[PATCH v4 6/7] drm/mgag200: Add vblank support

[Thomas Zimmermann]

Hi Jocelyn

Am 08.07.24 um 14:46 schrieb Jocelyn Falempe:
>
>
> On 05/07/2024 13:47, Thomas Zimmermann wrote:
>> There's no VBLANK interrupt on Matrox chipsets. The workaround that is
>> being used here and in other free Matrox drivers is to program 
>> <linecomp>
>> to the value of <vblkstr> and enable the VLINE interrupt. This triggers
>> an interrupt at the time when VBLANK begins.
>>
>> VLINE uses separate registers for enabling and clearing pending 
>> interrupts.
>> No extra synchronization between irq handler and the rest of the 
>> driver is
>> required.
>
> Thanks for this patch, I have a few comments below:

They make sense. I'll implement them.

[...]
>>   #define    MGAREG_IEN        0x1e1c
>> +#define MGAREG_IEN_VLINEIEN    BIT(5)
>>     #define    MGAREG_VCOUNT        0x1e20
>
>
> Do you know what happens if the IRQ doesn't work (ie not receiving any 
> IRQ)?

We'd miss the vblank timeout and the display would be slow or static.

> When testing the DMA, I remember that on a few machines, I never get 
> the Softrap IRQ. I don't know if it was a DMA engine problem, or if 
> it's the mgag200 IRQ that was not connected properly.

What to do depends. Not having an IRQ assigned is a problem in the BIOS 
configuration. All other DRM drivers expect the IRQ to be present and 
mgag200 would not be any different here. Users should assign an IRQ in 
BIOS. If there's really a hardware problem on some Matrox chips, we'd 
have to revert the patch. There's little use in handling separate vblank 
and non-vblank code paths. Most other patches in this patchset would 
still remain useful, I think.

OTOH other Matrox drivers have implemented vblank support since forever. 
I'd expect (well, hope at least :) that this simply isn't a problem.

>
> I will try to find this machine, and test this series on it.
>

Thanks a lot.

Best regards
Thomas

> Best regards,
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)