[PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems

[Russell King - ARM Linux]

On Thu, May 08, 2014 at 01:43:29PM -0400, Nicolas Pitre wrote:
> There might be some cases where precise timing is needed though.
> I thought I came across one such case in the past but I can't remember 
> which.

Anything which is expecting precise timings from udelay() is broken.
Firstly, udelay() does _not_ guarantee to give you a delay of at least
the requested period - it tries to give an _approximate_ delay.

The first thing to realise is that loops_per_jiffy is calibrated with
interrupts _on_, which means that the calculated loops_per_jiffy is
the number of iterations in a jiffy _minus_ the time it takes for the
timer interrupt to be processed.  This means loops_per_jiffy will
always be smaller than the number of loops that would be executed
within the same period.

This leads to udelay() always producing slightly shorter than
requested delays - this is quite measurable.

It gets worse when you consider the utter mess that the L2 cache code
is in - where on many platforms we calibrate udelay() with the cache
off, which results in loops_per_jiffy being smaller than it would
otherwise be (meaning shorter delays.)

So, that's two reasons there why loops_per_jiffy will be smaller than
it should be at boot, and therefore udelay() will be inaccurate.

Another reason udelay() can be unaccurate is if interrupts are on, and
you have USB present.  USB interrupt processing can take on the order
of 10s of milliseconds even on 800MHz or faster ARM CPUs.  If you
receive one of those mid-udelay(), your CPU will be occupied elsewhere.

Another reason is preempt.  If the kernel can preempt during udelay(),
your delay will also be much longer than you requested.  No, disabling
preemption in udelay() is not on, that would increase preemption
latency.

So, the only /real/ solution if you want proper delays is for udelay()
to use a timer or counter, and this is should always the preferred
method where it's available.  Quite rightly, we're not hacking udelay()
stuff to work around not having that, or if someone configures it out.

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