drm/simpledrm: Preallocate format-conversion buffer in atomic_check

Preallocate the format-conversion state's storage in the plane's
atomic_check function if a format conversion is necessary. Allows
the update to fail if no memory is available. Avoids the same
allocation within atomic_update, which may not fail.

Also inline drm_plane_helper_atomic_check() into the driver and thus
return early for invisible planes. Avoids memory allocation entirely
in this case.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231009141018.11291-6-tzimmermann@suse.de

drivers/gpu/drm/tiny/simpledrm.c

@@ -19,6 +19,7 @@
 #include <drm/drm_drv.h>
 #include <drm/drm_fbdev_generic.h>
 #include <drm/drm_format_helper.h>
+#include <drm/drm_framebuffer.h>
 #include <drm/drm_gem_atomic_helper.h>
 #include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_gem_shmem_helper.h>
@@ -579,6 +580,44 @@ static const uint64_t simpledrm_primary_plane_format_modifiers[] = {
 	DRM_FORMAT_MOD_INVALID
 };
 
+static int simpledrm_primary_plane_helper_atomic_check(struct drm_plane *plane,
+						       struct drm_atomic_state *state)
+{
+	struct drm_plane_state *new_plane_state = drm_atomic_get_new_plane_state(state, plane);
+	struct drm_shadow_plane_state *new_shadow_plane_state =
+		to_drm_shadow_plane_state(new_plane_state);
+	struct drm_framebuffer *new_fb = new_plane_state->fb;
+	struct drm_crtc *new_crtc = new_plane_state->crtc;
+	struct drm_crtc_state *new_crtc_state = NULL;
+	struct drm_device *dev = plane->dev;
+	struct simpledrm_device *sdev = simpledrm_device_of_dev(dev);
+	int ret;
+
+	if (new_crtc)
+		new_crtc_state = drm_atomic_get_new_crtc_state(state, new_crtc);
+
+	ret = drm_atomic_helper_check_plane_state(new_plane_state, new_crtc_state,
+						  DRM_PLANE_NO_SCALING,
+						  DRM_PLANE_NO_SCALING,
+						  false, false);
+	if (ret)
+		return ret;
+	else if (!new_plane_state->visible)
+		return 0;
+
+	if (new_fb->format != sdev->format) {
+		void *buf;
+
+		/* format conversion necessary; reserve buffer */
+		buf = drm_format_conv_state_reserve(&new_shadow_plane_state->fmtcnv_state,
+						    sdev->pitch, GFP_KERNEL);
+		if (!buf)
+			return -ENOMEM;
+	}
+
+	return 0;
+}
+
 static void simpledrm_primary_plane_helper_atomic_update(struct drm_plane *plane,
 							 struct drm_atomic_state *state)
 {
@@ -635,7 +674,7 @@ static void simpledrm_primary_plane_helper_atomic_disable(struct drm_plane *plan
 
 static const struct drm_plane_helper_funcs simpledrm_primary_plane_helper_funcs = {
 	DRM_GEM_SHADOW_PLANE_HELPER_FUNCS,
-	.atomic_check = drm_plane_helper_atomic_check,
+	.atomic_check = simpledrm_primary_plane_helper_atomic_check,
 	.atomic_update = simpledrm_primary_plane_helper_atomic_update,
 	.atomic_disable = simpledrm_primary_plane_helper_atomic_disable,
 };