drm: Enable PRIME import/export for all drivers

Call drm_gem_prime_handle_to_fd() and drm_gem_prime_fd_to_handle() by
default if no PRIME import/export helpers have been set. Both functions
are the default for almost all drivers.

DRM drivers implement struct drm_driver.gem_prime_import_sg_table
to import dma-buf objects from other drivers. Having the function
drm_gem_prime_fd_to_handle() functions set by default allows each
driver to import dma-buf objects to itself, even without support for
other drivers.

For drm_gem_prime_handle_to_fd() it is similar: using it by default
allows each driver to export to itself, even without support for other
drivers.

This functionality enables userspace to share per-driver buffers
across process boundaries via PRIME (e.g., wlroots requires this
functionality). The patch generalizes a pattern that has previously
been implemented by GEM VRAM helpers [1] to work with any driver.
For example, gma500 can now run the wlroots-based sway compositor.

v2:
	* clean up docs and TODO comments (Simon, Zack)
	* clean up style in drm_getcap()
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/dri-devel/20230302143502.500661-1-contact@emersion.fr/ # 1
Reviewed-by: Simon Ser <contact@emersion.fr>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230620080252.16368-2-tzimmermann@suse.de

drivers/gpu/drm/drm_ioctl.c

@@ -245,8 +245,7 @@ static int drm_getcap(struct drm_device *dev, void *data, struct drm_file *file_
 		req->value = 1;
 		return 0;
 	case DRM_CAP_PRIME:
-		req->value |= dev->driver->prime_fd_to_handle ? DRM_PRIME_CAP_IMPORT : 0;
-		req->value |= dev->driver->prime_handle_to_fd ? DRM_PRIME_CAP_EXPORT : 0;
+		req->value = DRM_PRIME_CAP_IMPORT | DRM_PRIME_CAP_EXPORT;
 		return 0;
 	case DRM_CAP_SYNCOBJ:
 		req->value = drm_core_check_feature(dev, DRIVER_SYNCOBJ);

drivers/gpu/drm/drm_prime.c

@@ -372,11 +372,12 @@ int drm_prime_fd_to_handle_ioctl(struct drm_device *dev, void *data,
 {
 	struct drm_prime_handle *args = data;
 
-	if (!dev->driver->prime_fd_to_handle)
-		return -ENOSYS;
+	if (dev->driver->prime_fd_to_handle) {
+		return dev->driver->prime_fd_to_handle(dev, file_priv, args->fd,
+						       &args->handle);
+	}
 
-	return dev->driver->prime_fd_to_handle(dev, file_priv,
-			args->fd, &args->handle);
+	return drm_gem_prime_fd_to_handle(dev, file_priv, args->fd, &args->handle);
 }
 
 static struct dma_buf *export_and_register_object(struct drm_device *dev,
@@ -518,15 +519,17 @@ int drm_prime_handle_to_fd_ioctl(struct drm_device *dev, void *data,
 {
 	struct drm_prime_handle *args = data;
 
-	if (!dev->driver->prime_handle_to_fd)
-		return -ENOSYS;
-
 	/* check flags are valid */
 	if (args->flags & ~(DRM_CLOEXEC | DRM_RDWR))
 		return -EINVAL;
 
-	return dev->driver->prime_handle_to_fd(dev, file_priv,
-			args->handle, args->flags, &args->fd);
+	if (dev->driver->prime_handle_to_fd) {
+		return dev->driver->prime_handle_to_fd(dev, file_priv,
+						       args->handle, args->flags,
+						       &args->fd);
+	}
+	return drm_gem_prime_handle_to_fd(dev, file_priv, args->handle,
+					  args->flags, &args->fd);
 }
 
 /**

include/drm/drm_drv.h

@@ -304,22 +304,14 @@ struct drm_driver {
 	/**
 	 * @prime_handle_to_fd:
 	 *
-	 * Main PRIME export function. Should be implemented with
-	 * drm_gem_prime_handle_to_fd() for GEM based drivers.
-	 *
-	 * For an in-depth discussion see :ref:`PRIME buffer sharing
-	 * documentation <prime_buffer_sharing>`.
+	 * PRIME export function. Only used by vmwgfx.
 	 */
 	int (*prime_handle_to_fd)(struct drm_device *dev, struct drm_file *file_priv,
 				uint32_t handle, uint32_t flags, int *prime_fd);
 	/**
 	 * @prime_fd_to_handle:
 	 *
-	 * Main PRIME import function. Should be implemented with
-	 * drm_gem_prime_fd_to_handle() for GEM based drivers.
-	 *
-	 * For an in-depth discussion see :ref:`PRIME buffer sharing
-	 * documentation <prime_buffer_sharing>`.
+	 * PRIME import function. Only used by vmwgfx.
 	 */
 	int (*prime_fd_to_handle)(struct drm_device *dev, struct drm_file *file_priv,
 				int prime_fd, uint32_t *handle);