DMAINIT(10.2) (x86) DMAINIT(10.2)
NAME
dmainit, dmasetup, dmadone, dmaend, dmacount - platform-
specific DMA support
SYNOPSIS
void dmainit(int chan)
long dmasetup(int chan, void *va, long len, int isread)
int dmadone(int chan)
void dmaend(int chan)
int dmacount(int chan)
DESCRIPTION
These functions manage DMA on a bus that uses ISA-style DMA
controllers. They were originally devised for the x86 plat-
form, but the same interface, and similar code, is used by
other platforms that use similar controllers. They compen-
sate as best they can for the limitations of older DMA
implementations (eg, alignment, boundary and length restric-
tions). There are 8 DMA channels: 0 to 3 are byte-oriented;
4 to 7 are word-oriented (16-bit words).
Dmainit must be called early in a driver's initialisation to
prepare chan for use. Amongst other things, it allocates a
page-sized buffer to help circumvent hardware restrictions
on DMA addressing.
Dmasetup prepares DMA channel chan for a transfer between a
device configured to use it and the virtual address va. (The
transfer is started by issuing a command to the device.) If
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 allo-
cated by dmainit. If isread is true (non-zero), data is to
be transferred from chan to va; if false, data is trans-
ferred from va to chan. In all cases, dmasetup returns the
number of bytes to be transferred. That value (rather than
len) must be given to the device in the read or write
request that starts the transfer.
Dmadone returns true (non-zero) if chan is idle.
Dmaend must be called at the end of every DMA operation. It
disables 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
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DMAINIT(10.2) (x86) DMAINIT(10.2)
the target buffer.
Dmacount returns the number of bytes that were last trans-
ferred by channel chan. The count is always even for word-
oriented DMA channels.
SOURCE
/os/pc/dma.c
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