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

[Helge Deller]

On 6/13/24 23:50, Javier Martinez Canillas wrote:
> Helge Deller <deller at 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 at suse.de>
>>> Fixes: 5e8ddcbe8692 ("Video mode probing support for the new x86 setup code")
>>> Cc: <stable at 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.

Really?

+static inline bool __screen_info_vbe_mode_nonvga(const struct screen_info *si)
+{
+	return si->vesa_attributes & BIT(5); // VGA if _not_ set
+}

At least the double negation "!nonvga()" breaks my head and the "//" comment
should be converted to /*..*/ IMHO.

Helge