linux-kernel - [PATCH 3.16 007/306] fbdev/efifb: Fix 16 color palette entry calculation

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Message-ID: <lsq.1487198500.815461472@decadent.org.uk>
Date:   Wed, 15 Feb 2017 22:41:40 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Max Staudt" <mstaudt@...e.de>,
        "Peter Jones" <pjones@...hat.com>,
        "Tomi Valkeinen" <tomi.valkeinen@...com>
Subject: [PATCH 3.16 007/306] fbdev/efifb: Fix 16 color palette entry
 calculation

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Max Staudt <mstaudt@...e.de>

commit d50b3f43db739f03fcf8c0a00664b3d2fed0496e upstream.

When using efifb with a 16-bit (5:6:5) visual, fbcon's text is rendered
in the wrong colors - e.g. text gray (#aaaaaa) is rendered as green
(#50bc50) and neighboring pixels have slightly different values
(such as #50bc78).

The reason is that fbcon loads its 16 color palette through
efifb_setcolreg(), which in turn calculates a 32-bit value to write
into memory for each palette index.
Until now, this code could only handle 8-bit visuals and didn't mask
overlapping values when ORing them.

With this patch, fbcon displays the correct colors when a qemu VM is
booted in 16-bit mode (in GRUB: "set gfxpayload=800x600x16").

Fixes: 7c83172b98e5 ("x86_64 EFI boot support: EFI frame buffer driver")  # v2.6.24+
Signed-off-by: Max Staudt <mstaudt@...e.de>
Acked-By: Peter Jones <pjones@...hat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/video/fbdev/efifb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/drivers/video/fbdev/efifb.c
+++ b/drivers/video/fbdev/efifb.c
@@ -52,9 +52,9 @@ static int efifb_setcolreg(unsigned regn
 		return 1;

 	if (regno < 16) {
-		red   >>= 8;
-		green >>= 8;
-		blue  >>= 8;
+		red   >>= 16 - info->var.red.length;
+		green >>= 16 - info->var.green.length;
+		blue  >>= 16 - info->var.blue.length;
 		((u32 *)(info->pseudo_palette))[regno] =
 			(red   << info->var.red.offset)   |
 			(green << info->var.green.offset) |