linux-kernel - Re: SCSI over USB showstopper bug?

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Date:	Wed, 1 Nov 2006 19:31:32 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Joerg Schilling <Joerg.Schilling@...us.fraunhofer.de>
Cc:	schilling@...us.fraunhofer.de, linux-kernel@...r.kernel.org,
	arjan@...radead.org
Subject: Re: SCSI over USB showstopper bug?

On Wed, Nov 01 2006, Joerg Schilling wrote:
> Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > > > > it looks as if SG_GET_RESERVED_SIZE & SG_SET_RESERVED_SIZE
> > > > > are not in interaction with the underlying SCSI transport.
> > > > > 
> > > > > Programs like readcd and cdda2wav that try to get very large SCSI
> > > > > transfer buffers get a confirmation for nearly any SCSI transfer size 
> > > > > but later when readcd/cdda2wav try to transfer data with an
> > > > > actual SCSI command, they fail with ENOMEM.
> > > > > 
> > > > > Correct fix: let sg.c make a callback to the underlying SCSI transport
> > > > > 		and let it get a confirmation tfor the buffer size.
> > > > > 
> > > > > Quick and dirty fix: reduce the maximum allowed DMA size to the smallest
> > > > > 		max DMA size of all SCSI transports.
> > > >
> > > > real good fix:
> > > >
> > > > use SG_IO on the device directly that checks this already
> > > 
> > > From looking into the source, this claim seems to be wrong.
> >
> > The block layer SG_IO entry point does what Arjan describes - it checks
> > the queue settings, which must match the hardware limits. It needs to,
> > since it won't accept a command larger than what the path to that device
> > will allow in one go. The SCSI sg variant may be more restricted, since
> > it should handle partial completions of such commands.
> 
> Then someone should change the source to match this statements.
> 
> From a report I have from the k3b Author, readcd and cdda2wav only work
> if you add a "ts=128k" option. 

Then please file (or have him/her file) a proper bug report. It may be a
usb specific bug, or it may just be something else.

-- 
Jens Axboe

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