Re: [PATCH] Revert 9fc2105aeaaf56b0cf75296a84702d0f9e64437b to fix pyaudio (and probably more)

[Pavel Machek <pavel@ucw.cz>]

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.pitre@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.pitre@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 <nico@linaro.org>

Acked-by: Pavel Machek <pavel@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)

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html