DMAINIT(10.2) 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 con‐
trollers. 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 limita‐
tions 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).
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 con‐
figured 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 allocated by
dmainit. If isread is true (non-zero), data is to be transferred from
chan to va; if false, data is transferred 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 con‐
tents to the appropriate part of the target buffer.
Dmacount returns the number of bytes that were last transferred by
channel chan. The count is always even for word-oriented DMA channels.
SOURCE
/os/pc/dma.c
x86 DMAINIT(10.2)
