Re: [linux-usb-devel] Re: Unaligned scatter-gather buffers and usb-storage

[Jens Axboe <axboe@suse.de>]

On Thu, Nov 20 2003, Alan Stern wrote:
> On Thu, 20 Nov 2003, Jens Axboe wrote:
> 
> > Then you just cannot do zero-copy dma, if the buffer isn't aligned
> > properly. It's as simple as that. And surely that wont kill you
> > performance wise.
> > 
> > Doing bounce buffering to just maintain the use of mapping user pages
> > into the kernel ls out right silly.
> > 
> > So no, I still don't think we have to add anything. Just don't do_dio if
> > the alignment doesn't allow it. Ditto for sg.
> 
> The answer seems very simple.  There should be a host template entry for
> dma buffer alignment (there's already a dma_boundary member).  It would be
> copied into the device's request queue stucture if it is nonzero,
> overriding the default value of 512.  sg and st should check the user
> buffer against the request queue's dma_alignment mask and avoid doing
> direct I/O if the alignment is wrong -- just fall back to normal I/O.
> 
> Any objections to this scheme?

Well yes, that's what my objection is against - adding that member. Did
you not read any of my mails? And it's quite simple why - basically
noone will add it, so it'll end up being 512 anyways.

I just don't see the point. It's a miniscule optimization. If you need
that last bit of performance, then align your buffers and noone loses.
See?

-- 
Jens Axboe