linux-kernel - Re: [RFC PATCH] sched_clock: also call register_current_timer

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Date:	Wed, 30 Apr 2014 13:48:00 +0100
From:	Will Deacon <will.deacon@....com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Russell King <linux@....linux.org.uk>,
	John Stultz <john.stultz@...aro.org>,
	Theodore Ts o <tytso@....edu>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] sched_clock: also call
 register_current_timer_delay() if possible

Hi Sebastian,

On Wed, Apr 30, 2014 at 01:23:34PM +0100, Sebastian Andrzej Siewior wrote:
> On ARM one has to call register_current_timer_delay() in order to use
> the (quick) timer based calibrarion instead of the a little slower loop
> based delay.
> The timer function specified in register_current_timer_delay() is also
> used by read_current_timer() which would otherwise return -ENXIO.
> And read_current_timer() timer is used by get_cycles() which in turn is
> used by random_get_entropy(). That means each sub-architecture returned
> here 0 if register_current_timer_delay() was no performed.
> 
> The parameters for for sched_clock_register() and
> register_current_timer_delay() are mostly the same. Instead of calling
> register_current_timer_delay() each time just after (or before)
> sched_clock_register() I thought it might easier by doing it once at
> sched_clock time since the parameter are the same. That means each ARM sub
> arch that working sched-clock would also have a working random_get_entropy()
> implementation.
> 
> Any comments?

As long as sched_clock is guaranteed to be a fixed frequency, always-on
clocksource then this could work, but it removes the flexibility of having
a separate delay clock and sched clock (is this useful?).

Looking at your patch, I noticed that we need to extend the
register_current_timer_delay function to deal with clocks that aren't as
wide as cycle_t, otherwise we don't delay() for long enough when the clock
overflows (this is potentially already an issue for architected timers <
64-bit). Could you cook a patch for that please?

Will
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