drm/panic: Add a drm panic handler

This module displays a user friendly message when a kernel panic
occurs. It currently doesn't contain any debug information,
but that can be added later.

v2
* Use get_scanout_buffer() instead of the drm client API.
(Thomas Zimmermann)
* Add the panic reason to the panic message (Nerdopolis)
* Add an exclamation mark (Nerdopolis)

v3
* Rework the drawing functions, to write the pixels line by line and
to use the drm conversion helper to support other formats.
(Thomas Zimmermann)

v4
* Use drm_fb_r1_to_32bit for fonts (Thomas Zimmermann)
* Remove the default y to DRM_PANIC config option (Thomas Zimmermann)
* Add foreground/background color config option
* Fix the bottom lines not painted if the framebuffer height
is not a multiple of the font height.
* Automatically register the device to drm_panic, if the function
get_scanout_buffer exists. (Thomas Zimmermann)

v5
* Change the drawing API, use drm_fb_blit_from_r1() to draw the font.
* Also add drm_fb_fill() to fill area with background color.
* Add draw_pixel_xy() API for drivers that can't provide a linear buffer.
* Add a flush() callback for drivers that needs to synchronize the buffer.
* Add a void *private field, so drivers can pass private data to
draw_pixel_xy() and flush().

v6
* Fix sparse warning for panic_msg and logo.

v7
* Add select DRM_KMS_HELPER for the color conversion functions.

v8
* Register directly each plane to the panic notifier (Sima)
* Add raw_spinlock to properly handle concurrency (Sima)
* Register plane instead of device, to avoid looping through plane
list, and simplify code.
* Replace get_scanout_buffer() logic with drm_panic_set_buffer()
(Thomas Zimmermann)
* Removed the draw_pixel_xy() API, will see later if it can be added back.

v9
* Revert to using get_scanout_buffer() (Sima)
* Move get_scanout_buffer() and panic_flush() to the plane helper
functions (Thomas Zimmermann)
* Register all planes with get_scanout_buffer() to the panic notifier
* Use drm_panic_lock() to protect against race (Sima)

v10
* Move blit and fill functions back in drm_panic (Thomas Zimmermann).
* Simplify the text drawing functions.
* Use kmsg_dumper instead of panic_notifier (Sima).

v12
* Use array for map and pitch in struct drm_scanout_buffer
to support multi-planar format later. (Thomas Zimmermann)
* Better indent struct drm_scanout_buffer declaration. (Thomas Zimmermann)

Signed-off-by: Jocelyn Falempe <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Acked-by: Daniel Vetter <[e-mail]>

Documentation/gpu/drm-kms.rst

···
 .. kernel-doc:: include/drm/drm_damage_helper.h
    :internal:
 
+Plane Panic Feature
+-------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_panic.c
+   :doc: overview
+
+Plane Panic Functions Reference
+-------------------------------
+
+.. kernel-doc:: include/drm/drm_panic.h
+   :internal:
+
+.. kernel-doc:: drivers/gpu/drm/drm_panic.c
+   :export:
+
 Display Modes Function Reference
 ================================
 

drivers/gpu/drm/Kconfig

···
 	help
 	  CRTC helpers for KMS drivers.
 
+config DRM_PANIC
+	bool "Display a user-friendly message when a kernel panic occurs"
+	depends on DRM && !FRAMEBUFFER_CONSOLE
+	select DRM_KMS_HELPER
+	select FONT_SUPPORT
+	help
+	  Enable a drm panic handler, which will display a user-friendly message
+	  when a kernel panic occurs. It's useful when using a user-space
+	  console instead of fbcon.
+	  It will only work if your graphic driver supports this feature.
+	  To support Hi-DPI Display, you can enable bigger fonts like
+	  FONT_TER16x32
+
+config DRM_PANIC_FOREGROUND_COLOR
+	hex "Drm panic screen foreground color, in RGB"
+	depends on DRM_PANIC
+	default 0xffffff
+
+config DRM_PANIC_BACKGROUND_COLOR
+	hex "Drm panic screen background color, in RGB"
+	depends on DRM_PANIC
+	default 0x000000
+
 config DRM_DEBUG_DP_MST_TOPOLOGY_REFS
         bool "Enable refcount backtrace history in the DP MST helpers"
 	depends on STACKTRACE_SUPPORT

