[PATCH v5 0/7] drm/mgag200: Implement VBLANK support

[Thomas Zimmermann]

Hi

thanks for your help.


Am 04.10.24 um 12:01 schrieb Ville Syrjälä:
> On Fri, Oct 04, 2024 at 11:17:02AM +0200, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 02.10.24 um 18:15 schrieb Luck, Tony:
>>>> Thanks for the bug report. Can you provide the output of 'sudo lspci
>>>> -vvv' for the graphics device?
>>> Thomas,
>>>
>>> Sure. Here's the output (run on the v6.11.0 kernel)
>> Thanks. It doesn't look much different from other systems. IRQ is also
>> assigned.
>>
>> Attached is a patch that fixes a possible off-by-one error in the
>> register settings. This would affect the bug you're reporting. If
>> possible, please apply the patch to your 6.12-rc1, test and report the
>> result.
> Didn't one of these weird variants have some bug where the
> CRTC startadd was not working? Is this one of those?

Yeah, but it seems unrelated.

> That to me sounds like maybe linecomp has internally been
> tied to be always active somehow. Perhaps that would
> also prevent it from generating the interrupt...

Linecomp is usually set to vtotal and that disables the irq. When set to 
vblank_start/vdisplay_end, it acts like a vblank IRQ. But the other 
matrox drivers I saw (fbdev, Xorg-video-matrox) set the value -1, while 
mgag200 doesn't. So there really is an off-by-one error.

>
> Anyways, sounds like someone should just double check whether
> the status bit ever get asserted or not. If yes, then the
> problem must be with interrupt delivery, otherwise the
> problem is that the internal interrupt is never even
> generated. In the latter case you could try using the
> vsync interrupt instead.

I didn't want to go into full debugging while there's a low-hanging fix 
to try first. I'll probably take that patch anyway even if it doesn't 
fix the reported bug.

Wrt. vsync: isn't that way to late for vblank events? Does DRM give any 
timing guarantees? (It doesn't AFAIK.) Or does it just mean that a 
vblank has happened at some point in the past?

Best regards
Thomas

>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
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