linux-kernel - [PATCH v4 01/11] drm/fourcc: Add warning for bad bpp

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Message-Id: <20250326-xilinx-formats-v4-1-322a300c6d72@ideasonboard.com>
Date: Wed, 26 Mar 2025 15:22:44 +0200
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Vishal Sagar <vishal.sagar@....com>, 
 Anatoliy Klymenko <anatoliy.klymenko@....com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
 Michal Simek <michal.simek@....com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org, 
 Geert Uytterhoeven <geert@...ux-m68k.org>, 
 Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>, 
 Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Subject: [PATCH v4 01/11] drm/fourcc: Add warning for bad bpp

drm_format_info_bpp() cannot be used for formats which do not have an
integer bits-per-pixel in a pixel block.

E.g. DRM_FORMAT_XV15's (not yet in upstream) plane 0 has three 10-bit
pixels (Y components), and two padding bits, in a 4 byte block. That is
10.666... bits per pixel when considering the whole 4 byte block, which
is what drm_format_info_bpp() does. Thus a driver that supports such
formats cannot use drm_format_info_bpp(),

It is a driver bug if this happens, but so handle wrong calls by
printing a warning and returning 0.

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
---
 drivers/gpu/drm/drm_fourcc.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
index 3a94ca211f9c..2f5781f5dcda 100644
--- a/drivers/gpu/drm/drm_fourcc.c
+++ b/drivers/gpu/drm/drm_fourcc.c
@@ -454,12 +454,20 @@ EXPORT_SYMBOL(drm_format_info_block_height);
  */
 unsigned int drm_format_info_bpp(const struct drm_format_info *info, int plane)
 {
+	unsigned int block_size;
+
 	if (!info || plane < 0 || plane >= info->num_planes)
 		return 0;
 
-	return info->char_per_block[plane] * 8 /
-	       (drm_format_info_block_width(info, plane) *
-		drm_format_info_block_height(info, plane));
+	block_size = drm_format_info_block_width(info, plane) *
+		     drm_format_info_block_height(info, plane);
+
+	if (info->char_per_block[plane] * 8 % block_size) {
+		pr_warn("unable to return an integer bpp\n");
+		return 0;
+	}
+
+	return info->char_per_block[plane] * 8 / block_size;
 }
 EXPORT_SYMBOL(drm_format_info_bpp);
 

-- 
2.43.0