drm_log is a simple logger that uses the drm_client API to print the
kmsg boot log on the screen. This is not a full replacement to fbcon,
as it will only print the kmsg. It will never handle user input, or a
terminal because this is better done in userspace.

Design decisions:
 * It uses the drm_client API, so it should work on all drm drivers
   from the start.
 * It doesn't scroll the message, that way it doesn't need to redraw
   the whole screen for each new message.
   It also means it doesn't have to keep drawn messages in memory, to
   redraw them when scrolling.
 * It uses the new non-blocking console API, so it should work well
   with PREEMPT_RT.

This patch also adds a Kconfig menu to select the drm client to use.
It can be overwritten on the kernel command line with:
drm_client_lib.default=log or drm_client_lib.default=fbdev

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
---

v2:
 * Use vmap_local() api, with that change, I've tested it successfully on simpledrm, virtio-gpu, amdgpu, and nouveau.
 * Stop drawing when the drm_master is taken. This avoid wasting CPU cycle if the buffer is not visible.
 * Use deferred probe. Only do the probe the first time there is a log to draw. With this, if you boot with quiet, drm_log won't do any modeset.
 * Add color support for the timestamp prefix, like what dmesg does.
 * Add build dependency on  disabling the fbdev emulation, as they are both drm_client, and there is no way to choose which one gets the focus.

v3:
 * Remove the work thread and circular buffer, and use the new write_thread() console API.
 * Register a console for each drm driver.

v4:
 * Can be built as a module, even if that's not really useful.
 * Rebased on top of "drm: Introduce DRM client library" series from Thomas Zimmermann.
 * Add a Kconfig menu to choose between drm client.

 drivers/gpu/drm/Kconfig            |  46 ++++
 drivers/gpu/drm/Makefile           |   2 +
 drivers/gpu/drm/drm_client_setup.c |  18 +-
 drivers/gpu/drm/drm_log.c          | 370 +++++++++++++++++++++++++++++
 drivers/gpu/drm/drm_log.h          |  11 +
 5 files changed, 444 insertions(+), 3 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_log.c
 create mode 100644 drivers/gpu/drm/drm_log.h

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index af8c0f4442f9..3c624276c2e2 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -291,6 +291,52 @@ config DRM_FBDEV_LEAK_PHYS_SMEM
 	  If in doubt, say "N" or spread the word to your closed source
 	  library vendor.
 
+config DRM_LOG
+	tristate "Print the kernel boot message on the screen"
+	depends on DRM_CLIENT_SELECTION
+	select DRM_DRAW
+	select DRM_CLIENT_SETUP
+	help
+	  This enable a drm logger, that will print the kernel messages to the
+	  screen until the userspace is ready to take over.
+
+	  If you only need logs, but no terminal, or if you prefer userspace
+	  terminal, say "Y".
+
+choice
+	prompt "Default DRM Client"
+	depends on DRM_CLIENT_SELECTION
+	default DRM_CLIENT_DEFAULT_FBDEV
+	help
+	  Selects the default drm client.
+
+	  The selection made here can be overridden by using the kernel
+	  command line 'drm_client_lib.default=fbdev' option.
+
+config DRM_CLIENT_DEFAULT_FBDEV
+	bool "fbdev"
+	depends on DRM_FBDEV_EMULATION
+	help
+	  Use fbdev emulation as default drm client. This is needed to have
+	  fbcon on top of a drm driver.
+
+config DRM_CLIENT_DEFAULT_LOG
+	bool "log"
+	depends on DRM_LOG
+	help
+	  Use drm log as default drm client. This will display boot logs on the
+	  screen, but doesn't implement a full terminal. For that you will need
+	  a userspace terminal using drm/kms.
+
+endchoice
+
+config DRM_CLIENT_DEFAULT
+       string
+       depends on DRM_CLIENT
+       default "fbdev" if DRM_CLIENT_DEFAULT_FBDEV
+       default "log" if DRM_CLIENT_DEFAULT_LOG
+       default ""
+
 endmenu
 
 config DRM_LOAD_EDID_FIRMWARE
diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index 3d1a704e0eec..35ba3ec6b55b 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -160,6 +160,8 @@ drm_client_lib-$(CONFIG_DRM_FBDEV_EMULATION) += \
 	drm_fb_helper.o
 obj-$(CONFIG_DRM_CLIENT_LIB) += drm_client_lib.o
 
