linux-kernel - Re: Limiting DMA speeds for individual IDE drives

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Message-ID: <20090908185306.0f30c07b@lxorguk.ukuu.org.uk>
Date:	Tue, 8 Sep 2009 18:53:06 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	linux-ide@...r.kernel.org,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Limiting DMA speeds for individual IDE drives

> 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...
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