[PATCH v5] drm: Use USB controller's DMA mask when importing dmabufs

[Alan Stern]

On Fri, Feb 26, 2021 at 10:26:47AM +0100, Thomas Zimmermann wrote:
> USB devices cannot perform DMA and hence have no dma_mask set in their
> device structure. Therefore importing dmabuf into a USB-based driver
> fails, which breaks joining and mirroring of display in X11.
> 
> For USB devices, pick the associated USB controller as attachment device.
> This allows the DRM import helpers to perform the DMA setup. If the DMA
> controller does not support DMA transfers, we're out of luck and cannot
> import. Our current USB-based DRM drivers don't use DMA, so the actual
> DMA device is not important.
> 
> Drivers should use DRM_GEM_SHMEM_DROVER_OPS_USB to initialize their
> instance of struct drm_driver.
> 
> Tested by joining/mirroring displays of udl and radeon un der Gnome/X11.
> 
> v5:
> 	* provide a helper for USB interfaces (Alan)
> 	* add FIXME item to documentation and TODO list (Daniel)

> --- a/drivers/usb/core/usb.c
> +++ b/drivers/usb/core/usb.c
> @@ -748,6 +748,37 @@ void usb_put_intf(struct usb_interface *intf)
>  }
>  EXPORT_SYMBOL_GPL(usb_put_intf);
>  
> +/**
> + * usb_get_dma_device - acquire a reference on the usb device's DMA endpoint
> + * @udev: usb device
> + *
> + * While a USB device cannot perform DMA operations by itself, many USB
> + * controllers can. A call to usb_get_dma_device() returns the DMA endpoint
> + * for the given USB device, if any. The returned device structure should be
> + * released with put_device().
> + *
> + * See also usb_intf_get_dma_device().
> + *
> + * Returns: A reference to the usb device's DMA endpoint; or NULL if none
> + *          exists.
> + */
> +struct device *usb_get_dma_device(struct usb_device *udev)
> +{
> +	struct device *dmadev;
> +
> +	if (!udev->bus)
> +		return NULL;
> +
> +	dmadev = get_device(udev->bus->sysdev);
> +	if (!dmadev || !dmadev->dma_mask) {
> +		put_device(dmadev);
> +		return NULL;
> +	}
> +
> +	return dmadev;
> +}
> +EXPORT_SYMBOL_GPL(usb_get_dma_device);

There's no point making this a separate function, since it has no
callers of its own.  Just make usb_intf_get_dma_device the only new
function.

> --- a/include/linux/usb.h
> +++ b/include/linux/usb.h
> @@ -711,6 +711,7 @@ struct usb_device {
>  	unsigned use_generic_driver:1;
>  };
>  #define	to_usb_device(d) container_of(d, struct usb_device, dev)
> +#define dev_is_usb(d)	((d)->bus == &usb_bus_type)
>  
>  static inline struct usb_device *interface_to_usbdev(struct usb_interface *intf)
>  {
> @@ -746,6 +747,29 @@ extern int usb_lock_device_for_reset(struct usb_device *udev,
>  extern int usb_reset_device(struct usb_device *dev);
>  extern void usb_queue_reset_device(struct usb_interface *dev);
>  
> +extern struct device *usb_get_dma_device(struct usb_device *udev);
> +
> +/**
> + * usb_intf_get_dma_device - acquire a reference on the usb interface's DMA endpoint
> + * @intf: the usb interface
> + *
> + * While a USB device cannot perform DMA operations by itself, many USB
> + * controllers can. A call to usb_intf_get_dma_device() returns the DMA endpoint
> + * for the given USB interface, if any. The returned device structure should be
> + * released with put_device().
> + *
> + * See also usb_get_dma_device().
> + *
> + * Returns: A reference to the usb interface's DMA endpoint; or NULL if none
> + *          exists.
> + */
> +static inline struct device *usb_intf_get_dma_device(struct usb_interface *intf)
> +{
> +	if (!intf)
> +		return NULL;

Why would intf ever be NULL?

> +	return usb_get_dma_device(interface_to_usbdev(intf));
> +}

Alan Stern