Re: [PATCH v3] drm: Use USB controller's DMA mask when importing dmabufs

On Tue, Feb 23, 2021 at 11:00:54AM -0500, Alan Stern wrote:

The impression I get is that Greg would like the USB core to export a function which takes struct usb_interface * as argument and returns the appropriate DMA mask value. Then instead of messing around with USB internals, drm_gem_prime_import_usb could just call this new function.

Adding such a utility function would be a sufficiently small change that it could go into the -stable kernels with no trouble.

The DMA mask value on its own is not useful. It needs to be paired with a device that the dma API functions can be called on.