linux-kernel - [PATCH] firmware: coreboot: framebuffer: Ignore reserved pixel color bits

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Date:   Sun, 22 Jan 2023 22:04:31 +0300
From:   Alper Nebi Yasak <alpernebiyasak@...il.com>
To:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Brian Norris <briannorris@...omium.org>,
        Wei-Ning Huang <wnhuang@...omium.org>,
        Samuel Holland <samuel@...lland.org>,
        Julius Werner <jwerner@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Alper Nebi Yasak <alpernebiyasak@...il.com>
Subject: [PATCH] firmware: coreboot: framebuffer: Ignore reserved pixel color bits

The coreboot framebuffer doesn't support transparency, its 'reserved'
bit field is merely padding for byte/word alignment of pixel colors [1].
When trying to match the framebuffer to a simplefb format, the kernel
driver unnecessarily requires the format's transparency bit field to
exactly match this padding, even if the former is zero-width.

Due to a coreboot bug [2] (fixed upstream), some boards misreport the
reserved field's size as equal to its position (0x18 for both on a
'Lick' Chromebook), and the driver fails to probe where it would have
otherwise worked fine with e.g. the a8r8g8b8 or x8r8g8b8 formats.

Remove the transparency comparison with reserved bits. When the
bits-per-pixel and other color components match, transparency will
already be in a subset of the reserved field. Not forcing it to match
reserved bits allows the driver to work on the boards which misreport
the reserved field. It also enables using simplefb formats that don't
have transparency bits, although this doesn't currently happen due to
format support and ordering in linux/platform_data/simplefb.h.

[1] https://review.coreboot.org/plugins/gitiles/coreboot/+/4.19/src/commonlib/include/commonlib/coreboot_tables.h#255
[2] https://review.coreboot.org/plugins/gitiles/coreboot/+/4.13/src/drivers/intel/fsp2_0/graphics.c#82

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@...il.com>
---

 drivers/firmware/google/framebuffer-coreboot.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/firmware/google/framebuffer-coreboot.c b/drivers/firmware/google/framebuffer-coreboot.c
index c6dcc1ef93ac..c323a818805c 100644
--- a/drivers/firmware/google/framebuffer-coreboot.c
+++ b/drivers/firmware/google/framebuffer-coreboot.c
@@ -43,9 +43,7 @@ static int framebuffer_probe(struct coreboot_device *dev)
 		    fb->green_mask_pos     == formats[i].green.offset &&
 		    fb->green_mask_size    == formats[i].green.length &&
 		    fb->blue_mask_pos      == formats[i].blue.offset &&
-		    fb->blue_mask_size     == formats[i].blue.length &&
-		    fb->reserved_mask_pos  == formats[i].transp.offset &&
-		    fb->reserved_mask_size == formats[i].transp.length)
+		    fb->blue_mask_size     == formats[i].blue.length)
 			pdata.format = formats[i].name;
 	}
 	if (!pdata.format)
-- 
2.39.0