Re: SCSI breakage on non-cache coherent architectures

Benjamin Herrenschmidt
2007-11-19T19:54:51Z

On Mon, 2007-11-19 at 09:09 -0600, James Bottomley wrote:
> > What other drivers do is DMA to their own allocation and then memcpy to
> > the sense buffer.
> >
> > There is a movement to allocate the sense data as its own sg list, but
> > I don't think that patch has even been posted yet.
>
> I'd like to be rid of it inside the command for various reasons: every
> command has one of these, and they're expensive in the allocation (at 96
> bytes). There's no reason we have to allocate and free that amount of
> space with every command. In theory, the number of these is bounded at
> the queue depth, in practice, there's usually only one, and this DMA
> alignment issue does requires most drivers to double copy.

And most drivers don't and break. Take USB storage, I -think- (code path
aren't trivial to follow) it just gets the sglist cooked up by the code
in scsi_error.c no ? That just points to the buffer in scsi_cmnd. It
then pass that for DMA to the USB stack.

Ben.