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Message-ID: <4789af9e05090813287f05e12a@mail.gmail.com>
Date: Thu, 8 Sep 2005 14:28:09 -0600
From: Jim Ramsay <jim.ramsay@gmail.com>
Reply-To: jim.ramsay@gmail.com
To: Matthew Dharm <mdharm-kernel@one-eyed-alien.net>,
       linux-usb-users@lists.sourceforge.net,
       Linux Kernel <linux-kernel@vger.kernel.org>, linux-scsi@vger.kernel.org
Subject: Re: [Linux-usb-users] Possible bug in usb storage (2.6.11 kernel)
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On 9/8/05, Jim Ramsay <jim.ramsay@gmail.com> wrote:
> On 9/8/05, Matthew Dharm <mdharm-kernel@one-eyed-alien.net> wrote:
> > On Thu, Sep 08, 2005 at 11:14:36AM -0600, Jim Ramsay wrote:
> > > I think I have found a possible bug:
> > > [...]
> > > I suppose the scsi code could be changed to guarantee that
> > > srb->request_buffer is page-aligned or cache-aligned, but that seems
> > > like the wrong solution for this bug.
> >
> > Fixing the SCSI layer is -exactly- the correct solution.  The SCSI layer is
> > supposed to guarantee us that those buffers are suitable for DMA'ing, and
> > apparently it's violating that promise.
> 
> Thanks, I'll check on what buffer I'm actually getting, where it's
> allocated, and post back what I find, or how I fixed it.

More information:

The error only occurrs during device sensing when the
srb->request_buffer is assigned as follows, by usb/storage/transport.c
in the routine usb_stor_invoke_transport:

old_request_buffer = srb->request_buffer;
srb->request_buffer = srb->sense_buffer;

Now, this is a problem because srb->sense_buffer is defined as follows
in the struct scsi_cmnd:

#define SCSI_SENSE_BUFFERSIZE   96
        unsigned char sense_buffer[SCSI_SENSE_BUFFERSIZE];

Since it is not allocated at runtime there is NO WAY the SCSI layer
can possibly guarantee it is page- or cache-aligned and ready for DMA.

Any suggestions on best fix for this?  Is it still a SCSI-layer issue?
 Or should USB step up in this case and ensure this buffer is dma-safe
itself?

-- 
Jim Ramsay
"Me fail English?  That's unpossible!"
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