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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 22.05.26 um 21:39 schrieb Ville Syrjälä:
> On Fri, May 22, 2026 at 03:43:26PM +0200, Thomas Zimmermann wrote:
>> 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.
>>>>>
>>>>> 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.
> Actual mirroring may not be possible due to different modes supported
> on each output. The whole multi-output fbdev thing in the drm fb helper
> is kind of a hack that's rather hard to make work 100% sensibly.

I know, but we have to support it to some extent. The current mirroring 
logic at least works, except that it assumes CRTC 0 to be present.

Best regards
Thomas

>
> For the panning possibly the only sensible thing is to use the max of
> hdisplay/vdisplay of all the crtcs as the xres/yres so it's clear
> how much things can actually be panned. Oh and tiled displays (assuming
> we would actually want the fbdev stuff to tile correctly) make the
> situation even more complicated. I think the current support for tiled
> displays in the fb helper is semi-busted.
>
>> 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)
>>

-- 
--
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)