drivers/gpu/drm/Makefile

···
 	drm_privacy_screen.o \
 	drm_privacy_screen_x86.o
 drm-$(CONFIG_DRM_ACCEL) += ../../accel/drm_accel.o
+drm-$(CONFIG_DRM_PANIC) += drm_panic.o
 obj-$(CONFIG_DRM)	+= drm.o
 
 obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o

drivers/gpu/drm/drm_drv.c

···
 #include <drm/drm_file.h>
 #include <drm/drm_managed.h>
 #include <drm/drm_mode_object.h>
+#include <drm/drm_panic.h>
 #include <drm/drm_print.h>
 #include <drm/drm_privacy_screen_machine.h>
 
···
 		if (ret)
 			goto err_unload;
 	}
+	drm_panic_register(dev);
 
 	DRM_INFO("Initialized %s %d.%d.%d %s for %s on minor %d\n",
 		 driver->name, driver->major, driver->minor,
···
 void drm_dev_unregister(struct drm_device *dev)
 {
 	dev->registered = false;
+
+	drm_panic_unregister(dev);
 
 	drm_client_dev_unregister(dev);
 

drivers/gpu/drm/drm_panic.c

···
+// SPDX-License-Identifier: GPL-2.0 or MIT
+/*
+ * Copyright (c) 2023 Red Hat.
+ * Author: Jocelyn Falempe <[e-mail]>
+ * inspired by the drm_log driver from David Herrmann <[e-mail]>
+ * Tux Ascii art taken from cowsay written by Tony Monroe
+ */
+
+#include <linux/font.h>
+#include <linux/iosys-map.h>
+#include <linux/kdebug.h>
+#include <linux/kmsg_dump.h>
+#include <linux/list.h>
+#include <linux/module.h>
+#include <linux/types.h>
+
+#include <drm/drm_drv.h>
+#include <drm/drm_format_helper.h>
+#include <drm/drm_fourcc.h>
+#include <drm/drm_framebuffer.h>
+#include <drm/drm_modeset_helper_vtables.h>
+#include <drm/drm_panic.h>
+#include <drm/drm_plane.h>
+#include <drm/drm_print.h>
+
+MODULE_AUTHOR("Jocelyn Falempe");
+MODULE_DESCRIPTION("DRM panic handler");
+MODULE_LICENSE("GPL");
+
+/**
+ * DOC: overview
+ *
+ * To enable DRM panic for a driver, the primary plane must implement a
+ * &drm_plane_helper_funcs.get_scanout_buffer helper function. It is then
+ * automatically registered to the drm panic handler.
+ * When a panic occurs, the &drm_plane_helper_funcs.get_scanout_buffer will be
+ * called, and the driver can provide a framebuffer so the panic handler can
+ * draw the panic screen on it. Currently only linear buffer and a few color
+ * formats are supported.
+ * Optionally the driver can also provide a &drm_plane_helper_funcs.panic_flush
+ * callback, that will be called after that, to send additional commands to the
+ * hardware to make the scanout buffer visible.
+ */
+
+/*
+ * This module displays a user friendly message on screen when a kernel panic
+ * occurs. This is conflicting with fbcon, so you can only enable it when fbcon
+ * is disabled.
+ * It's intended for end-user, so have minimal technical/debug information.
+ *
+ * Implementation details:
+ *
+ * It is a panic handler, so it can't take lock, allocate memory, run tasks/irq,
+ * or attempt to sleep. It's a best effort, and it may not be able to display
+ * the message in all situations (like if the panic occurs in the middle of a
+ * modesetting).
+ * It will display only one static frame, so performance optimizations are low
+ * priority as the machine is already in an unusable state.
+ */
+
+struct drm_panic_line {
+	u32 len;
+	const char *txt;
+};
+
+#define PANIC_LINE(s) {.len = sizeof(s) - 1, .txt = s}
+
+static struct drm_panic_line panic_msg[] = {
