On Sun 2015-01-04 23:51:31, Nicolas Pitre wrote:
> On Sun, 4 Jan 2015, Russell King - ARM Linux wrote:
>
> > On Sun, Jan 04, 2015 at 04:20:57PM -0500, Nicolas Pitre wrote:
> > > On Sun, 4 Jan 2015, Linus Torvalds wrote:
> > >
> > > > On Sun, Jan 4, 2015 at 12:56 PM, Nicolas Pitre
> > > > <nicolas.pi...@linaro.org> wrote:
> > > > >
> > > > > It wasted a lot of people's time before by simply being there and
> > > > > wrong
> > > > > before it was removed. It's only a matter of whose time you want to
> > > > > waste. Really.
> > > >
> > > > Really. Shut up.
> > > >
> > > > The whole "no regressions" thing is very much about the fact that we
> > > > don't waste users time.
> > >
> > > I was talking about users time all along.
> > >
> > > Never mind. I'm sorry for the NAK and sorry for attempting to start a
> > > discussion to find a better replacement.
> >
> > Nico,
> >
> > I encourage you *not* to back down like this. Linus is right in so far
> > as the regressions issue, but he is *totally* wrong to do the revert,
> > which IMHO has been done out of nothing more than spite.
> >
> > Either *with or without* the revert, the issue still remains, and needs
> > to be addressed properly.
> >
> > With the revert in place, we now have insanely small bogomips values
> > reported via /proc/cpuinfo when hardware timers are used. That needs
> > fixing.
>
> Here's my take on it. Taking a step back, it was stupid to mix bogomips
> with timer based delays.
>
> ----- >8
> From: Nicolas Pitre <nicolas.pi...@linaro.org>
> Date: Sun, 4 Jan 2015 22:28:58 -0500
> Subject: [PATCH] ARM: disentangle timer based delays and bogomips calibration
>
> The bogomips value is a pseudo CPU speed value originally used to calibrate
> loop-based small delays. It is also exported to user space through the
> /proc filesystem and some user space apps started relying on it.
>
> Modern hardware can vary their CPU clock at run time making the bogomips
> value less reliable for delay purposes. With the advent of high resolution
> timers, small delays migrated to timer polling loops instead. Strangely
> enough, the bogomips value calibration became timer based too, making it
> way more bogus than it already was as a CPU speed representation and people
> using it via /proc/cpuinfo started complaining.
>
> Since it was wrong for user space to rely on a "bogus" mips value to start
> with, the initial responce from kernel people was to remove it. This broke
> user space even more as some applications then refused to run altogether.
> The bogomips export was therefore reinstated in commit 4bf9636c39 ("Revert
> 'ARM: 7830/1: delay: don't bother reporting bogomips in /proc/cpuinfo'").
>
> Because the reported bogomips is orders of magnitude away from the
> traditionally expected value for a given CPU when timer based delays are
> in use, and because lumping bogomips and timer based delay loops is rather
> senseless anyway, let's calibrate bogomips using a CPU loop all the time
> even when timer based delays are available. Timer based delays don't
> need any calibration and /proc/cpuinfo will provide somewhat sensible
> values again.
>
> In practice, calls to __delay() will now always use the CPU based loop.
> Things remain unchanged for udelay() and its derivatives.
>
> Signed-off-by: Nicolas Pitre <n...@linaro.org>
Acked-by: Pavel Machek <pa...@ucw.cz>
(No, I did not test it, but going to "historical" value should not
hurt, and it actually makes code shorter).
> diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
> index dff714d886..7feeb163f5 100644
> --- a/arch/arm/include/asm/delay.h
> +++ b/arch/arm/include/asm/delay.h
> @@ -21,13 +21,12 @@ struct delay_timer {
> };
>
> extern struct arm_delay_ops {
> - void (*delay)(unsigned long);
> void (*const_udelay)(unsigned long);
> void (*udelay)(unsigned long);
> unsigned long ticks_per_jiffy;
> } arm_delay_ops;
>
> -#define __delay(n) arm_delay_ops.delay(n)
> +#define __delay(n) __loop_delay(n)
>
> /*
> * This function intentionally does not exist; if you see references to
> @@ -63,7 +62,6 @@ extern void __loop_udelay(unsigned long usecs);
> extern void __loop_const_udelay(unsigned long);
>
> /* Delay-loop timer registration. */
> -#define ARCH_HAS_READ_CURRENT_TIMER
> extern void register_current_timer_delay(const struct delay_timer *timer);
>
> #endif /* __ASSEMBLY__ */
> diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
> index 312d43eb68..d958886874 100644
> --- a/arch/arm/lib/delay.c
> +++ b/arch/arm/lib/delay.c
> @@ -30,7 +30,6 @@
> * Default to the loop-based delay implementation.
> */
> struct arm_delay_ops arm_delay_ops = {
> - .delay = __loop_delay,
> .const_udelay = __loop_const_udelay,
> .udelay = __loop_udelay,
> };
> @@ -86,12 +85,8 @@ void __init register_current_timer_delay(const struct
> delay_timer *timer)
> if (!delay_calibrated && (!delay_res || (res < delay_res))) {
> pr_info("Switching to timer-based delay loop, resolution
> %lluns\n", res);
> delay_timer = timer;
> - lpj_fine = timer->freq / HZ;
> delay_res = res;
> -
> - /* cpufreq may scale loops_per_jiffy, so keep a private copy */
> - arm_delay_ops.ticks_per_jiffy = lpj_fine;
> - arm_delay_ops.delay = __timer_delay;
> + arm_delay_ops.ticks_per_jiffy = timer->freq / HZ;
> arm_delay_ops.const_udelay = __timer_const_udelay;
> arm_delay_ops.udelay = __timer_udelay;
> } else {
> @@ -102,7 +97,9 @@ void __init register_current_timer_delay(const struct
> delay_timer *timer)
> unsigned long calibrate_delay_is_known(void)
> {
> delay_calibrated = true;
> - return lpj_fine;
> +
> + /* calibrate bogomips even when timer based delays are used */
> + return 0;
> }
>
> void calibration_delay_done(void)
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures)
http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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