Re: [PATCH RESEND number 2] libata: eliminate the home grown dma padding in favour of that provided by the block layer

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2008-02-05 at 09:06 +0900, Tejun Heo wrote:
> James Bottomley wrote:
> > Plus, tape devices are also ATAPI and since the problem seems to be
> > handling of ATAPI pio commands, they need all of this too.  It really
> > is, I think, better just to do the setup in the libata slave configure
> > if the device is atapi.
> 
> If no SCSI HBA needs it, fine.
> 
> > For all the rest, I think code demonstrates better ... well, except the
> > sata_fsl.c update ... you leave in qc->n_iter, but it's only ever set to
> > zero ... doesn't that cause this driver to break?
> 
> Probably.  I was gonna cross check with your patch while splitting.
> They seem mostly identical so I'll probably use your patch with some
> modifications.
> 
> > I analysed all the places you use the expanded data length.  To keep the
> > true length in the request and add the drain if necessary, I think
> > requires this update over my published libata drain patch.  I've also
> > added the request accessor to the drain buffer.  Could you verify this
> > actually works?  If it does, it's better than all the additions to block
> > and sr.  It would probably help to resend the series rebased against git
> > current.
> 
> No, it doesn't.  Drain needs to extend the sg table too 

The patches do extend the sg table.

> and it makes controllers lax about buffer overruns for commands they shouldn't be.

So adjust the qc->nbytes to show no drain element in libata ... although
the extra element gets mapped, the HBA will be none the wiser if there's
zero length allocated to it.  Thus the entire system behaves exactly
like the drain element isn't present.  The block layer is really just
making the drain element available ... you don't have to use it.

> Gee.. What's the deal with extra four bytes?  If you're worried about
> the size of struct request, chopping off PC request part into a separate
> struct would be far better strategy than making what's already confusing
> more confusing.

Well, we are trying to slim down the request structure and scsi_cmnd
structure.  However, there's no inherent advantage to counting this all
in the block layer.  Think of it just as a transparent passthrough.
You've told the block layer to make sure you have room for a drain
element when it constructs the SG list, which it does.  Whether you use
it (by adding blk_rq_drain_size() on to the actual length you hand down)
or not is entirely up to you ... there's no need to push the decision on
that one into the block layer, since use or not is pretty much a cost
free thing.

James