UAS: fix alignment of scatter/gather segments

[Oliver Neukum <oneukum@suse.com>]

On Mo, 2019-04-29 at 15:06 +0000, David Laight wrote:
> From: Oliver Neukum
> > On Mo, 2019-04-29 at 14:19 +0000, David Laight wrote:

> > AFAICT controllers do not export that property.
> 
> Perhaps they need to ....

Feel free to make a patch.

> > > Even if you decide the code is 'good enough' (I don't know what the
> > > cost is of enforcing a 2k alignment instead of 512 bytes)
> > > the comment is just plain wrong.
> > 
> > Usually block IO will be pages. They are 4K aligned.
> > In terms of performance this code is unlikely to matter.
> 
> Presumably there are some cases where the buffer isn't 4k aligned.
> I'm guessing things like 'dd' of raw disks?

Possibly.

> If the buffer has the wrong alignment then I presume a bounce buffer
> has to be allocated?
> The USB controller drivers are likely to need to be able to do that
> anyway because buffers from the network stack can have almost
> arbitrary alignment (I remember that code being horrid, I don't
> remember a data copy in the TX path).

You can ask the network layer to undo scatter/gather.
I don't see an issue.

> > But it is needed for correctness.
> 
> If you are doing it for 'correctness' then you need the correct size.

Why? Any larger size will do.

> If you are doing it because you've seen too small an alignment you
> should be mentioning the failing system.

Why?

> > What would you want for the comment?
> 
> You need to be more specific about the alignment requirements than
> the old comment, not far less specific.

But the statement the old comment made are no longer correct.

	Regards
		Oliver