Create powersaving guide

POWERSAVING.md

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+# Asus Zephyrus G14 GPU powersaving
+
+In order to get the best battery life, the dedicated GPU must be disabled. 
+
+Simply unloading the driver or enabling powersaving feaures isn't enough.
+
+In the future, the nvidia driver may allow for entering D3 (full power off) on it's own, but for now, there's another solution: acpi calls. This requires the `acpi_call` kernel module, on Arch this can be installed with the `acpi_call` or `acpi_call-dkms` package.
+
+Thanks to /u/FailSpai on reddit for finding the correct ACPI call. The GPU can be powered off and on with the call "\\_SB.PCI0.GPP0.PG00". For example, to turn off the GPU completely, run `sudo sh -c 'echo "\\_SB.PCI0.GPP0.PG00._OFF" > /proc/acpi/call'`. Similarly, to turn it back on, `sudo sh -c 'echo "\\_SB.PCI0.GPP0.PG00._ON" > /proc/acpi/call'`. You can verify the state of the GPU by running `sudo sh -c 'echo "\\_SB.PCI0.GPP0.PEGP.SGST" > /proc/acpi/call'`, then running `sudo cat /proc/acpi/call`. 0x0 means off, and anything above 0 means on. Alternatively, just look at the power draw, `cat /sys/class/power_supply/BAT0/power_now` and if it's under 10-11w then it's off.
+
+This allows for the same, or better, idle power draw than Windows, as long as you have done other tweaks (disable boost, change CPU governor, etc).
+
+If the GPU is powered off during suspend, the system will hang. This can be solved by using a systemd hook to turn the GPU back on before suspend, then turning it back off after. For example, place this script in an executable file in `/usr/lib/systemd/system-sleep/`:
+
+```
+#!/bin/bash
+
+if [ "${1}" == "pre" ]; then
+	sudo sh -c 'echo "\\_SB.PCI0.GPP0.PG00._ON" > /proc/acpi/call'
+    sleep 1 # The GPU needs a bit of time to ensure that it's on before suspend
+elif [ "${1}" == "post" ]; then
+	if xrandr --listproviders | grep NVIDIA; then
+	    echo "GPU is in use for PRIME, keeping on"
+    else
+	    sudo sh -c 'echo "\\_SB.PCI0.GPP0.PG00._OFF" > /proc/acpi/call'
+        echo "GPU powered off"
+    fi  
+fi
+```
+
+As the script suggests, you can't power off the GPU while it's in use. I would recommend using optimus-manager-amd (available on the AUR as optimus-manager-amd-git) to automatically handle switching Xorg sessions and automatically depowering the GPU when it's not used. You can see my config files for optimus-manager-amd [here](https://git.deck.sh/shark/g14gpu/-/tree/master/etc/optimus-manager). When in `amd` mode, I only draw 5.5-6w on idle, and it bounces around up to 10w when using firefox with the second half of the cores disabled.
+
+Again, thanks to /u/FailSpai for digging through the ACPI tables to find the right call, you can find his comment about it [here](https://www.reddit.com/r/VFIO/comments/hx5j8q/success_with_laptop_gpu_passthrough_on_asus_rog/g0m3kvh/).