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

[Jani Nikula <jani.nikula@intel.com>]

On Fri, 07 Mar 2025, Thomas Zimmermann <tzimmermann@suse.de> wrote:
> 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>

> ---
>  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

-- 
Jani Nikula, Intel