drm/fb_dma: Add generic get_scanout_buffer() for drm_panic

This was initialy done for imx6, but should work on most drivers
using drm_fb_dma_helper.

v8:
 * Replace get_scanout_buffer() logic with drm_panic_set_buffer()
   (Thomas Zimmermann)

v9:
 * go back to get_scanout_buffer()
 * move get_scanout_buffer() to plane helper functions

v12:
 * Rename drm_panic_gem_get_scanout_buffer to drm_fb_dma_get_scanout_buffer
   (Thomas Zimmermann)
 * Remove the #ifdef CONFIG_DRM_PANIC, and build it unconditionnaly, as
   it's a small function. (Thomas Zimmermann)
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240409163432.352518-6-jfalempe@redhat.comAcked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_fb_dma_helper.c

@@ -15,6 +15,7 @@
 #include <drm/drm_framebuffer.h>
 #include <drm/drm_gem_dma_helper.h>
 #include <drm/drm_gem_framebuffer_helper.h>
+#include <drm/drm_panic.h>
 #include <drm/drm_plane.h>
 #include <linux/dma-mapping.h>
 #include <linux/module.h>
@@ -148,3 +149,44 @@ void drm_fb_dma_sync_non_coherent(struct drm_device *drm,
 	}
 }
 EXPORT_SYMBOL_GPL(drm_fb_dma_sync_non_coherent);
+
+/**
+ * drm_fb_dma_get_scanout_buffer - Provide a scanout buffer in case of panic
+ * @plane: DRM primary plane
+ * @drm_scanout_buffer: scanout buffer for the panic handler
+ * Returns: 0 or negative error code
+ *
+ * Generic get_scanout_buffer() implementation, for drivers that uses the
+ * drm_fb_dma_helper. It won't call vmap in the panic context, so the driver
+ * should make sure the primary plane is vmapped, otherwise the panic screen
+ * won't get displayed.
+ */
+int drm_fb_dma_get_scanout_buffer(struct drm_plane *plane,
+				  struct drm_scanout_buffer *sb)
+{
+	struct drm_gem_dma_object *dma_obj;
+	struct drm_framebuffer *fb;
+
+	fb = plane->state->fb;
+	/* Only support linear modifier */
+	if (fb->modifier != DRM_FORMAT_MOD_LINEAR)
+		return -ENODEV;
+
+	dma_obj = drm_fb_dma_get_gem_obj(fb, 0);
+
+	/* Buffer should be accessible from the CPU */
+	if (dma_obj->base.import_attach)
+		return -ENODEV;
+
+	/* Buffer should be already mapped to CPU */
+	if (!dma_obj->vaddr)
+		return -ENODEV;
+
+	iosys_map_set_vaddr(&sb->map[0], dma_obj->vaddr);
+	sb->format = fb->format;
+	sb->height = fb->height;
+	sb->width = fb->width;
+	sb->pitch[0] = fb->pitches[0];
+	return 0;
+}
+EXPORT_SYMBOL(drm_fb_dma_get_scanout_buffer);

include/drm/drm_fb_dma_helper.h

@@ -7,6 +7,7 @@
 struct drm_device;
 struct drm_framebuffer;
 struct drm_plane_state;
+struct drm_scanout_buffer;
 
 struct drm_gem_dma_object *drm_fb_dma_get_gem_obj(struct drm_framebuffer *fb,
 	unsigned int plane);
@@ -19,5 +20,8 @@ void drm_fb_dma_sync_non_coherent(struct drm_device *drm,
 				  struct drm_plane_state *old_state,
 				  struct drm_plane_state *state);
 
+int drm_panic_gem_get_scanout_buffer(struct drm_plane *plane,
+				     struct drm_scanout_buffer *sb);
+
 #endif