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

[Ville Syrjälä]

On Mon, Oct 07, 2024 at 03:37:40PM +0200, Thomas Zimmermann wrote:
> Hi
> 
> 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?
> > 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...
> 
> Impressive debugging skills! The broken chip has vendor id 0x0522 
> according to commit 21e74bf99596 ("drm/mgag200: Store HW_BUG_NO_STARTADD 
> flag in device info"). And that's the same type the Tony reported. [1] 
> I'm just not sure if it's worth special casing the chip again or simply 
> revert vblank irqs.

Heh. Though I'm not sure if my theory is quite right. It
seems I've been confused about linecomp all these years;
I thought the split screen effect affected both VGA and
MGA modes (at least on the older chips), but looks like
it never affected MGA mode. I tested it here on a 2064w
based card, which is almost as old as you can go (I do
have an older Athena based card somewhere as well but
didn't bother digging it up).

-- 
Ville Syrjälä
Intel