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

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Date:	Thu, 8 Jan 2015 10:39:12 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	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 Wed, Jan 07, 2015 at 10:14:07PM +0000, Catalin Marinas wrote:
> On 7 January 2015 at 20:53, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > Now, if I understand Linus correctly, what irks him is when someone
> > upgrades a kernel on a platform, and some userland breaks.  That's
> > something which I've said multiple times I don't have a problem
> > agreeing with, and I suspect no one in this thread would disagree
> > that this is a serious failing, and one which needs fixing ASAP.
> 
> Agree. But I assume you refer to the fact that we removed the BogoMIPS
> reporting. It's fine to have it reverted.
> 
> > However, if running userland on platform A works, and but it doesn't
> > work on platform B.  The breakage may well be due to platform A reporting
> > 300 bogomips because it's using the kernel software loop, and platform
> > B reporting 6 bogomips because its using a hardware timer, but the CPU
> > is actually faster.  However, this is not a kernel problem, and it
> > certainly is not a regression.  It's a userspace bug which needs
> > userspace to fix.
> 
> We need to look back at the point we added timer-based delay about 2.5
> years ago. Prior to commit d0a533b18235d362, platform A reported
> bogomips 300. After that commit, the *same* platform A (not B),
> started reported 6.

Correct.

> Is the above considered user breakage? That's what Nico is trying to
> solve. If we are fine with it, than we can close this thread, no
> further changes needed.

It's not a regression - yet.  No one has shown that userspace has broken
according to the definition of the first quote above, and that's the
whole point.

> We can document it as Linus suggests and say that prior to whatever
> version we had 2.5 years ago, BogoMIPS was based on a busy loop. In
> more recent kernels, it is based on a timer delay. User space should
> make use of such information when interpreting BogoMIPS.

We don't need to document it - we just need to point people at this
URL:

http://en.wikipedia.org/wiki/BogoMips

which already describes it fully, including the existing ARM timer
behaviour.

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