On 6/14/24 09:23, Thomas Zimmermann wrote:
Hi
Am 14.06.24 um 00:24 schrieb Helge Deller:
On 6/13/24 23:50, Javier Martinez Canillas wrote:
Helge Deller <deller@xxxxxx> 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?
Yes, please. Decoding screen_info is a science on it's own. I've added several of these helpers to access because such code was open-coded and duplicated all over the graphics code. And several places got something wrong. The patch here is just one of many such cases.
Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
Fixes: 5e8ddcbe8692 ("Video mode probing support for the new x86 setup code")
Cc: <stable@xxxxxxxxxxxxxxx> # 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?
Yep.
+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.
The non-VGA bit is specified by VESA. So the helper does the correct thing. We can make a better comment though.
If we want to simplify usage of this helper, I'd say we should kick vga_compat from vesafb and rework that code.
Ok, then I suggest to keep your patch as-is, with just the "//" comment
replaced by a /**/ comment.
Helge
