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usb_alloc_coherent(9) [centos man page]


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)

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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 DESCRIPTION
Return value is either < 0 (indicating no buffers could be mapped), or the number of DMA mapping array entries in the scatterlist. 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 2.6. July 2010 USB_BUFFER_MAP_SG(9)
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