backend-drm: import GBM bo to scanout device if necessary

If the GBM bo was allocated on a different device than the device that is used
for the fb, we have to import the fd first and update the handle.

Use drmPrimeFDToHandle directly instead of using a gbm device for the scanout
device, since a gbm device would require a gbm implementation, which is often
not available for devices that only support scanout.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>

libweston/backend-drm/drm-internal.h

@@ -300,6 +300,11 @@ struct drm_device {
 		dev_t devnum;
 	} drm;
 
+	/* Track the GEM handles if the device does not have a gbm device, which
+	 * tracks the handles for us.
+	 */
+	struct hash_table *gem_handle_refcnt;
+
 	/* drm_crtc::link */
 	struct wl_list crtc_list;
 
@@ -378,6 +383,8 @@ enum drm_fb_type {
 struct drm_fb {
 	enum drm_fb_type type;
 
+	struct drm_device *scanout_device;
+
 	int refcnt;
 
 	uint32_t fb_id, size;

libweston/backend-drm/drm.c

@@ -51,6 +51,7 @@
 #include <libweston/backend-drm.h>
 #include <libweston/weston-log.h>
 #include "drm-internal.h"
+#include "shared/hash.h"
 #include "shared/helpers.h"
 #include "shared/timespec-util.h"
 #include "shared/string-helpers.h"

libweston/backend-drm/fb.c

@@ -38,6 +38,7 @@
 #include <libweston/backend-drm.h>
 #include <libweston/pixel-formats.h>
 #include <libweston/linux-dmabuf.h>
+#include "shared/hash.h"
 #include "shared/helpers.h"
 #include "shared/weston-drm-fourcc.h"
 #include "drm-internal.h"
@@ -68,6 +69,138 @@ drm_fb_destroy_dumb(struct drm_fb *fb)
 	drm_fb_destroy(fb);
 }
 
+#ifdef BUILD_DRM_GBM
+static int gem_handle_get(struct drm_device *device, int handle)
+{
+	unsigned int *ref_count;
+
+	ref_count = hash_table_lookup(device->gem_handle_refcnt, handle);
+	if (!ref_count) {
+		ref_count = zalloc(sizeof(*ref_count));
+		hash_table_insert(device->gem_handle_refcnt, handle, ref_count);
+	}
+	(*ref_count)++;
+
+	return handle;
+}
+
+static void gem_handle_put(struct drm_device *device, int handle)
+{
+	unsigned int *ref_count;
+
+	if (handle == 0)
+		return;
+
+	ref_count = hash_table_lookup(device->gem_handle_refcnt, handle);
+	if (!ref_count) {
+		weston_log("failed to find GEM handle %d for device %s\n",
+			   handle, device->drm.filename);
+		return;
+	}
+	(*ref_count)--;
+
+	if (*ref_count == 0) {
+		hash_table_remove(device->gem_handle_refcnt, handle);
+		free(ref_count);
+		drmCloseBufferHandle(device->drm.fd, handle);
+	}
+}
+
+static int
+drm_fb_import_plane(struct drm_device *device, struct drm_fb *fb, int plane)
+{
+	int bo_fd;
+	uint32_t handle;
+	int ret;
+
+	bo_fd = gbm_bo_get_fd_for_plane(fb->bo, plane);
+	if (bo_fd < 0)
+		return bo_fd;
+
+	/*
+	 * drmPrimeFDToHandle is dangerous, because the GEM handles are
+	 * not reference counted by the kernel and user space needs a
+	 * single reference counting implementation to avoid double
+	 * closing of GEM handles.
+	 *
+	 * It is not desirable to use a GBM device here, because this
+	 * requires a GBM device implementation, which might not be
+	 * available for simple or custom DRM devices that only support
+	 * scanout and no rendering.
+	 *
+	 * We are only importing the buffers from the render device to
+	 * the scanout device if the devices are distinct, since
+	 * otherwise no import is necessary. Therefore, we are the only
+	 * instance using the handles and we can implement reference
+	 * counting for the handles per device. See gem_handle_get and
+	 * gem_handle_put for the implementation.
+	 */
+	ret = drmPrimeFDToHandle(fb->fd, bo_fd, &handle);
+	if (ret)
+		goto out;
+
+	fb->handles[plane] = gem_handle_get(device, handle);
+
+out:
+	close(bo_fd);
+	return ret;
+}
+#endif
+
+/*
+ * If the fb is using a GBM surface, there is a possibility that the GBM
+ * surface has been created on a different device than the device which
+ * should be used for the fb. We have to import the fd of the GBM bo
+ * into the scanout device.
+ */
+static int
+drm_fb_maybe_import(struct drm_device *device, struct drm_fb *fb)
+{
+#ifndef BUILD_DRM_GBM
+	/*
+	 * Without GBM support, the fb is always allocated on the scanout device
+	 * and import is never necessary.
+	 */
+	return 0;
+#else
+	struct gbm_device *gbm_device;
+	int ret = 0;
+	int plane;
+
+	/* No import possible, if there is no gbm bo */
+	if (!fb->bo)
+		return 0;
+
+	/* No import necessary, if the gbm bo and the fb use the same device */
+	gbm_device = gbm_bo_get_device(fb->bo);
+	if (gbm_device_get_fd(gbm_device) == fb->fd)
+		return 0;
+
+	if (fb->fd != device->drm.fd) {
+		weston_log("fb was not allocated for scanout device %s\n",
+			   device->drm.filename);
+		return -1;
+	}
+
+	for (plane = 0; plane < gbm_bo_get_plane_count(fb->bo); plane++) {
+		ret = drm_fb_import_plane(device, fb, plane);
+		if (ret)
+			goto err;
+	}
+
+	fb->scanout_device = device;
+
+	return 0;
+err:
+	for (; plane >= 0; plane--) {
+		gem_handle_put(device, fb->handles[plane]);
+		fb->handles[plane] = 0;
+	}
+
+	return ret;
+#endif
+}
+
 static int
 drm_fb_addfb(struct drm_device *device, struct drm_fb *fb)
 {
@@ -75,6 +208,10 @@ drm_fb_addfb(struct drm_device *device, struct drm_fb *fb)
 	uint64_t mods[4] = { };
 	size_t i;
 
+	ret = drm_fb_maybe_import(device, fb);
+	if (ret)
+		return ret;
+
 	/* If we have a modifier set, we must only use the WithModifiers
 	 * entrypoint; we cannot import it through legacy ioctls. */
 	if (device->fb_modifiers && fb->modifier != DRM_FORMAT_MOD_INVALID) {
@@ -209,10 +346,21 @@ drm_fb_destroy_gbm(struct gbm_bo *bo, void *data)
 static void
 drm_fb_destroy_dmabuf(struct drm_fb *fb)
 {
+	int i;
+
 	/* We deliberately do not close the GEM handles here; GBM manages
 	 * their lifetime through the BO. */
 	if (fb->bo)
 		gbm_bo_destroy(fb->bo);
+
+	/*
+	 * If we imported the dmabuf into a scanout device, we are responsible
+	 * for closing the GEM handle.
+	 */
+	for (i = 0; i < 4; i++)
+		if (fb->scanout_device && fb->handles[i] != 0)
+			gem_handle_put(fb->scanout_device, fb->handles[i]);
+
 	drm_fb_destroy(fb);
 }
 

libweston/backend-drm/meson.build

@@ -36,6 +36,7 @@ deps_drm = [
 	dep_egl, # optional
 	dep_libm,
 	dep_libdl,
+	dep_libshared,
 	dep_libweston_private,
 	dep_session_helper,
 	dep_libdrm,