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[PATCH] MIPS: allow 24Hz timer frequency

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  [PATCH] MIPS: allow 24Hz timer frequency Paul Burton <paul.burton@imgtec.com> - 2015-09-22 19:20 +0200
    Re: [PATCH] MIPS: allow 24Hz timer frequency Ralf Baechle <ralf@linux-mips.org> - 2015-09-22 20:10 +0200

#1230518 — [PATCH] MIPS: allow 24Hz timer frequency

A boundary exists beyond which the timer frequency becomes high enough
that timer interrupts saturate the system and either cause it to slow to
a crawl or stop functioning entirely. Where that boundary lies depends
upon a number of factors such as the overhead of each interrupt and the
overall speed of the CPU, but correlates strongly with the clock
frequency at which the CPU runs. When running on emulators during
bringup or debug of a CPU that clock frequency is very low, which
results in the boundary at which the timer frequency becomes
unsustainable being very low. The current minimum of 48Hz pushes against
boundary in certain situations in current systems. Allow the kernel to
be configured for a 24Hz timer frequency in order to avoid problems on
such slow running systems.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
---

 arch/mips/Kconfig | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index e3aa5b0..79cac90 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2529,6 +2529,9 @@ choice
 	help
 	 Allows the configuration of the timer frequency.
 
+	config HZ_24
+		bool "24 HZ" if SYS_SUPPORTS_24HZ || SYS_SUPPORTS_ARBIT_HZ
+
 	config HZ_48
 		bool "48 HZ" if SYS_SUPPORTS_48HZ || SYS_SUPPORTS_ARBIT_HZ
 
@@ -2552,6 +2555,9 @@ choice
 
 endchoice
 
+config SYS_SUPPORTS_24HZ
+	bool
+
 config SYS_SUPPORTS_48HZ
 	bool
 
@@ -2575,13 +2581,18 @@ config SYS_SUPPORTS_1024HZ
 
 config SYS_SUPPORTS_ARBIT_HZ
 	bool
-	default y if !SYS_SUPPORTS_48HZ && !SYS_SUPPORTS_100HZ && \
-		     !SYS_SUPPORTS_128HZ && !SYS_SUPPORTS_250HZ && \
-		     !SYS_SUPPORTS_256HZ && !SYS_SUPPORTS_1000HZ && \
+	default y if !SYS_SUPPORTS_24HZ && \
+		     !SYS_SUPPORTS_48HZ && \
+		     !SYS_SUPPORTS_100HZ && \
+		     !SYS_SUPPORTS_128HZ && \
+		     !SYS_SUPPORTS_250HZ && \
+		     !SYS_SUPPORTS_256HZ && \
+		     !SYS_SUPPORTS_1000HZ && \
 		     !SYS_SUPPORTS_1024HZ
 
 config HZ
 	int
+	default 24 if HZ_24
 	default 48 if HZ_48
 	default 100 if HZ_100
 	default 128 if HZ_128
-- 
2.5.3

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#1230621

On Tue, Sep 22, 2015 at 10:16:39AM -0700, Paul Burton wrote:

> A boundary exists beyond which the timer frequency becomes high enough
> that timer interrupts saturate the system and either cause it to slow to
> a crawl or stop functioning entirely. Where that boundary lies depends
> upon a number of factors such as the overhead of each interrupt and the
> overall speed of the CPU, but correlates strongly with the clock
> frequency at which the CPU runs. When running on emulators during
> bringup or debug of a CPU that clock frequency is very low, which
> results in the boundary at which the timer frequency becomes
> unsustainable being very low. The current minimum of 48Hz pushes against
> boundary in certain situations in current systems. Allow the kernel to
> be configured for a 24Hz timer frequency in order to avoid problems on
> such slow running systems.

The current minimum of 48Hz in the MIPS kconfig files predates the
rewrites to use clocksource/clockevent device in 2.6.27 or so.  The
value of 48Hz is the lowest value at which the old time code was still
working properly.  Afair the change was submitted by Kevin Kissel or
Chris Dearman to improve performance on with emulated CPUs which by that
time were running at like 500kHz (I think it was an IKOS) clock rate.
Presumably below 48Hz the math in the kernel's time code was falling
apart but anyway, afaics the value was derived experimentally.

  Ralf
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