+obj-$(CONFIG_DRM_LOG) += drm_log.o
+
 #
 # Drivers and the rest
 #
diff --git a/drivers/gpu/drm/drm_client_setup.c b/drivers/gpu/drm/drm_client_setup.c
index c14221ca5a0d..fa44dbee3def 100644
--- a/drivers/gpu/drm/drm_client_setup.c
+++ b/drivers/gpu/drm/drm_client_setup.c
@@ -6,6 +6,14 @@
 #include <drm/drm_fourcc.h>
 #include <drm/drm_print.h>
 
+#include "drm_log.h"
+
+static char drm_client_default[16] = CONFIG_DRM_CLIENT_DEFAULT;
+module_param_string(client, drm_client_default, sizeof(drm_client_default), 0444);
+MODULE_PARM_DESC(client,
+		 "Choose which drm client to start, default is"
+		 CONFIG_DRM_CLIENT_DEFAULT "]");
+
 /**
  * drm_client_setup() - Setup in-kernel DRM clients
  * @dev: DRM device
@@ -26,9 +34,13 @@ void drm_client_setup(struct drm_device *dev, const struct drm_format_info *form
 {
 	int ret;
 
-	ret = drm_fbdev_client_setup(dev, format);
-	if (ret)
-		drm_warn(dev, "Failed to set up DRM client; error %d\n", ret);
+	if (!strcmp(drm_client_default, "log")) {
+		drm_log_register(dev);
+	} else if (!strcmp(drm_client_default, "fbdev")) {
+		ret = drm_fbdev_client_setup(dev, format);
+		if (ret)
+			drm_warn(dev, "Failed to set up DRM client; error %d\n", ret);
+	}
 }
 EXPORT_SYMBOL(drm_client_setup);
 
