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

[Helge Deller <deller@gmx.de>]

On 6/13/24 11:02, Thomas Zimmermann wrote:
> 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.

Nice catch, Thomas!

But do we really need this additional helper?


>
> 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+

> ---
>   drivers/video/fbdev/vesafb.c | 2 +-
>   include/linux/screen_info.h  | 5 +++++
>   2 files changed, 6 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);

Instead maybe just this: ?
  +	/* mode is VGA-compatible if BIT 5 is _NOT_ set */
  +	vga_compat = (si->vesa_attributes & BIT(5)) == 0;

I suggest to make patch small, esp. if you ask for backport to v2.6.23+.

Helge

>   	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..95f2a339de329 100644
> --- a/include/linux/screen_info.h
> +++ b/include/linux/screen_info.h
> @@ -49,6 +49,11 @@ 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)
> +{
> +	return si->vesa_attributes & BIT(5); // VGA if _not_ set
> +}
> +
>   static inline unsigned int __screen_info_video_type(unsigned int type)
>   {
>   	switch (type) {