linux-kernel - [PATCH 6/6] drm/panic: Fix 24bit pixel crossing page boundaries

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Message-ID: <20251009122955.562888-7-jfalempe@redhat.com>
Date: Thu,  9 Oct 2025 14:24:53 +0200
From: Jocelyn Falempe <jfalempe@...hat.com>
To: 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>,
	Jocelyn Falempe <jfalempe@...hat.com>,
	Javier Martinez Canillas <javierm@...hat.com>,
	linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Cc: stable@...r.kernel.org
Subject: [PATCH 6/6] drm/panic: Fix 24bit pixel crossing page boundaries

When using page list framebuffer, and using RGB888 format, some
pixels can cross the page boundaries, and this case was not handled,
leading to writing 1 or 2 bytes on the next virtual address.

Add a check and a specific function to handle this case.

Fixes: c9ff2808790f0 ("drm/panic: Add support to scanout buffer as array of pages")
Signed-off-by: Jocelyn Falempe <jfalempe@...hat.com>
---
 drivers/gpu/drm/drm_panic.c | 46 +++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_panic.c b/drivers/gpu/drm/drm_panic.c
index bc5158683b2b..d4b6ea42db0f 100644
--- a/drivers/gpu/drm/drm_panic.c
+++ b/drivers/gpu/drm/drm_panic.c
@@ -174,6 +174,33 @@ static void drm_panic_write_pixel24(void *vaddr, unsigned int offset, u32 color)
 	*p = color & 0xff;
 }
 
+/*
+ * Special case if the pixel crosses page boundaries
+ */
+static void drm_panic_write_pixel24_xpage(void *vaddr, struct page *next_page,
+					  unsigned int offset, u32 color)
+{
+	u8 *vaddr2;
+	u8 *p = vaddr + offset;
+
+	vaddr2 = kmap_local_page_try_from_panic(next_page);
+
+	*p++ = color & 0xff;
+	color >>= 8;
+
+	if (offset == PAGE_SIZE - 1)
+		p = vaddr2;
+
+	*p++ = color & 0xff;
+	color >>= 8;
+
+	if (offset == PAGE_SIZE - 2)
+		p = vaddr2;
+
+	*p = color & 0xff;
+	kunmap_local(vaddr2);
+}
+
 static void drm_panic_write_pixel32(void *vaddr, unsigned int offset, u32 color)
 {
 	u32 *p = vaddr + offset;
@@ -231,7 +258,14 @@ static void drm_panic_blit_page(struct page **pages, unsigned int dpitch,
 					page = new_page;
 					vaddr = kmap_local_page_try_from_panic(pages[page]);
 				}
-				if (vaddr)
+				if (!vaddr)
+					continue;
+
+				// Special case for 24bit, as a pixel might cross page boundaries
+				if (cpp == 3 && offset + 3 > PAGE_SIZE)
+					drm_panic_write_pixel24_xpage(vaddr, pages[page + 1],
+								      offset, fg32);
+				else
 					drm_panic_write_pixel(vaddr, offset, fg32, cpp);
 			}
 		}
@@ -321,7 +355,15 @@ static void drm_panic_fill_page(struct page **pages, unsigned int dpitch,
 				page = new_page;
 				vaddr = kmap_local_page_try_from_panic(pages[page]);
 			}
-			drm_panic_write_pixel(vaddr, offset, color, cpp);
+			if (!vaddr)
+				continue;
+
+			// Special case for 24bit, as a pixel might cross page boundaries
+			if (cpp == 3 && offset + 3 > PAGE_SIZE)
+				drm_panic_write_pixel24_xpage(vaddr, pages[page + 1],
+							      offset, color);
+			else
+				drm_panic_write_pixel(vaddr, offset, color, cpp);
 		}
 	}
 	if (vaddr)
-- 
2.51.0