diff --git a/drivers/gpu/drm/drm_log.c b/drivers/gpu/drm/drm_log.c
new file mode 100644
index 000000000000..376ee173d99d
--- /dev/null
+++ b/drivers/gpu/drm/drm_log.c
@@ -0,0 +1,370 @@
+// SPDX-License-Identifier: GPL-2.0 or MIT
+/*
+ * Copyright (c) 2024 Red Hat.
+ * Author: Jocelyn Falempe <jfalempe@redhat.com>
+ */
+
+#include <linux/console.h>
+#include <linux/font.h>
+#include <linux/init.h>
+#include <linux/iosys-map.h>
+#include <linux/module.h>
+#include <linux/types.h>
+
+#include <drm/drm_client.h>
+#include <drm/drm_drv.h>
+#include <drm/drm_fourcc.h>
+#include <drm/drm_framebuffer.h>
+#include <drm/drm_print.h>
+
+#include "drm_draw.h"
+#include "drm_log.h"
+
+MODULE_AUTHOR("Jocelyn Falempe");
+MODULE_DESCRIPTION("DRM boot logger");
+MODULE_LICENSE("GPL");
+
+/**
+ * DOC: overview
+ *
+ * This is a simple graphic logger, to print the kernel message on screen, until
+ * a userspace application is able to take over.
+ * It is only for debugging purpose.
+ */
+
+struct drm_log_scanout {
+	struct drm_client_buffer *buffer;
+	const struct font_desc *font;
+	u32 rows;
+	u32 columns;
+	u32 line;
+	u32 format;
+	u32 px_width;
+	u32 front_color;
+};
+
+struct drm_log {
+	struct mutex lock;
+	struct drm_client_dev client;
+	struct console con;
+	bool probed;
+	u32 n_scanout;
+	struct drm_log_scanout *scanout;
+};
+
+static struct drm_log *client_to_drm_log(struct drm_client_dev *client)
+{
+	return container_of(client, struct drm_log, client);
+}
+
+static struct drm_log *console_to_drm_log(struct console *con)
+{
+	return container_of(con, struct drm_log, con);
+}
+
+static void drm_log_blit(struct iosys_map *dst, unsigned int dst_pitch,
+			 const u8 *src, unsigned int src_pitch,
+			 u32 height, u32 width, u32 scale, u32 px_width, u32 color)
+{
+	switch (px_width) {
+	case 2:
+		drm_draw_blit16(dst, dst_pitch, src, src_pitch, height, width, scale, color);
+		break;
+	case 3:
+		drm_draw_blit24(dst, dst_pitch, src, src_pitch, height, width, scale, color);
+		break;
+	case 4:
+		drm_draw_blit32(dst, dst_pitch, src, src_pitch, height, width, scale, color);
+		break;
+	default:
+		WARN_ONCE(1, "Can't blit with pixel width %d\n", px_width);
+	}
+}
+
+static void drm_log_clear_line(struct drm_log_scanout *scanout, u32 line)
+{
+	struct drm_framebuffer *fb = scanout->buffer->fb;
+	unsigned long height = scanout->font->height;
+	struct iosys_map map;
+	struct drm_rect r = DRM_RECT_INIT(0, line * height, fb->width, height);
+
+	if (drm_client_buffer_vmap_local(scanout->buffer, &map))
+		return;
+	iosys_map_memset(&map, r.y1 * fb->pitches[0], 0, height * fb->pitches[0]);
+	drm_client_buffer_vunmap_local(scanout->buffer);
+	drm_client_framebuffer_flush(scanout->buffer, &r);
+}
+
+static void drm_log_draw_line(struct drm_log_scanout *scanout, const char *s,
+			      unsigned int len)
+{
+	struct drm_framebuffer *fb = scanout->buffer->fb;
+	struct iosys_map map;
+	const struct font_desc *font = scanout->font;
+	size_t font_pitch = DIV_ROUND_UP(font->width, 8);
+	const u8 *src;
+	u32 px_width = fb->format->cpp[0];
+	struct drm_rect r = DRM_RECT_INIT(0, scanout->line * font->height,
+					  fb->width, (scanout->line + 1) * font->height);
+	u32 i;
+
+	if (drm_client_buffer_vmap_local(scanout->buffer, &map))
+		return;
+
+	iosys_map_incr(&map, r.y1 * fb->pitches[0]);
+	for (i = 0; i < len && i < scanout->columns; i++) {
+		src = drm_draw_get_char_bitmap(font, s[i], font_pitch);
+		drm_log_blit(&map, fb->pitches[0], src, font_pitch, font->height, font->width,
+			     1, px_width, scanout->front_color);
+		iosys_map_incr(&map, font->width * px_width);
+	}
+
+	scanout->line++;
+	if (scanout->line >= scanout->rows)
+		scanout->line = 0;
+	drm_client_buffer_vunmap_local(scanout->buffer);
+	drm_client_framebuffer_flush(scanout->buffer, &r);
+}
+
+static void drm_log_draw_new_line(struct drm_log_scanout *scanout,
+				  const char *s, unsigned int len)
+{
+	if (scanout->line == 0) {
+		drm_log_clear_line(scanout, 0);
+		drm_log_clear_line(scanout, 1);
+		drm_log_clear_line(scanout, 2);
+	} else if (scanout->line + 2 < scanout->rows)
+		drm_log_clear_line(scanout, scanout->line + 2);
+
+	drm_log_draw_line(scanout, s, len);
+}
+
+static void drm_log_draw_kmsg_record(struct drm_log_scanout *scanout,
+				     const char *s, unsigned int len)
+{
+	/* do not print the ending \n character */
+	if (s[len - 1] == '\n')
+		len--;
+
+	while (len > scanout->columns) {
+		drm_log_draw_new_line(scanout, s, scanout->columns);
+		s += scanout->columns;
+		len -= scanout->columns;
+	}
+	if (len)
+		drm_log_draw_new_line(scanout, s, len);
+}
+
+static u32 drm_log_find_usable_format(struct drm_plane *plane)
+{
+	int i;
+
+	for (i = 0; i < plane->format_count; i++)
+		if (drm_draw_color_from_xrgb8888(0xffffff, plane->format_types[i]) != 0)
+			return plane->format_types[i];
+	return DRM_FORMAT_INVALID;
+}
+
+static int drm_log_setup_modeset(struct drm_client_dev *client,
+				 struct drm_mode_set *mode_set,
+				 struct drm_log_scanout *scanout)
+{
+	struct drm_crtc *crtc = mode_set->crtc;
+	u32 width = mode_set->mode->hdisplay;
+	u32 height = mode_set->mode->vdisplay;
+	u32 format;
+
+	scanout->font = get_default_font(width, height, NULL, NULL);
