Linux - Revision history

Gary.bisson at 15:49, 7 December 2021

2021-12-07T15:49:16Z

Gary.bisson at 16:44, 26 March 2021

2021-03-26T16:44:24Z

Gary.bisson: Created page with "This page includes everything you need to know in order to use a GNU/Linux OS. All the articles below are generic for all platforms and therefore apply whether you use Yocto..."

2021-03-26T16:35:13Z

Created page with "This page includes everything you need to know in order to use a GNU/Linux OS. All the articles below are generic for all platforms and therefore apply whether you use Yocto..."

New page

This page includes everything you need to know in order to use a GNU/Linux OS.

All the articles below are generic for all platforms and therefore apply whether you use Yocto, Buildroot, OpenWRT, Ubuntu, Debian and so on.

1. Kernel

2. Init and init scripts

3. Simple commands

4. Connectivity interfaces

5. Multimedia interfaces

6. USB devices and gadgets

7. GPIO interface

8. SPI interface

9. I2C interface

10. Serial interface (TTY)

11. PCIe

12. SDIO

13. Power management

14. Display management

15. GUI interfaces

← Older revision		Revision as of 15:49, 7 December 2021
Line 42:		Line 42:
	== Power management ==		== Power management ==

		+	This section will cover a few topics related to power management under Linux.
		+
		+	First, regarding the CPU frequencies, the Linux kernel offers the [https://www.kernel.org/doc/html/latest/admin-guide/pm/cpufreq.html cpufreq] (CPU Frequency scaling) subsystem.
		+
		+	This subsystem is supported on all our platforms and we will see below how it can be used.
		+
		+	You can see the different frequencies supported by the CPU as follows (tested on Nitrogen8MP):
		+	<pre># cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_frequencies
		+	1200000 1600000 1800000</pre>
		+	Values are in '''kHz''' so the above means that the CPU supports to run at 1.2, 1.6 or 1.8 GHz.
		+
		+	You can check the current CPU frequency:
		+	<pre># cat /sys/devices/system/cpu/cpufreq/policy0/scaling_cur_freq
		+	1200000</pre>
		+
		+	Then the CPU can use different '''scaling governors''' which implement algorithms to estimate the required CPU capacity. As a rule, each governor implements one, possibly parametrized, scaling algorithm.
		+	<pre># cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_governors
		+	conservative ondemand userspace powersave performance schedutil </pre>
		+	The governors are defined by kernel:
		+	* '''ondemand''': uses CPU load as a CPU frequency selection metric
		+	* '''conservative''': uses CPU load as a CPU frequency selection metric like ondemand but with a different algorithm
		+	* '''schedutil''': uses CPU utilization data available from the CPU scheduler instead of the load
		+	* '''performance''': causes the highest frequency, within the scaling_max_freq policy limit, to be requested
		+	* '''powersave''': causes the lowest frequency, within the scaling_min_freq policy limit, to be requested
		+	* '''userspace''': allows user space to set the CPU frequency for the policy it is attached to
		+
		+	You can get/set the current governor as shown below:
		+	<pre># cat /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
		+	ondemand
		+	# echo powersave > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
		+	# cat /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
		+	powersave</pre>
		+	Another interesting metric is the time spent in each frequency:
		+	<pre># cat /sys/devices/system/cpu/cpufreq/policy0/stats/time_in_state
		+	1200000 298542
		+	1600000 47
		+	1800000 118</pre>
		+	In the above we can see that the CPU spends most of its time @1.2GHz which means its load is pretty low.
		+
		+	For more details about cpufreq, best is to go through the [https://www.kernel.org/doc/html/latest/admin-guide/pm/cpufreq.html cpufreq kernel documentation].

	== Display management ==		== Display management ==

@@ Line 1: / Line 1: @@
 This page includes everything you need to know in order to use a GNU/Linux OS.
 All the articles below are generic for all platforms and therefore apply whether you use Yocto, Buildroot, OpenWRT, Ubuntu, Debian and so on.
-. Kernel
+== Kernel ==
-. Init and init scripts
+== Init and init scripts ==
-. Simple commands
+== Simple commands ==
-. Connectivity interfaces
+== Connectivity interfaces ==
-. Multimedia interfaces
+== Multimedia interfaces ==
-. USB devices and gadgets
+==  USB devices and gadgets ==
-. GPIO interface
+==  GPIO interface ==
-. SPI interface
+==  SPI interface ==
-. I2C interface
+== I2C interface ==
-. Serial interface (TTY)
+== Serial interface (TTY) ==
-. PCIe
+== PCIe ==
-. SDIO
+== SDIO ==
-. Power management
+== Power management ==
-. Display management
+== Display management ==
-. GUI interfaces
+== GUI interfaces ==