drm/prime: Support dedicated DMA device for dma-buf imports

Importing dma-bufs via PRIME requires a DMA-capable device. Devices on
peripheral busses, such as USB, often cannot perform DMA by themselves.
Without DMA-capable device PRIME import fails. DRM drivers for USB
devices already use a separate DMA device for dma-buf imports. Make the
mechanism generally available.

Besides the case of USB, there are embedded DRM devices without DMA
capability. DMA is performed by a separate controller. DRM drivers should
set this accordingly.

Add the field dma_dev to struct drm_device to refer to the device's DMA
device. For USB this should be the USB controller. Use dma_dev in the
PRIME import helpers, if set.

v2:
- acquire internal reference on dma_dev (Jani)
- add DMA-controller usecase to docs (Maxime)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250307080836.42848-2-tzimmermann@suse.de

1 files changed, 41 insertions, 0 deletions

diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index 6ea54a578cda..e2f894f1b90a 100644
--- a/include/drm/drm_device.h
+++ b/include/drm/drm_device.h
@@ -65,6 +65,28 @@ struct drm_device {
 	struct device *dev;
 
 	/**
+	 * @dma_dev:
+	 *
+	 * Device for DMA operations. Only required if the device @dev
+	 * cannot perform DMA by itself. Should be NULL otherwise. Call
+	 * drm_dev_dma_dev() to get the DMA device instead of using this
+	 * field directly. Call drm_dev_set_dma_dev() to set this field.
+	 *
+	 * DRM devices are sometimes bound to virtual devices that cannot
+	 * perform DMA by themselves. Drivers should set this field to the
+	 * respective DMA controller.
+	 *
+	 * Devices on USB and other peripheral busses also cannot perform
+	 * DMA by themselves. The @dma_dev field should point the bus
+	 * controller that does DMA on behalve of such a device. Required
+	 * for importing buffers via dma-buf.
+	 *
+	 * If set, the DRM core automatically releases the reference on the
+	 * device.
+	 */
+	struct device *dma_dev;
+
+	/**
 	 * @managed:
 	 *
 	 * Managed resources linked to the lifetime of this &drm_device as
@@ -327,4 +349,23 @@ struct drm_device {
 	struct dentry *debugfs_root;
 };
 
+void drm_dev_set_dma_dev(struct drm_device *dev, struct device *dma_dev);
+
+/**
+ * drm_dev_dma_dev - returns the DMA device for a DRM device
+ * @dev: DRM device
+ *
+ * Returns the DMA device of the given DRM device. By default, this
+ * the DRM device's parent. See drm_dev_set_dma_dev().
+ *
+ * Returns:
+ * A DMA-capable device for the DRM device.
+ */
+static inline struct device *drm_dev_dma_dev(struct drm_device *dev)
+{
+	if (dev->dma_dev)
+		return dev->dma_dev;
+	return dev->dev;
+}
+
 #endif