[Patch] fix MTD CFI/LPDDR flash driver huge latency bug

[David Woodhouse]

On Mon, 2010-03-15 at 14:24 +0000, Jamie Lokier wrote:
> > > > The problem is the memcpy operation which is very slow. A cond_resched
> > > > wouldn't help, since the cpu bus is blocked during the transfer of the
> > > > word.
> > > 
> > > I mean split the memcpy into multiple smaller memcpys, so that the
> > > total time in each memcpy is limited to something reasonable.
> > > 
> > > The check in cond_resched() is fast, especially once cached.  memcpy
> > > speed depends a lot on the attached flash and how everything's
> > > configured, varying from 2.5MB/s up to hundreds of MB/s.  So how about
> > > doing cond_resched() every 256 bytes?
> > > 
> > > -- Jamie
> > 
> > I thoght about this aporoach and i don't like this idea. Why not using a
> > preemptible kernel?
> 
> Because it introduces too many risks to enable CONFIG_PREEMPT in a
> stable rolled out device which isn't using it already.  Especially on
> devices where it's not well tested by other people, and with drivers
> that nobody ever used with CONFIG_PREEMPT before.
> 
> And because CONFIG_PREEMPT isn't always better.  (Why do you think
> it's a config option?)
> 
> As a bug fix for observed high scheduling latency when a flash I/O is
> occurring, splitting the memcpys is a good choice.  I will be trying
> it on my kernels, even if it doesn't get mainlined.  Thanks for the idea ;-)

Rather than pulling a number our of our posterior like "every 256 bytes"
which might _really_ screw up performance of some architectures' memcpy
routines, I suspect we might want the platform to provide an optimised
"sleepable_memcpy" function which does it at whatever interval is
appropriate for the memcpy routine in use. Or magically makes it
preemptable. Or uses a DMA engine. Or whatever.

I wonder where else we could use such a function...

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse at intel.com                              Intel Corporation