Linux Architecture Diagrams
===========================

Rendering Stack
---------------

.. image:: ../images/linux-rendering-stack.png

Networking Stack
----------------

.. image:: ../images/linux-networking-stack.png

- NAPI_ (New API): poll packets from the NIC in bulk so that it does not
  interrupt the CPU too much

- Netlink_: we use this to communicate with the kernel and configure the network
  stack

.. _NAPI: https://docs.kernel.org/networking/napi.html
.. _Netlink: https://www.kernel.org/doc/html/next/userspace-api/netlink/intro.html)

Storage Stack
-------------

.. image:: ../images/linux-storage-stack.png

To manage read and writes to the storage device, the Linux kernel uses the `bio`
structure which connects the filesystem to a particular storage device.

In order to not issue many commands to the storage device, the `bio` manages a
page cache. This is a copy-on-write cache which gets checked before accessing a
page. If data gets written to a page, the cache gets dirty, and it can be
flushed to the device.

- `Storage Performance Development Kit`_: fast and modern API to interact with
  NVMe devices

.. _Storage Performance Development Kit: https://spdk.io/)

Unique Kernel Contributors per Subsystem
----------------------------------------

+-----------------------+---------------+-------------------------------------+
| Subsystem             | Percentage    | Info                                |
+=======================+===============+=====================================+
| Drivers (General)     | ~55% - 60%    | This includes GPU (about 30% of all |
|                       |               | drivers), Network, Multimedia, and  |
|                       |               | Sound.                              |
+-----------------------+---------------+-------------------------------------+
| Networking (net/)     | 8% - 10%      | Massive corporate focus from Meta,  |
|                       |               | Google, and Mellanox/Nvidia. This   |
|                       |               | is the core of the cloud.           |
+-----------------------+---------------+-------------------------------------+
| Filesystems (fs/)     | ~7% - 8%      | This is the "Data Integrity" tier.  |
|                       |               | Recent spikes come from Bcachefs    |
|                       |               | (Kent Overstreet) and cloud-scale   |
|                       |               | optimizations for NVMe.             |
+-----------------------+---------------+-------------------------------------+
| Core Kernel (kernel/, | ~5%           | The "Priesthood." This is the       |
| mm/)                  |               | smallest group of developers, but   |
|                       |               | they have the highest gatekeeping   |
|                       |               | standards. It includes scheduling   |
|                       |               | and memory management.              |
+-----------------------+---------------+-------------------------------------+
| eBPF (kernel/bpf)     | ~4%           | This is the fastest-growing niche.  |
|                       |               | eBPF is becoming the universal      |
|                       |               | "glue" for observability and        |
|                       |               | networking, drawing in engineers    |
|                       |               | from Distributed Systems.           |
+-----------------------+---------------+-------------------------------------+
| Arch Specific (arch/) | ~10%          | Mostly ARM64 and RISC-V churn.      |
|                       |               | RISC-V is currently seeing a "gold  |
|                       |               | rush" of first-time contributors.   |
+-----------------------+---------------+-------------------------------------+

Source: gemini.

Build systems
-------------

.. image:: ../images/linux-build-systems.png

- `Linux Kernel Module`_: raw kernel development quickstart

- LKDE_: light kernel dev framework

- tmp105-driver_: buildroot based driver

- yocto: TODO

.. _Linux Kernel Module: https://github.com/San7o/linux-kernel-module
.. _LKDE: https://github.com/San7o/lkde-tool
.. _tmp105-driver: https://github.com/San7o/tmp105-driver/