Re: urbs are REFERENCE COUNTED!!!

On Mon, Apr 02, 2007 at 12:06:45PM -0700, David Brownell wrote:
> 
> > > That can certainly work, and it has the advantages of simplicity.
> > > I wouldn't use that strategy in every driver of course, but I'd
> > > guess it's fine for the serial and usblp cases mentioned here.
> > 
> > Why would you not want it in every driver?
> 
> For starters, your pre-emptive claim that performance isn't a factor
> is erroneous.  PCs are resource-rich -- in CPU and memory -- but Linux
> isn't just x86, and even x86 hits resource limits.
> 
> The CPU costs for hitting the heap twice (free then allocate) vs. just
> reusing an URB will add up in low-MIPS systems.  When those costs get
> paid in IRQ context, they impact IRQ latency.  When they get paid on
> systems with small batteries, they reduce time-till-recharge.  When
> they replace cache lines with less useful information, they increase
> the system's cycle-per-instruction ratio.  While the cost to one driver
> may not be high, system-wide costs do add up.

Ok, then a urb pool might be more applicable for systems like that, but
even then, we can't test for the status flag.

But I still think, given the advancement of processor speed over time,
trading simplicity in the driver by using fire-and-forget will work
well.  But benchmarks would be the best way to test this...

> > As Oliver is finding out, a 
> > lot of these "simple" drivers have races all due to their use of a
> > pre-allocated urb.  For what kind of situation would you feel this would
> > not work?
> 
> Not-simple drivers.  ;)

Then we should make them simpler :)

> Engineering tradeoffs always need to be made.  My point was less about
> "not work" and more about "good tradeoff".  One size does not fit all.
> A driver which prioritizes simplicity isn't necessarily going to handle
> all the corner cases robustly and efficiently.  Resource shortages are
> a *HUGE* category of corner cases.

Agreed.

thanks,

greg k-h

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