Re: Limiting DMA speeds for individual IDE drives

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

> I've got a situation where a drive claims to be capable of supporting
> UDMA/100, but it's in a noisy environment and gets lots of errors at
> that speed.  I'd like to limit it to UDMA/66 or even UDMA/33.

That should never occur with a proper cable and I would be concerned the
fault might be something more problematic such as speed misconfiguration
or an incompatibility. Which driver is in use ?

> The hdparm command should be able to do this but I can't run it until 
> the system has booted, by which time a bunch of CRC and possibly other 
> errors have already occurred.  Ideally it should be possible to limit 

Only the data transfers are CRC protected and at high speed, but noise at
low speed would be a real concern as the commands are sent low speed but
without protection on PATA devices - so a bit flip can send a DMA to the
wrong sector.

> the speed starting as early as device detection, but I can't find any 
> way to do it.  Is there support for such a thing or will I have to hack 
> it in?

You can disallow DMA but not clip DMA to UDMA33 with the old driver. You
could disallow DMA at boot and reallow it with a speed set by hdparm in
your boot scripts...