+	PANIC_LINE("KERNEL PANIC !"),
+	PANIC_LINE(""),
+	PANIC_LINE("Please reboot your computer."),
+};
+
+static const struct drm_panic_line logo[] = {
+	PANIC_LINE("     .--.        _"),
+	PANIC_LINE("    |o_o |      | |"),
+	PANIC_LINE("    |:_/ |      | |"),
+	PANIC_LINE("   //   \\ \\     |_|"),
+	PANIC_LINE("  (|     | )     _"),
+	PANIC_LINE(" /'\\_   _/`\\    (_)"),
+	PANIC_LINE(" \\___)=(___/"),
+};
+
+static void drm_panic_fill32(struct iosys_map *map, unsigned int pitch,
+			     unsigned int height, unsigned int width,
+			     u32 color)
+{
+	unsigned int y, x;
+
+	for (y = 0; y < height; y++)
+		for (x = 0; x < width; x++)
+			iosys_map_wr(map, y * pitch + x * sizeof(u32), u32, color);
+}
+
+static void drm_panic_blit32(struct iosys_map *dmap, unsigned int dpitch,
+			     const u8 *sbuf8, unsigned int spitch,
+			     unsigned int height, unsigned int width,
+			     u32 fg32, u32 bg32)
+{
+	unsigned int y, x;
+	u32 val32;
+
+	for (y = 0; y < height; y++) {
+		for (x = 0; x < width; x++) {
+			val32 = (sbuf8[(y * spitch) + x / 8] & (0x80 >> (x % 8))) ? fg32 : bg32;
+			iosys_map_wr(dmap, y * dpitch + x * sizeof(u32), u32, val32);
+		}
+	}
+}
+
+static const u8 *get_char_bitmap(const struct font_desc *font, char c, size_t font_pitch)
+{
+	return font->data + (c * font->height) * font_pitch;
+}
+
+static unsigned int get_max_line_len(const struct drm_panic_line *lines, int len)
+{
+	int i;
+	unsigned int max = 0;
+
+	for (i = 0; i < len; i++)
+		max = max(lines[i].len, max);
+	return max;
+}
+
+/*
+ * Draw a text in a rectangle on a framebuffer. The text is truncated if it overflows the rectangle
+ */
+static void draw_txt_rectangle(struct drm_scanout_buffer *sb,
+			       const struct font_desc *font,
+			       const struct drm_panic_line *msg,
+			       unsigned int msg_lines,
+			       bool centered,
+			       struct drm_rect *clip,
+			       u32 fg_color,
+			       u32 bg_color)
+{
+	int i, j;
+	const u8 *src;
+	size_t font_pitch = DIV_ROUND_UP(font->width, 8);
+	struct iosys_map dst;
+	unsigned int px_width = sb->format->cpp[0];
+	int left = 0;
+
+	msg_lines = min(msg_lines,  drm_rect_height(clip) / font->height);
+	for (i = 0; i < msg_lines; i++) {
+		size_t line_len = min(msg[i].len, drm_rect_width(clip) / font->width);
+
+		if (centered)
+			left = (drm_rect_width(clip) - (line_len * font->width)) / 2;
+
+		dst = sb->map[0];
+		iosys_map_incr(&dst, (clip->y1 + i * font->height) * sb->pitch[0] +
+				     (clip->x1 + left) * px_width);
+		for (j = 0; j < line_len; j++) {
+			src = get_char_bitmap(font, msg[i].txt[j], font_pitch);
+			drm_panic_blit32(&dst, sb->pitch[0], src, font_pitch,
+					 font->height, font->width,
+					 fg_color, bg_color);
+			iosys_map_incr(&dst, font->width * px_width);
+		}
+	}
+}
+
+/*
+ * Draw the panic message at the center of the screen
+ */
+static void draw_panic_static(struct drm_scanout_buffer *sb)
+{
+	size_t msg_lines = ARRAY_SIZE(panic_msg);
+	size_t logo_lines = ARRAY_SIZE(logo);
+	u32 fg_color = CONFIG_DRM_PANIC_FOREGROUND_COLOR;
+	u32 bg_color = CONFIG_DRM_PANIC_BACKGROUND_COLOR;
+	const struct font_desc *font = get_default_font(sb->width, sb->height, NULL, NULL);
+	struct drm_rect r_logo, r_msg;
+
+	if (!font)
