Add about CPU/GPU overclocking

Signed-off-by: Dongjin Kim <[email protected]>

guides/overclock.md

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+# CPU/GPU frequency and governor
+> :warning: The commands in this section only affects to the product with S905X3 and
+S922X.
+
+## CPU
+The min/max CPU frequency and governor can be changed by editing
+'/etc/default/cpufrequtils' and it will be applied from next booting.
+
+```bash
+$ cat /etc/default/cpufrequtils
+GOVERNOR=performance
+MIN_SPEED="1000MHZ"
+MAX_SPEED="2016MHZ"
+```
+
+The change in '/etc/default/cpufrequtils' will affect to big.LITTLE cores at
+the same time and will use the most closet value if given freqency does not
+exist. Valid frequencies are able to check with this command.
+```bash
+$ cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_frequencies
+$ cat /sys/devices/system/cpu/cpufreq/policy2/scaling_available_frequencies
+```
+
+### Overclocking
+
+CPU overclock can happen if you change the value of 'MAX_SPEED' in
+/etc/defaults/cpufrequtils but could lead malfunction or shutdown
+unexpected since they are not officially supported by SoC vendor.
+The feasible freqencies varies to the CPU.
+
+#### ODROID-N2 ####
+* 1908MHz
+* 2004MHz
+
+#### ODROID-N2Plus ####
+* 2304MHz
+* 2400MHz
+
+#### ODROID-C4/HC4 ####
+* 2016MHz
+* 2100MHz
+
+## GPU
+
+The min/max GPU frequency and governor can be changed by editing
+'/etc/default/gpufrequtils' and it will be applied from next booting.
+
+> :warning: The commands in this section only after Linux kernel 5.10.16.
+
+```bash
+$ uname -a
+Linux focal-server 5.10.0-odroid-arm64 #1 SMP PREEMPT Ubuntu 5.10.16-202102191318~groovy (2021-02-19) aarch64 aarch64 aarch64 GNU/Linux
+```
+
+```bash
+$ cat /etc/default/gpufrequtils
+# GOVERNORS:    [userspace powersave performance simple_ondemand]
+# FREQUENCIES : [124999998 249999996 285714281 399999994
+#                499999992 666666656 799999987]
+GOVERNOR=performance
+MIN_FREQ=499999992
+MAX_FREQ=799999987
+```
+The change in '/etc/default/gpufrequtils' will affect to GPU and will use the
+most closest value if given frequency does not exist. Valid frequencies are
+able to check with this command.
+
+#### With Mali Bifrost driver
+<ul>
+<li>/sys/devices/platform/soc/ffe40000.gpu/misc/mali0/device/devfreq/ffe40000.gpu/governor</li>
+<li>/sys/devices/platform/soc/ffe40000.gpu/misc/mali0/device/devfreq/ffe40000.gpu/min_freq</li>
+<li>/sys/devices/platform/soc/ffe40000.gpu/misc/mali0/device/devfreq/ffe40000.gpu/max_freq</li>
+</ul>
+
+#### With Mali Panfrost driver
+<ul>
+<li>/sys/devices/platform/soc/ffe40000.gpu/devfreq/ffe40000.gpu/governor</li>
+<li>/sys/devices/platform/soc/ffe40000.gpu/devfreq/ffe40000.gpu/max_freq</li>
+<li>/sys/devices/platform/soc/ffe40000.gpu/devfreq/ffe40000.gpu/min_freq</li>
+</ul>
+
+### Overclocking
+
+GPU overclock can happen if you change the value of 'MAX_SPEED' in
+/etc/defaults/gpufrequtils but could lead malfunction or shutdown
+unexpected since they are not officially supported by SoC vendor.
+The fasible frequency for overclocking is only 1000MHz.
+
+```bash
+MAX_FREQ=1000000000
+```