On 5 January 2015 at 04:51, Nicolas Pitre <nicolas.pi...@linaro.org> wrote:
> 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.
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.mari...@arm.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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