+	if (!scanout->font)
+		return -ENOENT;
+
+	format = drm_log_find_usable_format(crtc->primary);
+	if (format == DRM_FORMAT_INVALID)
+		return -EINVAL;
+
+	scanout->buffer = drm_client_framebuffer_create(client, width, height, format);
+	if (IS_ERR(scanout->buffer)) {
+		drm_warn(client->dev, "drm_log can't create framebuffer %d %d %p4cc\n",
+			 width, height, &format);
+		return -ENOMEM;
+	}
+	mode_set->fb = scanout->buffer->fb;
+	scanout->rows = height / scanout->font->height;
+	scanout->columns = width / scanout->font->width;
+	scanout->front_color = drm_draw_color_from_xrgb8888(0xffffff, format);
+	return 0;
+}
+
+static int drm_log_count_modeset(struct drm_client_dev *client)
+{
+	struct drm_mode_set *mode_set;
+	int count = 0;
+
+	mutex_lock(&client->modeset_mutex);
+	drm_client_for_each_modeset(mode_set, client)
+		count++;
+	mutex_unlock(&client->modeset_mutex);
+	return count;
+}
+
+static void drm_log_init_client(struct drm_log *dlog)
+{
+	struct drm_client_dev *client = &dlog->client;
+	struct drm_mode_set *mode_set;
+	int i, max_modeset;
+	int n_modeset = 0;
+
+	dlog->probed = true;
+
+	if (drm_client_modeset_probe(client, 0, 0))
+		return;
+
+	max_modeset = drm_log_count_modeset(client);
+	if (!max_modeset)
+		return;
+
+	dlog->scanout = kcalloc(max_modeset, sizeof(*dlog->scanout), GFP_KERNEL);
+	if (!dlog->scanout)
+		return;
+
+	mutex_lock(&client->modeset_mutex);
+	drm_client_for_each_modeset(mode_set, client) {
+		if (!mode_set->mode)
+			continue;
+		if (drm_log_setup_modeset(client, mode_set, &dlog->scanout[n_modeset]))
+			continue;
+		n_modeset++;
+	}
+	mutex_unlock(&client->modeset_mutex);
+	if (n_modeset == 0)
+		goto err_nomodeset;
+
+	if (drm_client_modeset_commit(client))
+		goto err_failed_commit;
+
+	dlog->n_scanout = n_modeset;
+	return;
+
+err_failed_commit:
+	for (i = 0; i < n_modeset; i++)
+		drm_client_framebuffer_delete(dlog->scanout[i].buffer);
+
+err_nomodeset:
+	kfree(dlog->scanout);
+	dlog->scanout = NULL;
+}
+
+static void drm_log_free_scanout(struct drm_client_dev *client)
+{
+	struct drm_log *dlog = client_to_drm_log(client);
+	int i;
+
+	if (dlog->n_scanout) {
+		for (i = 0; i < dlog->n_scanout; i++)
+			drm_client_framebuffer_delete(dlog->scanout[i].buffer);
+		dlog->n_scanout = 0;
+		kfree(dlog->scanout);
+		dlog->scanout = NULL;
+	}
+}
+
+static void drm_log_client_unregister(struct drm_client_dev *client)
+{
+	struct drm_log *dlog = client_to_drm_log(client);
+	struct drm_device *dev = client->dev;
+
+	unregister_console(&dlog->con);
+
+	mutex_lock(&dlog->lock);
+	drm_log_free_scanout(client);
+	drm_client_release(client);
+	mutex_unlock(&dlog->lock);
+	kfree(dlog);
+	drm_info(dev, "Unregistered with drm log\n");
+}
+
+static int drm_log_client_hotplug(struct drm_client_dev *client)
+{
+	struct drm_log *dlog = client_to_drm_log(client);
+
+	mutex_lock(&dlog->lock);
+	drm_log_free_scanout(client);
+	dlog->probed = false;
+	mutex_unlock(&dlog->lock);
+	return 0;
+}
+
+static const struct drm_client_funcs drm_log_client_funcs = {
+	.owner		= THIS_MODULE,
+	.unregister	= drm_log_client_unregister,
+	.hotplug	= drm_log_client_hotplug,
+};
+
+static void drm_log_write_thread(struct console *con, struct nbcon_write_context *wctxt)
+{
+	struct drm_log *dlog = console_to_drm_log(con);
+	int i;
+
+	if (!dlog->probed)
+		drm_log_init_client(dlog);
+
+	for (i = 0; i < dlog->n_scanout; i++)
+		drm_log_draw_kmsg_record(&dlog->scanout[i], wctxt->outbuf, wctxt->len);
+}
+
+static void drm_log_lock(struct console *con, unsigned long *flags)
+{
+	struct drm_log *dlog = console_to_drm_log(con);
+
+	mutex_lock(&dlog->lock);
+	migrate_disable();
+}
+
+static void drm_log_unlock(struct console *con, unsigned long flags)
+{
+	struct drm_log *dlog = console_to_drm_log(con);
+
+	migrate_enable();
+	mutex_unlock(&dlog->lock);
+}
+
+static void drm_log_register_console(struct console *con)
+{
+	strscpy(con->name, "drm_log");
+	con->write_thread = drm_log_write_thread;
+	con->device_lock = drm_log_lock;
+	con->device_unlock = drm_log_unlock;
+	con->flags = CON_PRINTBUFFER | CON_NBCON;
+	con->index = -1;
+
+	register_console(con);
+}
+
+/**
+ * drm_log_register() - Register a drm device to drm_log
+ * @dev: the drm device to register.
+ */
+void drm_log_register(struct drm_device *dev)
+{
+	struct drm_log *new;
+
+	new = kzalloc(sizeof(*new), GFP_KERNEL);
+	if (!new)
+		goto err_warn;
+
+	mutex_init(&new->lock);
+	if (drm_client_init(dev, &new->client, "drm_log", &drm_log_client_funcs))
+		goto err_free;
+
+	drm_client_register(&new->client);
+
+	drm_log_register_console(&new->con);
+
+	drm_info(dev, "Registered with drm log as %s\n", new->con.name);
+	return;
+
+err_free:
+	kfree(new);
+err_warn:
+	drm_warn(dev, "Failed to register with drm log\n");
+}
diff --git a/drivers/gpu/drm/drm_log.h b/drivers/gpu/drm/drm_log.h
new file mode 100644
index 000000000000..3a4ea8150947
--- /dev/null
+++ b/drivers/gpu/drm/drm_log.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 or MIT */
+#ifndef __DRM_LOG_H__
+#define __DRM_LOG_H__
+
+#ifdef CONFIG_DRM_LOG
+void drm_log_register(struct drm_device *dev);
+#else
+static inline void drm_log_register(struct drm_device *dev) {}
+#endif
+
+#endif
-- 
2.46.2

