[PATCH] firmware/sysfb: Fix VESA format selection

[Pierre Asselin <pa@panix.com>]

Some legacy BIOSes report no reserved bits in their 32-bit rgb mode,
breaking the calculation of bits_per_pixel in commit f35cd3fa7729
[firmware/sysfb: Fix EFI/VESA format selection].  However they report
lfb_depth correctly for those modes.  Recompute bits_per_pixel from the
color and reserved bit numbers and positions, but also from lfb_depth,
keeping the larger result.

Link: https://lore.kernel.org/r/4Psm6B6Lqkz1QXM@panix3.panix.com
Link: https://lore.kernel.org/r/20230412150225.3757223-1-javierm@redhat.com
Fixes: f35cd3fa7729 [firmware/sysfb: Fix EFI/VESA format selection]
Signed-off-by: Pierre Asselin <pa@panix.com>
---
 drivers/firmware/sysfb_simplefb.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/sysfb_simplefb.c b/drivers/firmware/sysfb_simplefb.c
index 82c64cb9f531..05dc25a524c8 100644
--- a/drivers/firmware/sysfb_simplefb.c
+++ b/drivers/firmware/sysfb_simplefb.c
@@ -51,15 +51,17 @@ __init bool sysfb_parse_mode(const struct screen_info *si,
 	 *
 	 * It's not easily possible to fix this in struct screen_info,
 	 * as this could break UAPI. The best solution is to compute
-	 * bits_per_pixel here and ignore lfb_depth. In the loop below,
+	 * bits_per_pixel from the color bits, reserved bits and
+	 * reported lfb_depth, whichever is highest.  In the loop below,
 	 * ignore simplefb formats with alpha bits, as EFI and VESA
 	 * don't specify alpha channels.
 	 */
 	if (si->lfb_depth > 8) {
-		bits_per_pixel = max(max3(si->red_size + si->red_pos,
+		bits_per_pixel = max3(max3(si->red_size + si->red_pos,
 					  si->green_size + si->green_pos,
 					  si->blue_size + si->blue_pos),
-				     si->rsvd_size + si->rsvd_pos);
+				     si->rsvd_size + si->rsvd_pos,
+				     si->lfb_depth);
 	} else {
 		bits_per_pixel = si->lfb_depth;
 	}
-- 
2.39.2