Re: [REGRESSION] VGA output with AST 2600 graphics.

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 07.06.22 um 13:02 schrieb Thorsten Leemhuis:
> Hi, this is your Linux kernel regression tracker.
> 
> On 01.06.22 14:29, Jocelyn Falempe wrote:
>> On 01/06/2022 12:33, Thomas Zimmermann wrote:
>>> Am 01.06.22 um 11:33 schrieb Jocelyn Falempe:
>>>>
>>>> I've found a regression in the ast driver, for AST2600 hardware.
>>>>
>>>> before the upstream commit f9bd00e0ea9d
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9bd00e0ea9d9b04140aa969a9a13ad3597a1e4e
>>>>
>>>>
>>>> The ast driver handled AST 2600 chip like an AST 2500.
>>>>
>>>> After this commit, it uses some default values, more like the older
>>>> AST chip.
>>>>
>>>> There are a lot of places in the driver like this:
>>>> https://elixir.bootlin.com/linux/v5.18.1/source/drivers/gpu/drm/ast/ast_post.c#L82
>>>>
>>>> where it checks for (AST2300 || AST2400 || AST2500) but not for AST2600.
>>>>
>>>> This makes the VGA output, to be blurred and flickered with whites
>>>> lines on AST2600.
>>>>
>>>> The issue is present since v5.11
>>>>
>>>> For v5.11~v5.17 I propose a simple workaround (as there are no other
>>>> reference to AST2600 in the driver):
>>>> --- a/drivers/gpu/drm/ast/ast_main.c
>>>> +++ b/drivers/gpu/drm/ast/ast_main.c
>>>> @@ -146,7 +146,8 @@ static int ast_detect_chip(struct drm_device
>>>> *dev, bool *need_post)
>>>>
>>>>        /* Identify chipset */
>>>>        if (pdev->revision >= 0x50) {
>>>> -        ast->chip = AST2600;
>>>> +        /* Workaround to use the same codepath for AST2600 */
>>>> +        ast->chip = AST2500;
>>>
>>> The whole handling of different models in this driver is broken by
>>> design and needs to be replaced.  I don't have much of the affected
>>> hardware, so such things are going slowly. :(
>>>
>>> For an intermediate fix, it would be better to change all tests for
>>> AST2500 to include AST2600 as well. There aren't too many IIRC.
>>
>> I feel a bit uncomfortable doing this, because I don't know if this
>> settings are good for AST2600 or not. I just know that AST2500 settings
>> are better than the "default".
> 
> KuoHsiang Chou, you wrote the commit causing this regression. Could you
> maybe take care of the idea Thomas outlined to get this fixed relative
> quickly? Or do you have a better idea?

Thanks for the reminder. I've sent out a patch for this problem. [1]

Best regards
Thomas

[1] 
https://lore.kernel.org/dri-devel/20220607120248.31716-1-tzimmermann@suse.de/T/#u

> 
>> Also it may not apply cleanly up to v5.11
> 
> FWIW, 5.11 is EOL anyway.
> 
>> I will do a test patch and see what it gives.
>>
>> Another solution would be to just revert f9bd00e0ea9d for v5.11 to v5.17 ?
> 
> That might cause a regression for users that depend on something
> supported thx to this change. :-/
> 
> Ciao, Thorsten

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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