[PATCH] usb: ehci: make HC see up-to-date qh/qtd descriptor ASAP

[Mark Salter]

On Wed, 2011-08-31 at 20:35 +0100, Will Deacon wrote:
> On Wed, Aug 31, 2011 at 07:19:33PM +0100, Rob Herring wrote:
> > On 08/31/2011 12:51 PM, Will Deacon wrote:
> > > Another thing that Marc and I tried on OMAP4 was not bringing up the secondary
> > > CPU during boot (by commenting out most of smp_init). In this case, I/O
> > > performance was good until we tried to online the secondary CPU. The online
> > > failed but after that the I/O performance was certainly degraded.
> > > 
> > 
> > Was the SCU enabled at that point? One diff between nosmp boot and
> > offlining the 2nd core would be that the SCU remains enabled in the
> > latter case. I think the SCU does not get enabled for nosmp.
> 
> Our rudimentary test (printing out the SCU control register during boot)
> showed that it *was* enabled for nosmp. I think this is due to the secure
> world having to do that on OMAP so it's probably not true for other
> platforms.

I've done a little test and found that turning on the MMU of the second
core causes the problem to show up. I patched head.S so I stopped the
second core in an infinite loop just before turning on the MMU. The
system continues booting on core#0 and I see ~20MB/s with hdparm -t to
an attached usb disk. Same setup but with second core being stopped with
infinite loop just after MMU is enabled shows ~5MB/s. So whatever is
going wrong, its not because of anything the second core is doing beyond
turning on its MMU and doing an empty loop.

--Mark