Re: New USB stack on FreeBSD w/Linux emulation layer coming

On Wed, 16 May 2007 10:57:04 -0400 (EDT), Alan Stern <[email protected]> wrote:

> It's worth pointing out that there already are drivers which
> preallocate URBs and memory buffers and then share them among multiple
> endpoints.  One example is usb-storage.

This is for CBI transport, right? Honestly, I forgot about that.
But aren't control transfers short, just commands and replies?
Their buffers can be dealiased, *if* need arises.

> We have discussed in the past the notion of making URB allocation 
> per-bus.  That would have less impact; not too many drivers share URBs 
> among different devices (although I think the Bluetooth driver does).  
> The advantage is that the allocation could then automatically 
> incorporate an HCD-private area; currently this has to be allocated 
> separately every time an URB is submitted.

Right. Per-endpoint with a size limits also allows TDs to be
preallocated.

> > The requirement for massive on-the-fly allocations is a part of Linux
> > stack that bothered me for a long time. This is especially annoying
> > for block devices when pressure builds up in the VM, and dirty data
> > has to be written out to a block storage device attached by USB.
> > This is somewhat analogous to the problem which exists with NFS.
> 
> I don't see how either of these scenarios would be affected.  Whether
> you do all the other allocations when the URB and its buffers are
> allocated or you wait until the URB is submitted, the memory pressure
> would still be just as bad.

If URB, hcd_priv, and TDs are preallocated, the allocation happens
when there's no pressure, and not when all of your memory is dirtied.
It can also happen on a process context.

> > I suppose we could do one last effort:
> >  - redefine usb_buffer_alloc to allow for endpoint argument
> >  - implement methods in HCDs to allocate and free QPs and xxx_priv
> >  - spell out in documentation very clearly that drivers should
> >    NOT use these facilities by default and only use them when
> >    memory pressure invokes these URBs
> > 
> > This is something which, I think, only Greg Kroah can decide on doing
> > or not doing.
> 
> I am not in favor of such a large change.

Yeah...

I mentioned in my reply to David that I would like to explore more
narrow approaches, like a threadless ub-like usb-storage. This won't
help Hans in any way, though.

-- Pete

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