.TH DMAINIT 9
.SH NAME
dmainit, dmasetup, dmadone, dmaend, dmacount \- platform-specific DMA support
.SH SYNOPSIS
.ta \w'\fLushort 'u
.B
void	dmainit(int chan)
.PP
.B
long	dmasetup(int chan, void *va, long len, int isread)
.PP
.B
int	dmadone(int chan)
.PP
.B
void	dmaend(int chan)
.PP
.B
int	dmacount(int chan)
.PP
.SH DESCRIPTION
These functions manage DMA on a bus that uses ISA-style DMA controllers.
They were originally devised for the x86 platform, but the same interface, and similar code,
is used by other platforms that use similar controllers.
They compensate as best they can for the limitations of older DMA implementations
(eg, alignment, boundary and length restrictions).
There are 8 DMA channels:
0 to 3 are byte-oriented; 4 to 7 are word-oriented (16-bit words).
.PP
.I Dmainit
must be called early in a driver's initialisation to prepare
.I chan
for use.
Amongst other things, it allocates a page-sized buffer to help circumvent hardware
restrictions on DMA addressing.
.PP
.I Dmasetup
prepares DMA channel
.IR chan
for a transfer between a device configured to use it
and the virtual address
.IR va .
(The transfer is started by issuing a command to the device.)
If
.I va
lies outside the kernel address space,
the transfer crosses a 64k boundary,
or exceeds the 16 Mbyte limit imposed by some DMA controllers,
the transfer will be split into page-sized transfers using the buffer previously allocated by
.IR dmainit .
If
.I isread
is true (non-zero), data is to be transferred from
.I chan
to
.IR va ;
if false, data is transferred from
.I va
to
.IR chan .
In all cases,
.I dmasetup
returns the number of bytes to be transferred.
That value (rather than
.IR len )
must be given to the device in the read or write request that starts the transfer.
.PP
.I Dmadone
returns true (non-zero) if
.I chan
is idle.
.PP
.I Dmaend
must be called at the end of every DMA operation.
It disables
.IR chan ,
preventing further access to the previously associated memory and,
if a low-memory buffer was required for input, transfers its contents
to the appropriate part of the target buffer.
.PP
.I Dmacount
returns the number of bytes that were last transferred by channel
.IR chan .
The count is always even for word-oriented DMA channels.
.SH SOURCE
.B /sys/src/9/pc/dma.c