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Re: [Xen-devel] Re: usbback cleanup code

FWIW, I took a look at USB over IP. It looks pretty reasonable to me (plus, it's already in -mm). At this point, I'm convinced we in the very least want to share code (even if we don't use IP as the actual transport). It already handles all of the nasty protocol marshaling stuff. No reason to have two bits of code doing the same thing.


Anthony Liguori

Mark Williamson wrote:
I was able to do a little review of the patch a while back but never had
to time finish looking through it properly. It looked much closer to
mergeable, but there still seemed to be quite a lot of abstraction code.
I think in general, folks were hoping to see a minimum amount of
abstraction code with the USB driver instead using the driver APIs
As far as I'm aware, the USB code is using the driver API correctly
(except possibly for any bugs or where the API may have changed since
the last patch I released).

Sorry, didn't mean to imply it wasn't correctly using it now. I meant to say "directly", which is not at all the same thing.

I think we have a fairly fundamental disconnect about abstraction.  For
me, abstraction is a necessary part of good software engineering.  Just
as I assume you wouldn't write machine code where you could use assembly
and wouldn't write assembly where you could write C, I wouldn't write
code at a low level of abstraction where it was possible to use a higher
level of abstraction.  Abstraction is useful to manage complexity and
useful to write software which is easier to reason about and easier to

Quite. But it can be a problem where there's just one client, going through many layers of abstractions.

There were a lot of files added by your patch which appeared to be utility code / abstractions. This is fine in general, but the other drivers seem to get away with much less of this kind of thing without suffering unduly in terms of complexity. I didn't have time to study the code in detail, but I wasn't convinced they were all strictly necessary.

If you don't want to do any more work on it, then maybe it would make a
good project for somebody.
If anyone wants to pick it up, they are more than welcome but I think it
might be worthwhile to wait until some Xen drivers have been
successfully merged upstream with Linux since I suspect that there may
be some more significant churn in the xenbus/xenstore area before this

Maybe, but I suspect upstream merge is still quite a long way off.

Personally, I've found that the Xenbus APIs are now sufficiently simple to work with that it's very little work to establish a shared memory page (I hacked up one very quickly for DCSS), after which you don't have to worry about them anylonger. I don't think keeping up with the control plane is prohibitive now, although it was at one stage.

Isochronous is implemented but untested as I couldn't get the
isochronous devices I bought for testing working under native Linux.


The most difficult remaining work is to fix the protocol to correctly
stall URBs during error recovery.  I was involved in some discussion
about this on the USB mailing list and there was a proposal for a
solution but it is fairly tricky.  Stalling URBs is required when there
is a queue of URBs and an URB fails.  If the URBs are not stalled then
they may be submitted to the device out-of-order which is a
data-integrity exposure.

Any reason not just to fail all the URBs on the queue? It's not the ideal response, but I wouldn't see a need to handle error recovery fully initially, although it'd be nice in the long run.

Also I would expect the Linux USB stack to have changed again.

2.6's APIs do change fairly flexibly, but I don't remember there being any major changes to the USB stack for some time now.




On May 1 2006, Harry Butterworth wrote:
I haven't done any more work on the USB code since the last patch I
posted to xen-devel.  There wasn't any feedback and it wasn't committed.
I think people were too busy with the release.

I have stopped working on USB.  I have done several versions now with no
success at getting it merged.  I think it will be easier to see what is
required once there are some examples of drivers that have been merged
with Linux.

On Sat, 2006-04-29 at 19:43 +0000, sanjay kumar wrote:
Hi Harry,
Do you know by what time the USB virtualization code will be commited
in the xen-unstable tree?


On 4/3/06, Harry Butterworth <harry@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
        The code is supposed to work with isochronous devices but it's
        so probably doesn't.


PhD Student, Georgia Tech
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