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

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Date:	Wed, 7 Jan 2015 18:11:15 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>,
	Marc Zyngier <marc.zyngier@....com>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Revert 9fc2105aeaaf56b0cf75296a84702d0f9e64437b to fix
 pyaudio (and probably more)

On 5 January 2015 at 04:51, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> From: Nicolas Pitre <nicolas.pitre@...aro.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.

I think this makes sense since __delay() expects the number of loops
as argument rather than a duration scaled by some factor (based on the
generic timer frequency).

FWIW:

Acked-by: Catalin Marinas <catalin.marinas@....com>

Minor comment below:

>  unsigned long calibrate_delay_is_known(void)
>  {
>         delay_calibrated = true;
> -       return lpj_fine;
> +
> +       /* calibrate bogomips even when timer based delays are used */
> +       return 0;
>  }

Do we need to remove delay_calibrated = true as well? We do it further
down again in calibration_delay_done() .

-- 
Catalin
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