linux-kernel - Re: [PATCH 0/3] Fixing udelay() on SMP (and non-SMP too)

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Date:	Wed, 22 Sep 2010 14:54:47 -0700
From:	Daniel Walker <dwalker@...eaurora.org>
To:	Russell King <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Kevin Hilman <khilman@...prootsystems.com>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Saravana Kannan <skannan@...eaurora.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Colin Cross <ccross@...roid.com>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH 0/3] Fixing udelay() on SMP (and non-SMP too)


Russell

Is this series something you would be willing to pull into your tree?

Daniel

On Tue, 2010-09-07 at 11:23 -0700, Stephen Boyd wrote:
> (Sorry, resending due to stray comma...)
> 
> These patches are another attempt at fixing the udelay()
> issue pointed out on arm-lkml[1][2]. A quick recap: some SMP
> machines can scale their CPU frequencies independent of one
> another. loops_per_jiffy is calibrated globally and used in
> __const_udelay(). If one CPU is running faster than what the
> loops_per_jiffy is calculated (or scaled) for, udelay() will
> be incorrect and not wait long enough (or too long). A similar
> problem occurs if the cpu frequency is scaled during a udelay()
> call.
> 
> We could fix this issue a couple ways, wholesale replacement
> of __udelay() and __const_udelay() (see [2] for that approach),
> or replacement of __delay() (this series). Option 1 can fail if
> anybody uses udelay() before memory is mapped and also duplicates
> most of the code in asm/delay.h. It also needs to hardcode the
> timer tick frequency, which can sometimes be inaccurate. The
> benefit is that loops_per_jiffy stays the same and thus BogoMIPS
> is unchanged.  Option 2 can't fail since the __delay() loop is
> replaced after memory is mapped in, but it suffers from a low
> BogoMIPS when timers are clocked slowly. It also more accurately
> calculates the timer tick frequency through the use of
> calibrate_delay_direct().
> 
> -- Reference --
> [1] http://article.gmane.org/gmane.linux.kernel/977567
> [2] http://article.gmane.org/gmane.linux.ports.arm.kernel/78496 
> 
> Stephen Boyd (3):
>   [ARM] Translate delay.S into (mostly) C
>   [ARM] Allow machines to override __delay()
>   [ARM] Implement a timer based __delay() loop
> 
>  arch/arm/include/asm/delay.h |    5 ++-
>  arch/arm/kernel/armksyms.c   |    4 --
>  arch/arm/lib/delay.S         |   65 ------------------------------
>  arch/arm/lib/delay.c         |   90 ++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 94 insertions(+), 70 deletions(-)
>  delete mode 100644 arch/arm/lib/delay.S
>  create mode 100644 arch/arm/lib/delay.c
> 

-- 

Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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