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usb_buffer_map_sg(9) [centos man page]

USB_BUFFER_MAP_SG(9)						   USB Core APIs					      USB_BUFFER_MAP_SG(9)

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)

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USB_SG_INIT(9)							   USB Core APIs						    USB_SG_INIT(9)

usb_sg_init - initializes scatterlist-based bulk/interrupt I/O request SYNOPSIS
int usb_sg_init(struct usb_sg_request * io, struct usb_device * dev, unsigned pipe, unsigned period, struct scatterlist * sg, int nents, size_t length, gfp_t mem_flags); ARGUMENTS
io request block being initialized. until usb_sg_wait returns, treat this as a pointer to an opaque block of memory, dev the usb device that will send or receive the data pipe endpoint "pipe" used to transfer the data period polling rate for interrupt endpoints, in frames or (for high speed endpoints) microframes; ignored for bulk sg scatterlist entries nents how many entries in the scatterlist length how many bytes to send from the scatterlist, or zero to send every byte identified in the list. mem_flags SLAB_* flags affecting memory allocations in this call DESCRIPTION
Returns zero for success, else a negative errno value. This initializes a scatter/gather request, allocating resources such as I/O mappings and urb memory (except maybe memory used by USB controller drivers). The request must be issued using usb_sg_wait, which waits for the I/O to complete (or to be canceled) and then cleans up all resources allocated by usb_sg_init. The request may be canceled with usb_sg_cancel, either before or after usb_sg_wait is called. COPYRIGHT
Kernel Hackers Manual 2.6. July 2010 USB_SG_INIT(9)

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