+		return;
+
+	r_logo = DRM_RECT_INIT(0, 0,
+			       get_max_line_len(logo, logo_lines) * font->width,
+			       logo_lines * font->height);
+	r_msg = DRM_RECT_INIT(0, 0,
+			      min(get_max_line_len(panic_msg, msg_lines) * font->width, sb->width),
+			      min(msg_lines * font->height, sb->height));
+
+	/* Center the panic message */
+	drm_rect_translate(&r_msg, (sb->width - r_msg.x2) / 2, (sb->height - r_msg.y2) / 2);
+
+	/* Fill with the background color, and draw text on top */
+	drm_panic_fill32(&sb->map[0], sb->pitch[0], sb->height, sb->width, bg_color);
+
+	if ((r_msg.x1 >= drm_rect_width(&r_logo) || r_msg.y1 >= drm_rect_height(&r_logo)) &&
+	    drm_rect_width(&r_logo) < sb->width && drm_rect_height(&r_logo) < sb->height) {
+		draw_txt_rectangle(sb, font, logo, logo_lines, false, &r_logo, fg_color, bg_color);
+	}
+	draw_txt_rectangle(sb, font, panic_msg, msg_lines, true, &r_msg, fg_color, bg_color);
+}
+
+/*
+ * drm_panic_is_format_supported()
+ * @format: a fourcc color code
+ * Returns: true if supported, false otherwise.
+ *
+ * Check if drm_panic will be able to use this color format.
+ */
+static bool drm_panic_is_format_supported(const struct drm_format_info *format)
+{
+	if (format->num_planes != 1)
+		return false;
+	return format->format == DRM_FORMAT_XRGB8888;
+}
+
+static void draw_panic_plane(struct drm_plane *plane)
+{
+	struct drm_scanout_buffer sb;
+	int ret;
+	unsigned long flags;
+
+	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)) {
+		draw_panic_static(&sb);
+		if (plane->helper_private->panic_flush)
+			plane->helper_private->panic_flush(plane);
+	}
+	drm_panic_unlock(plane->dev, flags);
+}
+
+static struct drm_plane *to_drm_plane(struct kmsg_dumper *kd)
+{
+	return container_of(kd, struct drm_plane, kmsg_panic);
+}
+
+static void drm_panic(struct kmsg_dumper *dumper, enum kmsg_dump_reason reason)
+{
+	struct drm_plane *plane = to_drm_plane(dumper);
+
+	if (reason == KMSG_DUMP_PANIC)
+		draw_panic_plane(plane);
+}
+
+/**
+ * drm_panic_register() - Initialize DRM panic for a device
+ * @dev: the drm device on which the panic screen will be displayed.
+ */
+void drm_panic_register(struct drm_device *dev)
+{
+	struct drm_plane *plane;
+	int registered_plane = 0;
+
+	if (!dev->mode_config.num_total_plane)
+		return;
+
+	drm_for_each_plane(plane, dev) {
+		if (!plane->helper_private || !plane->helper_private->get_scanout_buffer)
+			continue;
+		plane->kmsg_panic.dump = drm_panic;
+		plane->kmsg_panic.max_reason = KMSG_DUMP_PANIC;
+		if (kmsg_dump_register(&plane->kmsg_panic))
+			drm_warn(dev, "Failed to register panic handler\n");
+		else
+			registered_plane++;
+	}
+	if (registered_plane)
+		drm_info(dev, "Registered %d planes with drm panic\n", registered_plane);
+}
+EXPORT_SYMBOL(drm_panic_register);
+
+/**
+ * drm_panic_unregister()
+ * @dev: the drm device previously registered.
+ */
+void drm_panic_unregister(struct drm_device *dev)
+{
+	struct drm_plane *plane;
+
+	if (!dev->mode_config.num_total_plane)
+		return;
+
+	drm_for_each_plane(plane, dev) {
+		if (!plane->helper_private || !plane->helper_private->get_scanout_buffer)
+			continue;
+		kmsg_dump_unregister(&plane->kmsg_panic);
+	}
+}
+EXPORT_SYMBOL(drm_panic_unregister);

