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

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

Helge Deller <deller@gmx.de> writes:

> 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+.
>

I prefer the helper. It's a static inline anyways and having it as a
function makes it much easier to read / understand.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat