'\" t
.\"     Title: usb_alloc_coherent
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: USB Core APIs
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "USB_ALLOC_COHERENT" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "USB Core APIs"
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.SH "NAME"
usb_alloc_coherent \- allocate dma\-consistent buffer for URB_NO_xxx_DMA_MAP
.SH "SYNOPSIS"
.HP \w'void\ *\ usb_alloc_coherent('u
.BI "void * usb_alloc_coherent(struct\ usb_device\ *\ " "dev" ", size_t\ " "size" ", gfp_t\ " "mem_flags" ", dma_addr_t\ *\ " "dma" ");"
.SH "ARGUMENTS"
.PP
\fIdev\fR
.RS 4
device the buffer will be used with
.RE
.PP
\fIsize\fR
.RS 4
requested buffer size
.RE
.PP
\fImem_flags\fR
.RS 4
affect whether allocation may block
.RE
.PP
\fIdma\fR
.RS 4
used to return DMA address of buffer
.RE
.SH "RETURN"
.PP
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)\&.
.SH "NOTE"
.PP
These buffers are used with URB_NO_xxx_DMA_MAP set in urb\->transfer_flags to avoid behaviors like using
\(lqDMA bounce buffers\(rq, 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\&.
.PP
When the buffer is no longer used, free it with
\fBusb_free_coherent\fR\&.
.SH "COPYRIGHT"
.br