usb_alloc_coherent(9) [centos man page]

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 3.10					     June 2014						     USB_ALLOC_COHERENT(9)

USB_BUFFER_MAP_SG(9)						   USB Core APIs					      USB_BUFFER_MAP_SG(9)

NAME

       usb_buffer_map_sg - create scatterlist DMA mapping(s) for an endpoint

SYNOPSIS

       int usb_buffer_map_sg(const struct usb_device * dev, int is_in, struct scatterlist * sg, int nents);

ARGUMENTS

       dev
	   device to which the scatterlist will be mapped

       is_in
	   mapping transfer direction

       sg
	   the scatterlist to map

       nents
	   the number of entries in the scatterlist

RETURN

       Either < 0 (indicating no buffers could be mapped), or the number of DMA mapping array entries in the scatterlist.

NOTE

       The caller is responsible for placing the resulting DMA addresses from the scatterlist into URB transfer buffer pointers, and for setting
       the URB_NO_TRANSFER_DMA_MAP transfer flag in each of those URBs.

       Top I/O rates come from queuing URBs, instead of waiting for each one to complete before starting the next I/O. This is particularly easy
       to do with scatterlists. Just allocate and submit one URB for each DMA mapping entry returned, stopping on the first error or when all
       succeed. Better yet, use the usb_sg_*() calls, which do that (and more) for you.

       This call would normally be used when translating scatterlist requests, rather than usb_buffer_map, since on some hardware (with IOMMUs) it
       may be able to coalesce mappings for improved I/O efficiency.

       Reverse the effect of this call with usb_buffer_unmap_sg.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014						      USB_BUFFER_MAP_SG(9)