drm: Add client-agnostic setup helper

DRM may support multiple in-kernel clients that run as soon as a DRM
driver has been registered. To select the client(s) in a single place,
introduce drm_client_setup().

Drivers that call the new helper automatically instantiate the kernel's
configured default clients. Only fbdev emulation is currently supported.
Later versions can add support for DRM-based logging, a boot logo or even
a console.

Some drivers handle the color mode for clients internally. Provide the
helper drm_client_setup_with_color_mode() for them.

Using the new interface requires the driver to select
DRM_CLIENT_SELECTION in its Kconfig. For now this only enables the
client-setup helpers if the fbdev client has been configured by the
user. A future patchset will further modularize client support and
rework DRM_CLIENT_SELECTION to select the correct dependencies for
all its clients.

v5:
- add CONFIG_DRM_CLIENT_SELECTION und DRM_CLIENT_SETUP
v4:
- fix docs for drm_client_setup_with_fourcc() (Geert)
v3:
- fix build error
v2:
- add drm_client_setup_with_fourcc() (Laurent)
- push default-format handling into actual clients

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-5-tzimmermann@suse.de

drivers/gpu/drm/Kconfig

@@ -210,6 +210,18 @@ config DRM_DEBUG_MODESET_LOCK
 
 	  If in doubt, say "N".
 
+config DRM_CLIENT_SELECTION
+	bool
+	depends on DRM
+	select DRM_CLIENT_SETUP if DRM_FBDEV_EMULATION
+	help
+	  Drivers that support in-kernel DRM clients have to select this
+	  option.
+
+config DRM_CLIENT_SETUP
+	bool
+	depends on DRM_CLIENT_SELECTION
+
 config DRM_FBDEV_EMULATION
 	bool "Enable legacy fbdev support for your modesetting driver"
 	depends on DRM

drivers/gpu/drm/Makefile

@@ -143,6 +143,8 @@ drm_kms_helper-y := \
 	drm_probe_helper.o \
 	drm_self_refresh_helper.o \
 	drm_simple_kms_helper.o
+drm_kms_helper-$(CONFIG_DRM_CLIENT_SETUP) += \
+	drm_client_setup.o
 drm_kms_helper-$(CONFIG_DRM_PANEL_BRIDGE) += bridge/panel.o
 drm_kms_helper-$(CONFIG_DRM_FBDEV_EMULATION) += \
 	drm_fbdev_client.o \

drivers/gpu/drm/drm_client_setup.c

@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: MIT
+
+#include <drm/drm_client_setup.h>
+#include <drm/drm_device.h>
+#include <drm/drm_fbdev_client.h>
+#include <drm/drm_fourcc.h>
+#include <drm/drm_print.h>
+
+/**
+ * drm_client_setup() - Setup in-kernel DRM clients
+ * @dev: DRM device
+ * @format: Preferred pixel format for the device. Use NULL, unless
+ *          there is clearly a driver-preferred format.
+ *
+ * This function sets up the in-kernel DRM clients. Restore, hotplug
+ * events and teardown are all taken care of.
+ *
+ * Drivers should call drm_client_setup() after registering the new
+ * DRM device with drm_dev_register(). This function is safe to call
+ * even when there are no connectors present. Setup will be retried
+ * on the next hotplug event.
+ *
+ * The clients are destroyed by drm_dev_unregister().
+ */
+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);
+}
+EXPORT_SYMBOL(drm_client_setup);
+
+/**
+ * drm_client_setup_with_fourcc() - Setup in-kernel DRM clients for color mode
+ * @dev: DRM device
+ * @fourcc: Preferred pixel format as 4CC code for the device
+ *
+ * This function sets up the in-kernel DRM clients. It is equivalent
+ * to drm_client_setup(), but expects a 4CC code as second argument.
+ */
+void drm_client_setup_with_fourcc(struct drm_device *dev, u32 fourcc)
+{
+	drm_client_setup(dev, drm_format_info(fourcc));
+}
+EXPORT_SYMBOL(drm_client_setup_with_fourcc);
+
+/**
+ * drm_client_setup_with_color_mode() - Setup in-kernel DRM clients for color mode
+ * @dev: DRM device
+ * @color_mode: Preferred color mode for the device
+ *
+ * This function sets up the in-kernel DRM clients. It is equivalent
+ * to drm_client_setup(), but expects a color mode as second argument.
+ *
+ * Do not use this function in new drivers. Prefer drm_client_setup() with a
+ * format of NULL.
+ */
+void drm_client_setup_with_color_mode(struct drm_device *dev, unsigned int color_mode)
+{
+	u32 fourcc = drm_driver_color_mode_format(dev, color_mode);
+
+	drm_client_setup_with_fourcc(dev, fourcc);
+}
+EXPORT_SYMBOL(drm_client_setup_with_color_mode);

include/drm/drm_client_setup.h

@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: MIT */
+
+#ifndef DRM_CLIENT_SETUP_H
+#define DRM_CLIENT_SETUP_H
+
+#include <linux/types.h>
+
+struct drm_device;
+struct drm_format_info;
+
+#if defined(CONFIG_DRM_CLIENT_SETUP)
+void drm_client_setup(struct drm_device *dev, const struct drm_format_info *format);
+void drm_client_setup_with_fourcc(struct drm_device *dev, u32 fourcc);
+void drm_client_setup_with_color_mode(struct drm_device *dev, unsigned int color_mode);
+#else
+static inline void drm_client_setup(struct drm_device *dev,
+				    const struct drm_format_info *format)
+{ }
+static inline void drm_client_setup_with_fourcc(struct drm_device *dev, u32 fourcc)
+{ }
+static inline void drm_client_setup_with_color_mode(struct drm_device *dev,
+						    unsigned int color_mode)
+{ }
+#endif
+
+#endif