drm/log: Introduce a new boot logger to draw the kmsg on the screen

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.active=log or drm_client_lib.active=fbdev

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de> # console API
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241204160014.1171469-3-jfalempe@redhat.com

1 files changed, 25 insertions, 4 deletions

diff --git a/drivers/gpu/drm/clients/drm_client_setup.c b/drivers/gpu/drm/clients/drm_client_setup.c
index 4b211a4812b5..e17265039ca8 100644
--- a/drivers/gpu/drm/clients/drm_client_setup.c
+++ b/drivers/gpu/drm/clients/drm_client_setup.c
@@ -7,6 +7,12 @@
 
 #include "drm_client_internal.h"
 
+static char drm_client_default[16] = CONFIG_DRM_CLIENT_DEFAULT;
+module_param_string(active, drm_client_default, sizeof(drm_client_default), 0444);
+MODULE_PARM_DESC(active,
+		 "Choose which drm client to start, default is"
+		 CONFIG_DRM_CLIENT_DEFAULT "]");
+
 /**
  * drm_client_setup() - Setup in-kernel DRM clients
  * @dev: DRM device
@@ -25,11 +31,26 @@
  */
 void drm_client_setup(struct drm_device *dev, const struct drm_format_info *format)
 {
-	int ret;
 
-	ret = drm_fbdev_client_setup(dev, format);
-	if (ret)
-		drm_warn(dev, "Failed to set up DRM client; error %d\n", ret);
+#ifdef CONFIG_DRM_FBDEV_EMULATION
+	if (!strcmp(drm_client_default, "fbdev")) {
+		int ret;
+
+		ret = drm_fbdev_client_setup(dev, format);
+		if (ret)
+			drm_warn(dev, "Failed to set up DRM client; error %d\n", ret);
+		return;
+	}
+#endif
+
+#ifdef CONFIG_DRM_CLIENT_LOG
+	if (!strcmp(drm_client_default, "log")) {
+		drm_log_register(dev);
+		return;
+	}
+#endif
+	if (strcmp(drm_client_default, ""))
+		drm_warn(dev, "Unknown DRM client %s\n", drm_client_default);
 }
 EXPORT_SYMBOL(drm_client_setup);