View source for User:Knuxify/Draft:Qualcomm/Adding a new SoC to mainline Linux

{{Subpage of|Qualcomm}}
{{Work-in-progress page|note=Currently in the middle of finding this out in real time :)}}

This guide will cover the process of adding support for a recent (~2020 or newer, todo?) Qualcomm SoC into mainline Linux. Most of the information contained within is also applicable to older chips, though more manual effort may be needed to get these to run. <!--For the U-Boot guide, see TODO.-->

This guide was tested with the SM7435. Some details might be different for other SoCs. If you have any hints, please contribute!

== High-level overview ==

{{todo|Some of this will have to be moved out to the top-level Qualcomm page}}

The basic components of a Qualcomm SoC are:

* The '''GCC''', or ''Global Clock Controller''; as the name suggests, it is the primary clock controller for most components.
* The other clock controllers: '''CAMCC''', '''DISPCC''', '''GPUCC''' and '''VIDEOCC'''.
* The '''RPM''', or ''Resource and Power Manager'' ('''RPMH''' or RPM ''Hardened'' in SDM845 and later); todo explanation from [https://wiki.postmarketos.org/wiki/Qualcomm_Glossary qcom glossary].
** Under the RPM is the '''RPM(H)CC''' (''RPM clock controller'') and '''RPM(H)PD''' (''RPM power domains'').
* '''QUP''', or ''Qualcomm Universal Peripheral''; the part of the SoC that provides I2C or SPI busses, as well as UART.
* '''TLMM''' (''Top Level Mode Multiplexer''); provides pinmux and GPIO pins.
* Many '''NOC''' interconnects (''Network on Chip'').
* A timer and an interrupt controller, both standard ARM parts.

These are just the most low-level components; from there, we can move on to the higher-level ones:

* Thermal monitoring sensor
* USB host controller and PHY
* Display controller and DSI PHY
* GPU
* etc.

This page will touch on mainline porting of most of these parts.

== Finding information about your SoC ==

In order to begin mainlining your SoC, you need to have the following bits of information:

* The codename of your SoC. If you're lucky, it's probably already somewhere out there online. This will be useful when looking things up in the downstream kernel, as it primarily uses the codename, whereas mainline uses the model number. (TODO - we should have a table of SoC codenames somewhere!)
* A copy of the downstream kernel; preferably the one for your device, but you can also use the kernel from another device with the same SoC, or - as a last resort - any kernel with the relevant SoC drivers (hint: search by codename).
* A copy of the <code>qcom_proprietary_devicetree</code> or similar repo with the DTSI source files (because they're not in the kernel repo for whatever reason...). Search for <code>(codename).dtsi</code> on GitHub and you'll find the right repository eventually.
* An extracted DTB from your device. Dump this from a running device using FDT (todo: instructions, would probably be good on a generic page, maybe subpage of [[Devicetree]]?
* Find a similar SoC that is already supported. Usually flagship SoCs are available in mainline; try to find a flagship from around the same time as the SoC you're mainlining. Find its DTSIs as well; then you can compare the differences between downstream and mainline for the upstreamed SoC, and correlate them with differences in your SoC.

== Adding the DTSI ==

todo

== Porting individual parts ==

=== Clock controllers ===

On recent Qualcomm SoCs, the downstream kernel uses the mainline Qualcomm clock driver with only ''some'' vendor-specific quirks. This means that the clock drivers and their relevant headers can pretty much be copied verbatim from downstream, with some slight modifications.

First, create the '''GCC''' driver. Open <code>driver/clk/qcom/Kconfig</code> and add the relevant Kconfig option for your SoC (remember to keep it ordered alphabetically):

<syntaxhighlight lang="kconfig">
config SM_GCC_xxxx
	tristate "SMxxxx Global Clock Controller"
	depends on ARM64 || COMPILE_TEST
	select QCOM_GDSC
	help
	  Support for the global clock controller on SMxxxx devices.
	  Say Y if you want to use peripheral devices such as UART,
	  SPI, I2C, USB, SD/UFS, PCIe etc.
</syntaxhighlight>

Open <code>driver/clk/qcom/Makefile</code> and add the relevant entry:

<syntaxhighlight lang="makefile">
obj-$(CONFIG_SM_GCC_xxxx) += gcc-smXXXX.o
</syntaxhighlight>

Open <code>driver/clk/qcom/gcc-smXXXX.c</code>.

Paste the initialization code from another GCC driver (<syntaxhighlight inline lang="c">static struct platform_driver gcc_smXXXX_driver</syntaxhighlight> and onwards.)

== Porting the PMIC(s) ==

Qualcomm devices use ''multiple'' PMICs for different purposes. Often, the PMIC is paired with an SoC or series of SoCs.