Re: [PATCH] fbdev: vesafb: Detect VGA compatibility from screen info's VESA attributes

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi Javier

Am 13.06.24 um 11:35 schrieb Javier Martinez Canillas:
> Thomas Zimmermann <tzimmermann@suse.de> writes:
>
> Hello Thomas,
>
>> Test the vesa_attributes field in struct screen_info for compatibility
>> with VGA hardware. Vesafb currently tests bit 1 in screen_info's
>> capabilities field, It sets the framebuffer address size and is
>> unrelated to VGA.
>>
>> Section 4.4 of the Vesa VBE 2.0 specifications defines that bit 5 in
>> the mode's attributes field signals VGA compatibility. The mode is
>> compatible with VGA hardware if the bit is clear. In that case, the
>> driver can access VGA state of the VBE's underlying hardware. The
>> vesafb driver uses this feature to program the color LUT in palette
>> modes. Without, colors might be incorrect.
>>
>> The problem got introduced in commit 89ec4c238e7a ("[PATCH] vesafb: Fix
>> incorrect logo colors in x86_64"). It incorrectly stores the mode
>> attributes in the screen_info's capabilities field and updates vesafb
>> accordingly. Later, commit 5e8ddcbe8692 ("Video mode probing support for
>> the new x86 setup code") fixed the screen_info, but did not update vesafb.
>> Color output still tends to work, because bit 1 in capabilities is
>> usually 0.
>>
> How did you find this ?

I was reading through vesafb and found that [1] and [2] look 
surprisingly similar, which makes no sense. So I started looking where 
bit 1 came from. The flag signals a 64-bit framebuffer address for EFI 
(see VIDEO_CAPABILITY_64BIT_BASE 
<https://elixir.bootlin.com/linux/latest/C/ident/VIDEO_CAPABILITY_64BIT_BASE>). 
But old VESA framebuffers are usually located within the first 32-bit 
range. So the bit is mostly 0 and vesafb works as expected.

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/video/fbdev/vesafb.c#L274
[2] 
https://elixir.bootlin.com/linux/latest/source/include/linux/screen_info.h#L26

>
>> Besides fixing the bug in vesafb, this commit introduces a helper that
>> reads the correct bit from screen_info.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> Fixes: 5e8ddcbe8692 ("Video mode probing support for the new x86 setup code")
>> Cc: <stable@vger.kernel.org> # v2.6.23+
>> ---
> The patch looks correct to me after your explanation in the commit message
> and looking at the mentioned commits.
>
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

Thanks a lot.

Best regards
Thomas

>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
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