Re: [PATCH v4] drm: Use USB controller's DMA mask when importing dmabufs

Alan Stern
2021-02-24T15:27:17Z

On Wed, Feb 24, 2021 at 10:23:04AM +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.
>
> v4:
> * implement workaround with USB helper functions (Greg)
> * use struct usb_device->bus->sysdev as DMA device (Takashi)
> v3:
> * drop gem_create_object
> * use DMA mask of USB controller, if any (Daniel, Christian, Noralf)
> v2:
> * move fix to importer side (Christian, Daniel)
> * update SHMEM and CMA helpers for new PRIME callbacks
>
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: <stable@vger.kernel.org> # v5.10+
> ---

> +struct drm_gem_object *drm_gem_prime_import_usb(struct drm_device *dev,
> + struct dma_buf *dma_buf)
> +{
> + struct usb_device *udev;
> + struct device *dmadev;
> + struct drm_gem_object *obj;
> +
> + if (!dev_is_usb(dev->dev))
> + return ERR_PTR(-ENODEV);
> + udev = interface_to_usbdev(to_usb_interface(dev->dev));
> +
> + dmadev = usb_get_dma_device(udev);

You can do it this way if you want, but I think usb_get_dma_device would
be easier to use if its argument was a pointer to struct usb_interface
or (even better) a pointer to a usb_interface's embedded struct device.
Then you wouldn't need to compute udev, and the same would be true for
other callers.

Alan Stern