer
>>>> SUSE Software Solutions Germany GmbH
>>>> Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
>>>> GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809,
>>>> AG Nürnberg)
>>>>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)