Hi Jocelyn,

kernel test robot noticed the following build errors:

[auto build test ERROR on 33c255312660653cf54f8019896b5dca28e3c580]

url:    https://github.com/intel-lab-lkp/linux/commits/Jocelyn-Falempe/drm-client-squash-of-drm-client-pending-series/20241011-225715
base:   33c255312660653cf54f8019896b5dca28e3c580
patch link:    https://lore.kernel.org/r/20241011105526.615812-4-jfalempe%40redhat.com
patch subject: [PATCH v4 3/7] drm/log: Introduce a new boot logger to draw the kmsg on the screen
config: i386-allmodconfig (https://download.01.org/0day-ci/archive/20241016/202410160342.rkkekxWK-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241016/202410160342.rkkekxWK-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202410160342.rkkekxWK-lkp@intel.com/

All errors (new ones prefixed by >>):

>> drivers/gpu/drm/drm_log.c:347:6: error: redefinition of 'drm_log_register'
     347 | void drm_log_register(struct drm_device *dev)
         |      ^~~~~~~~~~~~~~~~
   In file included from drivers/gpu/drm/drm_log.c:21:
   drivers/gpu/drm/drm_log.h:8:20: note: previous definition of 'drm_log_register' with type 'void(struct drm_device *)'
       8 | static inline void drm_log_register(struct drm_device *dev) {}
         |                    ^~~~~~~~~~~~~~~~

Kconfig warnings: (for reference only)
   WARNING: unmet direct dependencies detected for MODVERSIONS
   Depends on [n]: MODULES [=y] && !COMPILE_TEST [=y]
   Selected by [y]:
   - RANDSTRUCT_FULL [=y] && (CC_HAS_RANDSTRUCT [=n] || GCC_PLUGINS [=y]) && MODULES [=y]
   WARNING: unmet direct dependencies detected for GET_FREE_REGION
   Depends on [n]: SPARSEMEM [=n]
   Selected by [m]:
   - RESOURCE_KUNIT_TEST [=m] && RUNTIME_TESTING_MENU [=y] && KUNIT [=m]


vim +/drm_log_register +347 drivers/gpu/drm/drm_log.c

   342	
   343	/**
   344	 * drm_log_register() - Register a drm device to drm_log
   345	 * @dev: the drm device to register.
   346	 */
 > 347	void drm_log_register(struct drm_device *dev)

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki