Re: 2.1.111: IDE DMA disabled?

Richard B. Johnson (
Tue, 28 Jul 1998 22:46:41 -0400 (EDT)

On Tue, 28 Jul 1998, Doug Ledford wrote:

> Mark Lord wrote:
> > That's from the IDE (ATA) specs. 18" max.
> >
> > The timings with DMA are *identical* to the timings with PIO,
> > so if it fails with DMA, it will fail with PIO as well.
> >
> > Removing DMA from the kernel solves nothing there.
> As per Linus' comment in another email, it does make a difference, it not in
> the cable timings, then in other timings relating to the transfer. Probably
> not the least of which is memory congestion. In any case, it does make a
> difference.
> --

PIO and DMA, at the bus level, use the exact same mechanisms. The
advantage of DMA is that, if a driver is coded to handle this, the
CPU can do other things while the DMA transfer is occurring.

PIO uses the CPU for the transfer, so the CPU is busy. The Port/RAM
and RAM/Port speeds are identical if the CPU has a reasonable internal
clock (like 33MHz and greater).

Since the PC buses are not synchronous, the DMA Controller can't
cycle-steal as can be done in some other machines. So there is
usually no reason to use the DMA Controller at all. For instance
the BusLogic SCSI controller is very fast. The driver doesn't use DMA.

Script started on Tue Jul 28 22:42:37 1998

# cat /proc/dma
4: cascade
# cat /proc/scsi/*
cat: /proc/scsi/BusLogic: Bad file number
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: SEAGATE Model: ST32550W Rev: 0021

Dick Johnson
Penguin : Linux version 2.1.111 on an i586 machine (66.15 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.

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