On Mon, Mar 01, 2021 at 10:31:27AM +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.
>
> v6:
> * implement workaround in DRM drivers and hold reference to
> DMA device while USB device is in use
> * remove dev_is_usb() (Greg)
> * collapse USB helper into usb_intf_get_dma_device() (Alan)
> * integrate Daniel's TODO statement (Daniel)
> * fix typos (Greg)
> v5:
> * provide a helper for USB interfaces (Alan)
> * add FIXME item to documentation and TODO list (Daniel)
> 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@xxxxxxx>
> Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> Tested-by: Pavel Machek <pavel@xxxxxx>
> Acked-by: Christian König <christian.koenig@xxxxxxx>
> Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+
> Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
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