Am Sonntag, 1. Juli 2007 schrieb Alan Stern:
> It's not entirely clear what you're trying to accomplish by doing this.
>
> First, notice that pre-allocation is useless with one-shot URBs, since
> the resources would be allocated anyway as soon as the URB is
Yes, obviously.
> submitted. You gain an advantage only by avoiding having the resources
> deallocated when the URB completes, so that they can be reused when the
> URB is resubmitted. (Or possibly when a different URB is submitted for
> the same endpoint.) How will your changes affect deallocation?
It happens when the URB is destroyed.
> Notice also that the usb-storage doesn't reuse URBs for its large data
> transfers (ub might work differently; I don't know). Those are all
> done using one-shot URBs allocated by the scatter-gather library. Do
> you intend to change how that works?
That is a long term goal and would require some invasive changes in
the storage driver.
> Another thing worth noticing is that with reusable URBs, at allocation
> time you might not know the transfer lengths you will need. Drivers
> would have to guess at the maximum requirement and then live with that
> guess.
I doubt this is a common problem. Only rarely will a driver transfer
more than PAGE_SIZE.
> Finally, don't forget the advantages of avoiding preallocation and
> keeping resources in a pool, available for multiple uses.
What are these, beyond memory usage?
My goals are:
- speed up things for drivers that maintain pools of URBs:
some serial drivers, cdc-acm, video drivers ...
- reduce CPU usage in interrupt handlers and in general
- remove some nasty ENOMEM error cases
- guarantee the storage driver a fallback path without memory allocation
Regards
Oliver
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