[PATCH v2] fbdev: vesafb: Detect VGA compatibility from screen info's VESA attributes

[Thomas Zimmermann]

Am 17.06.24 um 13:06 schrieb Thomas Zimmermann:
> 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.
>
> Besides fixing the bug in vesafb, this commit introduces a helper that
> reads the correct bit from screen_info.
>
> v2:
> - clarify comment on non-VGA modes (Helge)
>
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> Fixes: 5e8ddcbe8692 ("Video mode probing support for the new x86 setup code")
> Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>
> Cc: <stable at vger.kernel.org> # v2.6.23+
> ---
>   drivers/video/fbdev/vesafb.c |  2 +-
>   include/linux/screen_info.h  | 10 ++++++++++
>   2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/vesafb.c b/drivers/video/fbdev/vesafb.c
> index 8ab64ae4cad3e..5a161750a3aee 100644
> --- a/drivers/video/fbdev/vesafb.c
> +++ b/drivers/video/fbdev/vesafb.c
> @@ -271,7 +271,7 @@ static int vesafb_probe(struct platform_device *dev)
>   	if (si->orig_video_isVGA != VIDEO_TYPE_VLFB)
>   		return -ENODEV;
>   
> -	vga_compat = (si->capabilities & 2) ? 0 : 1;
> +	vga_compat = !__screen_info_vbe_mode_nonvga(si);
>   	vesafb_fix.smem_start = si->lfb_base;
>   	vesafb_defined.bits_per_pixel = si->lfb_depth;
>   	if (15 == vesafb_defined.bits_per_pixel)
> diff --git a/include/linux/screen_info.h b/include/linux/screen_info.h
> index 75303c126285a..d21f8e4e9f4a4 100644
> --- a/include/linux/screen_info.h
> +++ b/include/linux/screen_info.h
> @@ -49,6 +49,16 @@ static inline u64 __screen_info_lfb_size(const struct screen_info *si, unsigned
>   	return lfb_size;
>   }
>   
> +static inline bool __screen_info_vbe_mode_nonvga(const struct screen_info *si)
> +{
> +	/*
> +	 * VESA modes typically run on VGA hardware. Set bit 5 signal that this

'signals'

> +	 * is not the case. Drivers can then not make use of VGA resources. See
> +	 * Sec 4.4 of the VBE 2.0 spec.
> +	 */
> +	return si->vesa_attributes & BIT(5);
> +}
> +
>   static inline unsigned int __screen_info_video_type(unsigned int type)
>   {
>   	switch (type) {

-- 
--
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)