Re: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[David Engraf]

Am 17.01.2017 um 10:13 schrieb Daniel Lezcano:
> On 11/01/2017 14:50, David Engraf wrote:
> > On newer boards the TC can be read as single 32 bit value without locking.
> > Thus the clock can be used as reference for sched_clock which is much more
> > accurate than the jiffies implementation.
> >
> > Tested on a Atmel SAMA5D2 board.
>
> Is this change backward compatible ?

The way how the driver reads the clock hasn't been changed. I've only 
introduced the call to sched_clock_register() which will use this clock 
for the scheduler as long as no better clock is already used. So I don't 
see a backward compatibility problem.

Best regards
- David

> > Signed-off-by: David Engraf <david.engraf@xxxxxxxxx>
> > ---
> >  drivers/clocksource/tcb_clksrc.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/clocksource/tcb_clksrc.c b/drivers/clocksource/tcb_clksrc.c
> > index d4ca996..745844e 100644
> > --- a/drivers/clocksource/tcb_clksrc.c
> > +++ b/drivers/clocksource/tcb_clksrc.c
> > @@ -10,6 +10,7 @@
> >  #include <linux/io.h>
> >  #include <linux/platform_device.h>
> >  #include <linux/atmel_tc.h>
> > +#include <linux/sched_clock.h>
> >
> >
> >  /*
> > @@ -56,11 +57,16 @@ static u64 tc_get_cycles(struct clocksource *cs)
> >  	return (upper << 16) | lower;
> >  }
> >
> > -static u64 tc_get_cycles32(struct clocksource *cs)
> > +static u32 tc_get_cv32(void)
> >  {
> >  	return __raw_readl(tcaddr + ATMEL_TC_REG(0, CV));
> >  }
> >
> > +static u64 tc_get_cycles32(struct clocksource *cs)
> > +{
> > +	return tc_get_cv32();
> > +}
> > +
> >  static struct clocksource clksrc = {
> >  	.name           = "tcb_clksrc",
> >  	.rating         = 200,
> > @@ -69,6 +75,11 @@ static struct clocksource clksrc = {
> >  	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
> >  };
> >
> > +static u64 notrace tc_read_sched_clock(void)
> > +{
> > +	return tc_get_cv32();
> > +}
> > +
> >  #ifdef CONFIG_GENERIC_CLOCKEVENTS
> >
> >  struct tc_clkevt_device {
> > @@ -339,6 +350,9 @@ static int __init tcb_clksrc_init(void)
> >  		clksrc.read = tc_get_cycles32;
> >  		/* setup ony channel 0 */
> >  		tcb_setup_single_chan(tc, best_divisor_idx);
> > +
> > +		/* register sched_clock on chips with single 32 bit counter */
> > +		sched_clock_register(tc_read_sched_clock, 32, divided_rate);
> >  	} else {
> >  		/* tclib will give us three clocks no matter what the
> >  		 * underlying platform supports.