linux-kernel - [PATCH] powernow-k6: set transition latency value so ondemand governor can be used

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Date:	Sat, 31 Oct 2009 11:03:01 +0100
From:	Krzysztof Helt <krzysztof.h1@...pl>
To:	Dave Jones <davej@...hat.com>
Cc:	cpufreq@...r.kernel.org, LKLM <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: [PATCH] powernow-k6: set transition latency value so ondemand
 governor can be used

From: Krzysztof Helt <krzysztof.h1@...pl>

Set the transition latency to value smaller than CPUFREQ_ETERNAL so
governors other than "performance" work (like the "ondemand" one).

The value is found in "AMD PowerNow! Technology Platform Design Guide for
Embedded Processors" dated December 2000 (AMD doc #24267A). There is the
answer to one of FAQs on page 40 which states that suggested complete
transition period is 200 us.

Tested on K6-2+ CPU with K6-3 core (model 13, stepping 4).

Signed-off-by: Krzysztof Helt <krzysztof.h1@...pl>
---
This is the second version of the patch with added comment into the source code.

 arch/x86/kernel/cpu/cpufreq/powernow-k6.c |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/cpufreq/powernow-k6.c b/arch/x86/kernel/cpu/cpufreq/powernow-k6.c
index f10dea4..15f84a6 100644
--- a/arch/x86/kernel/cpu/cpufreq/powernow-k6.c
+++ b/arch/x86/kernel/cpu/cpufreq/powernow-k6.c
@@ -163,8 +163,15 @@ static int powernow_k6_cpu_init(struct cpufreq_policy *policy)
 			clock_ratio[i].frequency = busfreq * f;
 	}
 
-	/* cpuinfo and default policy values */
-	policy->cpuinfo.transition_latency = CPUFREQ_ETERNAL;
+	/*
+	 * cpuinfo and default policy values
+	 *
+	 * The value is found in "AMD PowerNow! Technology Platform
+	 * Design Guide for Embedded Processors" dated December 2000
+	 * (AMD doc #24267A). There is the answer to one of FAQs on page 40
+	 * which states that suggested complete transition period is 200 us.
+	 */
+	policy->cpuinfo.transition_latency = 200000;
 	policy->cur = busfreq * max_multiplier;
 
 	result = cpufreq_frequency_table_cpuinfo(policy, clock_ratio);
-- 
1.6.4

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