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From: Scott Long <scottl@samsco.org>
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Date: Thu, 3 Sep 2009 23:31:07 -0600
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To: Matthew Dillon <dillon@apollo.backplane.com>
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Cc: Ryan Rogers <webmaster@doghouserepair.com>,
	Alexander Motin <mav@freebsd.org>, current@freebsd.org
Subject: Re: non aligned DMA transfer attempted
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On Sep 3, 2009, at 4:10 PM, Matthew Dillon wrote:

>    This is a known problem with physio and/or the ATA driver,  
> depending
>    on your viewpoint.  See kern/kern_physio.c.
>
>    The physio code directly maps the userland buffer via vmapbuf() and
>    supplies it as a BIO to the device.  The ATA driver does not use
>    BUSDMA (and never has)... it assumes BIOs are minimally aligned.
>

Wrong.

>    But BIOs generated from kern_physio.c use user addresses directly  
> and
>    thus might only be byte-aligned.
>
>    The CAM pass-through device has the same problem.
>

Huh?  Are you confused by the physaddr interface of CAM?

>    The problem occurs most often when running a cd/dvd player or  
> burner
>    which declares operational structures it intends to read() or  
> write()
>    on the stack, sometimes even with char[] arrays.
>
>    The solution I came up for with DFly was to implement bounce  
> buffers
>    manually in kern_physio.c and the CAM pass-through device for any
>    user data that was not 16-byte aligned.  I considered implementing
>    bounce buffers in all the disk drivers that were missing it but
>    there are other serious problems trying to bounce dma buffers  
> that deep
>    in the device hierarchy... you can hit low-memory deadlocks if the
>    driver isn't written carefully enough (and most aren't).  The  
> BUSDMA
>    API has never been easy to work with for anyone trying to use the
>    async data buffer bouncing load feature... most drivers just  
> force it
>    to run synchronously and thus hit the deadlock issues.
>

Wrong.

Scott