Re: 2.1.111: IDE DMA disabled?

Alan Cox (
Sun, 26 Jul 1998 15:11:19 +0100 (BST)

> > The configuration on which the filesystem corruption was observed is a
> > Tyan Tomcat III board + WD 2.2GB drive; that motherboard is using the
> > 430HX chipset, which pre-dates Ultra-DMA and doesn't support CRC check.
> Which implies that at _least_ we need to disable DMA automatically if it's
> one of those chipsets.

Only its nothing to do with the chipsets. non ultra DMA is so slow relatively
speaking it doesnt need bit checks. The other thing of course is that bit
errors dont cause the first 128 bytes of the block to be all zeros, they cause
bits to flip - just like on scsi.

non Ultra DMA works for the same reasons that scsi without parity works - its
a mathematically non interesting probability of bit errors. Ultra DMA is
a bit different due to the speed and also the desire to survive cowboy
installers. Again note _bit_ errors not writing complete crap over a block
of disk.

> Personally, I never understood why DMA was enabled by default anyway. As
> far as I can tell the only sane thing is to have it always disabled by
> default, and enable it with hdparm.

Its a factor of 10 speedup in effective throughput. It was turned on by
the BIOS - which may have configured aspects of the firmware we don't even
know about for DMA only operation.


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