From owner-freebsd-hackers@FreeBSD.ORG  Thu Jul  8 13:41:05 2004
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Date: Thu, 08 Jul 2004 15:40:21 +0200
From: Martin Nilsson <martin@gneto.com>
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To: Avleen Vig <lists-freebsd@silverwraith.com>
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cc: freebsd-hackers@freebsd.org
Subject: Re: Adding DMA support for a device
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Avleen Vig wrote:
> The drive supports DMA, and Windows sets it to use UDMA mode 5 (I
> think), but BSD does not.
> 
> Can anyone point me in the right direction for this?
> 
have you tried man ata ?

The following tunables are settable from the loader:
hw.ata.atapi_dma
      set to 1 for DMA access, 0 for PIO (default is PIO).

/Martin