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The cpufreq subsystem is a Linux kernel subsystem responsible for managing the CPU frequency. It's commonly used in laptops to reduce power usage when idle.
On the S3C2410, adjusting the CPU frequency changes the clocks for almost all devices on the chip. This means that, to write a cpufreq driver for the S3C2410, you have to also adjust some values on almost all devices.
This project aims to implement the cpufreq driver (which manages the CPU frequency transitions) and a cpufreq notifier for all the affected drivers (to do two things: quiesce the device before the change, and adjust the frequency after the change). Deciding which frequency to use is the responsability of code from somewhere else (either cpufreq governors or userspace).
The most recent version of the code can be found at http://repo.or.cz/w/linux-2.6/s3c2410-cpufreq.git on the s3c2410-cpufreq-om branch.
NOTE: Do not run on real hardware, unless you really know what you are doing. This code is mostly untested, and changes frequently. It might even not compile. |
You need a fully built OpenMoko tree for the cross-compiler and uboot tools.
.config
file
make ARCH=arm CROSS_COMPILE=arm-angstrom-linux-gnueabi- oldconfig
and answer correctly the questions
make ARCH=arm CROSS_COMPILE=arm-angstrom-linux-gnueabi- uImage
CONFIG_CPU_FREQ_DEBUG
and add cpufreq.debug=7
to the kernel command line to enable the relevant debug output. You can also enable it after boot by going to the correct place at /sys/modules and changing the values there.
git format-patch -o dir master-om..s3c2410-cpufreq-om
to generate a set of patches, and apply them all on top of the OpenMoko patches 2.6.22.5 kernel, except the framebuffer patch(es) (the 2.6.24.x code for the framebuffer is different from the 2.6.22.5 one).
cpufreq.debug=7
on the kernel command line and carefully inspect the kernel output for any incorrect output.
cpufreq.debug_ratelimit=0
; it can be set via /sys/modules/cpufreq/parameters
).
userspace
and switch to the second highest frequency. Check the kernel output again (be on the lookout in particular for frequency mismatch messages from the cpufreq core).
s3c2410_cpufreq_adjust_table
and s3c2410_cpufreq_adjust_pin
should be enough) to be able to test all frequencies. Start again from the top of the list; watch in particular for the SLOW mode ones (12000 and below). There's no need to go all the way down to 1000; all SLOW mode ones should work the same, and it's very annoying. Test going back from SLOW mode to the lowest non-SLOW frequency, and jumping from SLOW mode to the maximum frequency (this last case can cause misterious crashes if the registers were set out of order, causing the bus to go too fast).
conservative
governor.
ondemand
governor (it's the one which stresses the most the cpufreq mechanism).
Driver | Status (GTA01) | Status (GTA02) |
---|---|---|
core | Working | Untested |
timer | Working | Untested |
nand | Working | Untested |
serial | Working | Untested |
framebuffer | Working | Untested |
This code resides on arch/arm/mach-s3c2410/s3c2410-cpufreq.c
and is responsible for actually changing the frequency.
The list of available frequencies and their parameters (HCLK/PCLK dividers) can be found on this file.
Timer 4 is used by the kernel as the periodic tick timer. The cpufreq notifier on arch/arm/plat-s3c24xx/time.c
is responsible for adjusting the current and reload values of the timer to match the new frequency.
The cpufreq notifier on drivers/serial/s3c2410.c
is responsible for (if possible) pausing both sides of the serial transmission before the frequency change and reloading the baud generator (and unpausing the serial transmission) after the change.
Pausing the transmission is currently only possible with hardware flow control. Since both serial ports on GTA01 use hardware flow control, that's not a problem (unless you are using the serial console).
The cpufreq notifier on drivers/video/s3c2410fb.c
is responsible for dynamically changing the frequency divider used to derive the video clocks.
The cpufreq notifier on drivers/mtd/nand/s3c2410.c
is responsible for reprogramming the NAND timings after a frequency change.
Some drivers need specific frequencies. Currently there's no way to tell the cpufreq core of that, but at minimum a cpufreq notifier should be used to turn them off if they won't be able to work with that frequency.