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

2 files changed, 22 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 17fc5dc708f4..c9487bc88624 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -500,6 +500,25 @@ void drm_dev_unplug(struct drm_device *dev)
 }
 EXPORT_SYMBOL(drm_dev_unplug);
 
+/**
+ * drm_dev_set_dma_dev - set the DMA device for a DRM device
+ * @dev: DRM device
+ * @dma_dev: DMA device or NULL
+ *
+ * Sets the DMA device of the given DRM device. Only required if
+ * the DMA device is different from the DRM device's parent. After
+ * calling this function, the DRM device holds a reference on
+ * @dma_dev. Pass NULL to clear the DMA device.
+ */
+void drm_dev_set_dma_dev(struct drm_device *dev, struct device *dma_dev)
+{
+	dma_dev = get_device(dma_dev);
+
+	put_device(dev->dma_dev);
+	dev->dma_dev = dma_dev;
+}
+EXPORT_SYMBOL(drm_dev_set_dma_dev);
+
 /*
  * Available recovery methods for wedged device. To be sent along with device
  * wedged uevent.
@@ -654,6 +673,8 @@ static void drm_dev_init_release(struct drm_device *dev, void *res)
 {
 	drm_fs_inode_free(dev->anon_inode);
 
+	put_device(dev->dma_dev);
+	dev->dma_dev = NULL;
 	put_device(dev->dev);
 	/* Prevent use-after-free in drm_managed_release when debugging is
 	 * enabled. Slightly awkward, but can't really be helped. */
diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index a3d64f93a225..4b8c6075e46a 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -997,7 +997,7 @@ EXPORT_SYMBOL(drm_gem_prime_import_dev);
 struct drm_gem_object *drm_gem_prime_import(struct drm_device *dev,
 					    struct dma_buf *dma_buf)
 {
-	return drm_gem_prime_import_dev(dev, dma_buf, dev->dev);
+	return drm_gem_prime_import_dev(dev, dma_buf, drm_dev_dma_dev(dev));
 }
 EXPORT_SYMBOL(drm_gem_prime_import);