Re: [PATCH] drm/panic: Add support to scanout buffer as array of pages

Thomas Zimmermann <tzimmermann@xxxxxxx> · Mon, 17 Mar 2025 14:47:50 +0100

Hi

Am 13.03.25 um 10:42 schrieb Jocelyn Falempe:
Some drivers like virtio-gpu, don't map the scanout buffer in the
kernel. Calling vmap() in a panic handler is not safe, and writing an
atomic_vmap() API is more complex than expected [1].
So instead, pass the array of pages of the scanout buffer to the
panic handler, and map only one page at a time to draw the pixels.
This is obviously slow, but acceptable for a panic handler.

It seems this could still be optimized by writing multiple pixels at 
once if they are located on the same pages. Although it's not really 
necessary.


[1] https://lore.kernel.org/dri-devel/20250305152555.318159-1-ryasuoka@xxxxxxxxxx/

Signed-off-by: Jocelyn Falempe <jfalempe@xxxxxxxxxx>

Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

Best regards
Thomas

---
  drivers/gpu/drm/drm_panic.c | 139 ++++++++++++++++++++++++++++++++++--
  include/drm/drm_panic.h     |  10 ++-
  2 files changed, 142 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/drm_panic.c b/drivers/gpu/drm/drm_panic.c
index ab42a2b1567d..e10ab8fe847c 100644
--- a/drivers/gpu/drm/drm_panic.c
+++ b/drivers/gpu/drm/drm_panic.c
@@ -7,6 +7,7 @@
   */
  
  #include <linux/font.h>
+#include <linux/highmem.h>
  #include <linux/init.h>
  #include <linux/iosys-map.h>
  #include <linux/kdebug.h>
@@ -154,6 +155,87 @@ static void drm_panic_blit_pixel(struct drm_scanout_buffer *sb, struct drm_rect
  				sb->set_pixel(sb, clip->x1 + x, clip->y1 + y, fg_color);
  }
  
+static void drm_panic_write_pixel16(void *vaddr, unsigned int offset, u16 color)
+{
+	u16 *p = vaddr + offset;
+
+	*p = color;
+}
+
+static void drm_panic_write_pixel24(void *vaddr, unsigned int offset, u32 color)
+{
+	u8 *p = vaddr + offset;
+
+	*p++ = color & 0xff;
+	color >>= 8;
+	*p++ = color & 0xff;
+	color >>= 8;
+	*p = color & 0xff;
+}
+
+static void drm_panic_write_pixel32(void *vaddr, unsigned int offset, u32 color)
+{
+	u32 *p = vaddr + offset;
+
+	*p = color;
+}
+
+static void drm_panic_write_pixel(void *vaddr, unsigned int offset, u32 color, unsigned int cpp)
+{
+	switch (cpp) {
+	case 2:
+		drm_panic_write_pixel16(vaddr, offset, color);
+		break;
+	case 3:
+		drm_panic_write_pixel24(vaddr, offset, color);
+		break;
+	case 4:
+		drm_panic_write_pixel32(vaddr, offset, color);
+		break;
+	default:
+		DRM_WARN_ONCE("Can't blit with pixel width %d\n", cpp);
+	}
+}
+
+/*
+ * The scanout buffer pages are not mapped, so for each pixel,
+ * use kmap_local_page() to map the page, and write the pixel.
+ * Tries to keep the map from the previous pixel, to avoid too much map/unmap.
+ */
+static void drm_panic_blit_page(struct page **pages, unsigned int dpitch,
+				unsigned int cpp, const u8 *sbuf8,
+				unsigned int spitch, struct drm_rect *clip,
+				unsigned int scale, u32 fg32)
+{
+	unsigned int y, x;
+	unsigned int page = ~0;
+	unsigned int height = drm_rect_height(clip);
+	unsigned int width = drm_rect_width(clip);
+	void *vaddr = NULL;
+
+	for (y = 0; y < height; y++) {
+		for (x = 0; x < width; x++) {
+			if (drm_draw_is_pixel_fg(sbuf8, spitch, x / scale, y / scale)) {
+				unsigned int new_page;
+				unsigned int offset;
+
+				offset = (y + clip->y1) * dpitch + (x + clip->x1) * cpp;
+				new_page = offset >> PAGE_SHIFT;
+				offset = offset % PAGE_SIZE;
+				if (new_page != page) {
+					if (vaddr)
+						kunmap_local(vaddr);
+					page = new_page;
+					vaddr = kmap_local_page(pages[page]);
+				}
+				drm_panic_write_pixel(vaddr, offset, fg32, cpp);
+			}
+		}
+	}
+	if (vaddr)
+		kunmap_local(vaddr);
+}
+
  /*
   * drm_panic_blit - convert a monochrome image to a linear framebuffer
   * @sb: destination scanout buffer
@@ -177,6 +259,10 @@ static void drm_panic_blit(struct drm_scanout_buffer *sb, struct drm_rect *clip,
  	if (sb->set_pixel)
  		return drm_panic_blit_pixel(sb, clip, sbuf8, spitch, scale, fg_color);
  
