backend/drm: Fix hardware cursors when rendering to linear is not available

GPUs such as NVIDIA and VmWare do not support rendering to a linear
buffer, but require linear cursor buffers. This means we have to
render to some other format, and then convert it ourselves.

This change advertises any render format as an available cursor format.

backend/drm/drm.c

@@ -1005,6 +1005,7 @@ static bool drm_connector_set_cursor(struct wlr_output *output,
 	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
 	struct wlr_drm_backend *drm = conn->backend;
 	struct wlr_drm_crtc *crtc = conn->crtc;
+	bool ok = false;
 
 	if (!crtc) {
 		return false;
@@ -1033,34 +1034,62 @@ static bool drm_connector_set_cursor(struct wlr_output *output,
 			return false;
 		}
 
-		struct wlr_buffer *local_buf;
-		if (drm->parent) {
+		// First try importing our buffer
+		struct wlr_buffer *local_buf = wlr_buffer_lock(buffer);
+		ok = drm_fb_import(&conn->cursor_pending_fb, drm, local_buf,
+				&plane->formats);
+		wlr_buffer_unlock(local_buf);
+
+		if (!ok) {
+			// If this failed blit a compatible buffer. This will blit it to
+			// our mgpu surface in the case that we are a secondary device
 			struct wlr_drm_format format = {0};
+			// Try to find a common format/modifier
 			if (!drm_plane_pick_render_format(plane, &format, &drm->mgpu_renderer)) {
 				wlr_log(WLR_ERROR, "Failed to pick cursor plane format");
-				return false;
+				// If the above failed it may be because the modifier for this
+				// buffer is not able to be scanned out, as is the case on some
+				// GPUs.  If it failed try to do a linear copy. This will map
+				// the mgpu surface as a linear texture and read pixels from
+				// the buffer into it. This avoids a scenario where the
+				// hardware cannot render to linear textures but only linear
+				// textures are supported for cursors, as is the case with
+				// Nvidia and VmWare GPUs
+
+				// Create a default format with only the linear modifier
+				wlr_drm_format_init(&format, DRM_FORMAT_ARGB8888);
+				if (!wlr_drm_format_add(&format, 0)) {
+					wlr_drm_format_finish(&format);
+					return false;
+				}
 			}
 
-			bool ok = init_drm_surface(&plane->mgpu_surf, &drm->mgpu_renderer,
-				buffer->width, buffer->height, &format);
+			ok = init_drm_surface(&plane->mgpu_surf, &drm->mgpu_renderer,
+					buffer->width, buffer->height, &format);
 			wlr_drm_format_finish(&format);
 			if (!ok) {
 				return false;
 			}
 
+			// First try to blit our cursor image.
 			local_buf = drm_surface_blit(&plane->mgpu_surf, buffer);
+			// If this is not possible due to the GPU not being able to
+			// render to a supported cursor format, then fall back to a
+			// more expensive copy
 			if (local_buf == NULL) {
-				return false;
+				// use the primary GPU for this, which will either be the current DRM
+				// backend or the parent if it has one
+				struct wlr_drm_renderer *drm_renderer =
+					drm->parent ? &drm->parent->mgpu_renderer : &drm->mgpu_renderer;
+				local_buf = drm_cursor_copy(&plane->mgpu_surf, drm_renderer, buffer);
+				if (local_buf == NULL) {
+					return false;
+				}
 			}
-		} else {
-			local_buf = wlr_buffer_lock(buffer);
-		}
 
-		bool ok = drm_fb_import(&conn->cursor_pending_fb, drm, local_buf,
-			&plane->formats);
-		wlr_buffer_unlock(local_buf);
-		if (!ok) {
-			return false;
+			ok = drm_fb_import(&conn->cursor_pending_fb, drm, local_buf,
+					&plane->formats);
+			wlr_buffer_unlock(local_buf);
 		}
 
 		conn->cursor_enabled = true;
@@ -1069,7 +1098,7 @@ static bool drm_connector_set_cursor(struct wlr_output *output,
 	}
 
 	wlr_output_update_needs_frame(output);
-	return true;
+	return ok;
 }
 
 static bool drm_connector_move_cursor(struct wlr_output *output,

backend/drm/renderer.c

@@ -1,4 +1,5 @@
 #include <assert.h>
+#include <stdlib.h>
 #include <drm_fourcc.h>
 #include <wlr/render/swapchain.h>
 #include <wlr/render/wlr_renderer.h>
@@ -73,6 +74,82 @@ bool init_drm_surface(struct wlr_drm_surface *surf,
 	return true;
 }
 
+struct wlr_buffer *drm_cursor_copy(struct wlr_drm_surface *surf,
+		struct wlr_drm_renderer *parent_renderer, struct wlr_buffer *buffer) {
+	void *data, *src_data;
+	size_t stride, src_stride;
+	uint32_t drm_format = DRM_FORMAT_ARGB8888;
+
+	if (surf->swapchain->width != buffer->width ||
+			surf->swapchain->height != buffer->height) {
+		wlr_log(WLR_ERROR, "Surface size doesn't match buffer size");
+		return NULL;
+	}
+
+	struct wlr_texture *tex = wlr_texture_from_buffer(parent_renderer->wlr_rend, buffer);
+	if (tex == NULL) {
+		wlr_log(WLR_ERROR, "Failed to import cursor into multi-GPU renderer");
+		return NULL;
+	}
+
+	struct wlr_buffer *dst = wlr_swapchain_acquire(surf->swapchain, NULL);
+	if (!dst) {
+		wlr_log(WLR_ERROR, "Failed to acquire multi-GPU swapchain buffer");
+		goto error_tex;
+	}
+
+	if (!wlr_buffer_begin_data_ptr_access(dst, WLR_BUFFER_DATA_PTR_ACCESS_WRITE, &data,
+				&drm_format, &stride)) {
+		wlr_log(WLR_ERROR, "Failed to get data ptr access to DRM cursor surface");
+		goto error_dst;
+	}
+
+	// Allocate memory to store our pixel data
+	src_stride = tex->width * 4;
+	src_data = malloc(tex->height * src_stride);
+	if (data == NULL) {
+		goto end_access;
+	}
+
+	// Get our linear pixel data from the source texture
+	bool result = wlr_texture_read_pixels(tex, &(struct wlr_texture_read_pixels_options) {
+		.format = DRM_FORMAT_ARGB8888,
+		.stride = src_stride,
+		.data = src_data,
+	});
+
+	if (!result) {
+		wlr_log(WLR_ERROR, "Failed to get data ptr access to DRM cursor surface");
+		goto free_src_data;
+	}
+
+	if (stride != src_stride) {
+		wlr_log(WLR_ERROR, "Format/stride values for DRM cursor source and destination"
+				"buffers do not match");
+		goto free_src_data;
+	}
+
+	// Copy our linear pixels into our DRM surface
+	memcpy(data, src_data, stride * buffer->height);
+
+	free(src_data);
+	wlr_buffer_end_data_ptr_access(dst);
+	wlr_texture_destroy(tex);
+
+	return dst;
+
+free_src_data:
+	free(src_data);
+end_access:
+	wlr_buffer_end_data_ptr_access(dst);
+error_dst:
+	wlr_buffer_unlock(dst);
+error_tex:
+	wlr_texture_destroy(tex);
+
+	return NULL;
+}
+
 struct wlr_buffer *drm_surface_blit(struct wlr_drm_surface *surf,
 		struct wlr_buffer *buffer) {
 	struct wlr_renderer *renderer = surf->renderer->wlr_rend;