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)