+	if (sb->pages)
+		return drm_panic_blit_page(sb->pages, sb->pitch[0], sb->format->cpp[0],
+					   sbuf8, spitch, clip, scale, fg_color);
+
  	map = sb->map[0];
  	iosys_map_incr(&map, clip->y1 * sb->pitch[0] + clip->x1 * sb->format->cpp[0]);
  
@@ -209,6 +295,35 @@ static void drm_panic_fill_pixel(struct drm_scanout_buffer *sb,
  			sb->set_pixel(sb, clip->x1 + x, clip->y1 + y, color);
  }
  
+static void drm_panic_fill_page(struct page **pages, unsigned int dpitch,
+				unsigned int cpp, struct drm_rect *clip,
+				u32 color)
+{
+	unsigned int y, x;
+	unsigned int page = ~0;
+	void *vaddr = NULL;
+
+	for (y = clip->y1; y < clip->y2; y++) {
+		for (x = clip->x1; x < clip->x2; x++) {
+			unsigned int new_page;
+			unsigned int offset;
+
+			offset = y * dpitch + x * cpp;
+			new_page = offset >> PAGE_SHIFT;
+			offset = offset % PAGE_SIZE;
+			if (new_page != page) {
+				if (vaddr)
+					kunmap_local(vaddr);
+				page = new_page;
+				vaddr = kmap_local_page(pages[page]);
+			}
+			drm_panic_write_pixel(vaddr, offset, color, cpp);
+		}
+	}
+	if (vaddr)
+		kunmap_local(vaddr);
+}
+
  /*
   * drm_panic_fill - Fill a rectangle with a color
   * @sb: destination scanout buffer
@@ -225,6 +340,10 @@ static void drm_panic_fill(struct drm_scanout_buffer *sb, struct drm_rect *clip,
  	if (sb->set_pixel)
  		return drm_panic_fill_pixel(sb, clip, color);
  
+	if (sb->pages)
+		return drm_panic_fill_page(sb->pages, sb->pitch[0], sb->format->cpp[0],
+					   clip, color);
+
  	map = sb->map[0];
  	iosys_map_incr(&map, clip->y1 * sb->pitch[0] + clip->x1 * sb->format->cpp[0]);
  
@@ -714,16 +833,24 @@ static void draw_panic_plane(struct drm_plane *plane, const char *description)
  	if (!drm_panic_trylock(plane->dev, flags))
  		return;
  
+	ret = plane->helper_private->get_scanout_buffer(plane, &sb);
+
+	if (ret || !drm_panic_is_format_supported(sb.format))
+		goto unlock;
+
+	/* One of these should be set, or it can't draw pixels */
+	if (!sb.set_pixel && !sb.pages && iosys_map_is_null(&sb.map[0]))
+		goto unlock;
+
  	drm_panic_set_description(description);
  
-	ret = plane->helper_private->get_scanout_buffer(plane, &sb);
+	draw_panic_dispatch(&sb);
+	if (plane->helper_private->panic_flush)
+		plane->helper_private->panic_flush(plane);
  
-	if (!ret && drm_panic_is_format_supported(sb.format)) {
-		draw_panic_dispatch(&sb);
-		if (plane->helper_private->panic_flush)
-			plane->helper_private->panic_flush(plane);
-	}
  	drm_panic_clear_description();
+
+unlock:
  	drm_panic_unlock(plane->dev, flags);
  }
  
diff --git a/include/drm/drm_panic.h b/include/drm/drm_panic.h
index f4e1fa9ae607..8b91a13347b9 100644
--- a/include/drm/drm_panic.h
+++ b/include/drm/drm_panic.h
@@ -39,6 +39,14 @@ struct drm_scanout_buffer {
  	 */
  	struct iosys_map map[DRM_FORMAT_MAX_PLANES];
  
+	/**
+	 * @pages: Optional, if the scanout buffer is not mapped, set this field
+	 * to the array of pages of the scanout buffer. The panic code will use
+	 * kmap_local_page() to map one page at a time to write all the pixels.
+	 * The scanout buffer should be in linear format.
+	 */
+	struct page **pages;
+
  	/**
  	 * @width: Width of the scanout buffer, in pixels.
  	 */
@@ -57,7 +65,7 @@ struct drm_scanout_buffer {
  	/**
  	 * @set_pixel: Optional function, to set a pixel color on the
  	 * framebuffer. It allows to handle special tiling format inside the
-	 * driver.
+	 * driver. It takes precedence over the @map and @pages fields.
  	 */
  	void (*set_pixel)(struct drm_scanout_buffer *sb, unsigned int x,
  			  unsigned int y, u32 color);

base-commit: 9e75b6ef407fee5d4ed8021cd7ddd9d6a8f7b0e8

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)







