Re: code review request: simple usb stream interface (susi)

On Thu, 8 Feb 2007, Edwin Olson wrote:

> I forgot to ask an additional question:
> 
> Is it sensible to have multiple outstanding read URBs, in the way that 
> there are multiple outstanding write URBs?

Yes.

> On my embedded platform, it is fairly expensive to do write-merging in 
> order to send maximum length packets. (In other words, on my device, my 
> "write" syscalls have a 1:1 correspondence with USB packets; when these 
> packets are less than 64 bytes, I only get one packet per 1ms USB frame.)

Why only one per frame?  Why not more?  Is that a limitation of the host
controller driver on your platform?

> If the host had multiple read URBs pending, would it continue to send IN 
> tokens after it had already received a short packet in that frame?

Yes.

>  Could 
> I be safely assured that the bulk_read callbacks would occur in the 
> order already read, or would the driver need to do some explicit 
> fragment reassembly? 

I don't know what you mean by "fragment reassembly" -- USB has no notion
of "fragments".  I also don't know what you mean by "already read".  But
the completion callbacks would indeed occur in the order the URBs were
submitted.  The rule is simple: One accepted URB submission = one
callback.  (For 2.6 anyway; 2.4 had more a more complex rule.)

> (Could read callbacks be called out of order or 
> concurrently on an SMP machine?)

No.  Nor could write callbacks.  However read callbacks could be 
interspersed more or less arbitrarily among write callbacks -- although 
again, not concurrently.

For more information, read the kerneldoc in the source file 
drivers/usb/core/urb.c.

Alan Stern


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