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

[Javier Martinez Canillas <javierm@redhat.com>]

Thomas Zimmermann <tzimmermann@suse.de> writes:

> 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
>

I see. Thanks a lot for the explanation and references.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat