[RFC/NOT FOR MERGING] HACK: add global/private timers for A9

Felipe Balbi
2015-06-03T21:28:21Z

On Wed, Jun 03, 2015 at 04:04:55PM -0500, Felipe Balbi wrote:
> On Wed, Jun 03, 2015 at 10:55:27PM +0200, Arnd Bergmann wrote:
> > On Wednesday 03 June 2015 15:32:45 Felipe Balbi wrote:
> > >
> > > Hi Tony and Russell,
> > >
> > > AM43xx, even though it's a single processor A9, it still has TWD and global
> > > timer. I was doing some profiling with RT v4.0 and latency is 3.5x lower just
> > > by switching from gptimer to twd/global.
> > >
> > > The only problem is that currently, is_smp() check prevents me from using twd
> > > with AM43xx (that's why it's commented below, for testing purposes).
> > >
> > > In the hopes that we can start a, hopefully, small thread around the subject,
> > > I'm sending this HACK which I used to get TWD and global timer enabled so I
> > > could measure latencies with cyclictest.
> > >
> > > Is it so that TWD shouldn't be available on UP integrations of ARM's Cortex-A
> > > processors ?
> > >
> > >
> >
> > I wondered about this recently when looking at something unrelated
> > and noticed that the check had been introduced as part of
> > 904464b91eca8 ("ARM: 7655/1: smp_twd: make twd_local_timer_of_register()
> > no-op for nosmp").
> >
> > I suspect this was just the wrong fix at the time, and that the
> > real culprit is either alloc_percpu() or request_percpu_irq()
> > getting called too early on a machine without SMP support.
> >
> > Possibly the problem is already resolved independently, if you
> > didn't run into it.
>
> no, no splats, nothing at all. See [1]
>
> [1] http://hastebin.com/helekubutu

Adding Shawn

--
balbi
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