[RFC 2/3] USB: core: Add non-coherent buffer allocation helpers

On Fri, Aug 31, 2018 at 2:50 PM Christoph Hellwig [off-list ref] wrote:

On Thu, Aug 30, 2018 at 07:11:35PM -0300, Ezequiel Garcia wrote:

quoted

On Thu, 2018-08-30 at 10:58 -0700, Christoph Hellwig wrote:

quoted

Please don't introduce new DMA_ATTR_NON_CONSISTENT users, it is
a rather horrible interface, and I plan to kill it off rather sooner
than later.  I plan to post some patches for a better interface
that can reuse the normal dma_sync_single_* interfaces for ownership
transfers.  I can happily include usb in that initial patch set based
on your work here if that helps.

Please do. Until we have proper allocators that go thru the DMA API,
drivers will have to kmalloc the USB transfer buffers, and have
streaming mappings. Which in turns mean not using IOMMU or CMA.

dma_map_page will of course use the iommu.

Sure, dma_map*() will, but using kmalloc() defeats (half of) the
purpose of it, since contiguous memory would be allocated
unnecessarily, risking failures due to fragmentation.

Best regards,
Tomasz