include/drm/drm_modeset_helper_vtables.h

···
  * To make this clear all the helper vtables are pulled together in this location here.
  */
 
+struct drm_scanout_buffer;
 struct drm_writeback_connector;
 struct drm_writeback_job;
 
···
 	 */
 	void (*atomic_async_update)(struct drm_plane *plane,
 				    struct drm_atomic_state *state);
+
+	/**
+	 * @get_scanout_buffer:
+	 *
+	 * Get the current scanout buffer, to display a message with drm_panic.
+	 * The driver should do the minimum changes to provide a buffer,
+	 * that can be used to display the panic screen. Currently only linear
+	 * buffers are supported. Non-linear buffer support is on the TODO list.
+	 * The device &dev.mode_config.panic_lock is taken before calling this
+	 * function, so you can safely access the &plane.state
+	 * It is called from a panic callback, and must follow its restrictions.
+	 * Please look the documentation at drm_panic_trylock() for an in-depth
+	 * discussions of what's safe and what is not allowed.
+	 * It's a best effort mode, so it's expected that in some complex cases
+	 * the panic screen won't be displayed.
+	 * The returned &drm_scanout_buffer.map must be valid if no error code is
+	 * returned.
+	 *
+	 * Return:
+	 * %0 on success, negative errno on failure.
+	 */
+	int (*get_scanout_buffer)(struct drm_plane *plane,
+				  struct drm_scanout_buffer *sb);
+
+	/**
+	 * @panic_flush:
+	 *
+	 * It is used by drm_panic, and is called after the panic screen is
+	 * drawn to the scanout buffer. In this function, the driver
+	 * can send additional commands to the hardware, to make the scanout
+	 * buffer visible.
+	 * It is only called if get_scanout_buffer() returned successfully, and
+	 * the &dev.mode_config.panic_lock is held during the entire sequence.
+	 * It is called from a panic callback, and must follow its restrictions.
+	 * Please look the documentation at drm_panic_trylock() for an in-depth
+	 * discussions of what's safe and what is not allowed.
+	 */
+	void (*panic_flush)(struct drm_plane *plane);
 };
 
 /**

include/drm/drm_panic.h

···
 #ifndef __DRM_PANIC_H__
 #define __DRM_PANIC_H__
 
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/iosys-map.h>
+
 #include <drm/drm_device.h>
+#include <drm/drm_fourcc.h>
 /*
  * Copyright (c) 2024 Intel
  */
+
+/**
+ * struct drm_scanout_buffer - DRM scanout buffer
+ *
+ * This structure holds the information necessary for drm_panic to draw the
+ * panic screen, and display it.
+ */
+struct drm_scanout_buffer {
+	/**
+	 * @format:
+	 *
+	 * drm format of the scanout buffer.
+	 */
+	const struct drm_format_info *format;
+
+	/**
+	 * @map:
+	 *
+	 * Virtual address of the scanout buffer, either in memory or iomem.
+	 * The scanout buffer should be in linear format, and can be directly
+	 * sent to the display hardware. Tearing is not an issue for the panic
+	 * screen.
+	 */
+	struct iosys_map map[DRM_FORMAT_MAX_PLANES];
+
+	/**
+	 * @width: Width of the scanout buffer, in pixels.
+	 */
+	unsigned int width;
+
+	/**
+	 * @height: Height of the scanout buffer, in pixels.
+	 */
+	unsigned int height;
+
+	/**
+	 * @pitch: Length in bytes between the start of two consecutive lines.
+	 */
+	unsigned int pitch[DRM_FORMAT_MAX_PLANES];
+};
 
 /**
  * drm_panic_trylock - try to enter the panic printing critical section
···
  */
 #define drm_panic_unlock(dev, flags) \
 	raw_spin_unlock_irqrestore(&(dev)->mode_config.panic_lock, flags)
+
+#ifdef CONFIG_DRM_PANIC
+
+void drm_panic_register(struct drm_device *dev);
+void drm_panic_unregister(struct drm_device *dev);
+
+#else
+
+static inline void drm_panic_register(struct drm_device *dev) {}
+static inline void drm_panic_unregister(struct drm_device *dev) {}
+
+#endif
 
 #endif /* __DRM_PANIC_H__ */

include/drm/drm_plane.h

···
 
 #include <linux/list.h>
 #include <linux/ctype.h>
+#include <linux/kmsg_dump.h>
 #include <drm/drm_mode_object.h>
 #include <drm/drm_color_mgmt.h>
 #include <drm/drm_rect.h>
···
 	 * @hotspot_y_property: property to set mouse hotspot y offset.
 	 */
 	struct drm_property *hotspot_y_property;
+
+	/**
+	 * @kmsg_panic: Used to register a panic notifier for this plane
+	 */
+	struct kmsg_dumper kmsg_panic;
 };
 
 #define obj_to_plane(x) container_of(x, struct drm_plane, base)