[PATCH v2 1/5] drm/prime: Support dedicated DMA device for dma-buf imports

[Thomas Zimmermann]

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 at suse.de>
---
 drivers/gpu/drm/drm_drv.c   | 21 +++++++++++++++++++
 drivers/gpu/drm/drm_prime.c |  2 +-
 include/drm/drm_device.h    | 41 +++++++++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 1 deletion(-)

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 bdb51c8f262e..ed4e5f06b79d 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -998,7 +998,7 @@ 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);
 
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
@@ -64,6 +64,28 @@ struct drm_device {
 	/** @dev: Device structure of bus-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:
 	 *
@@ -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
-- 
2.48.1