linux-kernel - Limiting DMA speeds for individual IDE drives

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Message-ID: <Pine.LNX.4.44L0.0909081157520.2653-100000@iolanthe.rowland.org>
Date:	Tue, 8 Sep 2009 12:03:48 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	linux-ide@...r.kernel.org,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Limiting DMA speeds for individual IDE drives

Is there any simple way to force the old IDE driver to limit the DMA
speed for a particular device?

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.

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 
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?

Thanks,

Alan Stern

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