USB_ALLOC_COHERENT(9)                     USB Core APIs                     USB_ALLOC_COHERENT(9)



NAME
       usb_alloc_coherent - allocate dma-consistent buffer for URB_NO_xxx_DMA_MAP

SYNOPSIS
       void * usb_alloc_coherent(struct usb_device * dev, size_t size, gfp_t mem_flags,
                                 dma_addr_t * dma);

ARGUMENTS
       dev
           device the buffer will be used with

       size
           requested buffer size

       mem_flags
           affect whether allocation may block

       dma
           used to return DMA address of buffer

RETURN
       Either null (indicating no buffer could be allocated), or the cpu-space pointer to a
       buffer that may be used to perform DMA to the specified device. Such cpu-space buffers are
       returned along with the DMA address (through the pointer provided).

NOTE
       These buffers are used with URB_NO_xxx_DMA_MAP set in urb->transfer_flags to avoid
       behaviors like using “DMA bounce buffers”, or thrashing IOMMU hardware during URB
       completion/resubmit. The implementation varies between platforms, depending on details of
       how DMA will work to this device. Using these buffers also eliminates cacheline sharing
       problems on architectures where CPU caches are not DMA-coherent. On systems without
       bus-snooping caches, these buffers are uncached.

       When the buffer is no longer used, free it with usb_free_coherent.

COPYRIGHT
Kernel Hackers Manual 4.8.                 January 2017                     USB_ALLOC_COHERENT(9)