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:

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 ?

> 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>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat