Re: 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:
> 
> > 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.
> 
> All right, fine.  Host adapter drivers have the option of setting the 
> dma_alignment mask in their slave_configure() routines.  Doing it there 
> won't add anything to the host template, will leave the value at its 
> default 512 for host drivers that don't care, and will let drivers that do 
> care set it to the optimal value.
> 
> The important thing is that sg and st should check the alignment of the 
> actual buffer against the dma_alignment mask, which they currently don't 
> do.

Sure fine. I still don't think it's worth anybodies time messing with.

> > 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?
> 
> I'm not sure -- was that "you" directed at me personally or at userspace 
> program writers in general?  Telling me to align the user buffers won't 
> help anything; it's the program writers who need to be informed of the 
> restrictions.  And it's the host-driver writers who need to loosen those 
> restrictions where applicable.

You as in program writers, clearly you cannot do anything about it. Of
course sg/st/whatever cannot pass down badly aligned sg addresses,
that's a given. And I think they are currently broken that they don't
attempt to align to 512 bytes. I just think we've waisted way more time
discussion the issue than what will ever be saved in cpu time.

-- 
Jens Axboe



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