Add PR checklist document. (#9913)

* Add PR checklist document.

* Update docs/pr_checklist.md

Co-authored-by: James Young <18669334+noroadsleft@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Ryan <fauxpark@gmail.com>

* Reword the lower/raise/adjust suggestion somewhat for clarity.

* Add suggestion from @Didel for coding conventions.

Co-authored-by: James Young <18669334+noroadsleft@users.noreply.github.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

2 files changed, 126 insertions, 0 deletions

diff --git a/docs/_summary.md b/docs/_summary.md
index 63c54ec219..9ed55a3d06 100644
--- a/docs/_summary.md
+++ b/docs/_summary.md
@@ -111,6 +111,7 @@
     * [Velocikey](feature_velocikey.md)
 
 * Developing QMK
+  * [PR Checklist](pr_checklist.md)
   * Breaking Changes
     * [Overview](breaking_changes.md)
     * [My Pull Request Was Flagged](breaking_changes_instructions.md)
diff --git a/docs/pr_checklist.md b/docs/pr_checklist.md
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+# PR checklists
+
+This is a non-exhaustive checklist of what the QMK collaborators will be checking when reviewing submitted PRs.
+
+If there are any inconsistencies with these recommendations, you're best off [creating an issue](https://github.com/qmk/qmk_firmware/issues/new) against this document, or getting in touch with a QMK Collaborator on Discord.
+
+## General PRs
+
+- PR should be submitted using a non-`master` branch on the source repository
+    - This does not mean you target a different branch for your PR, rather that you're not working out of your own master branch
+    - If submitter _does_ use their own `master` branch, they'll be given a link to the ["how to git"](https://docs.qmk.fm/#/newbs_git_using_your_master_branch) page after merging -- (end of this document will contain the contents of the message)
+- Newly-added directories and filenames must be lowercase
+    - This rule may be relaxed if upstream sources originally had uppercase characters (e.g. ChibiOS, or imported files from other repositories etc.)
+    - If there is enough justification (i.e. consistency with existing core files etc.) this can be relaxed
+        - a board designer naming their keyboard with uppercase letters is not enough justification
+- Valid license headers on all `*.c` and `*.h` source files
+    - GPL2/GPL3 recommended for consistency
+    - Other licenses are permitted, however they must be GPL-compatible and must allow for redistribution. Using a different license will almost certainly delay a PR getting merged.
+- QMK codebase "best practices" followed
+    - This is not an exhaustive list, and will likely get amended as time goes by
+    - `#pragma once` instead of `#ifndef` include guards in header files
+    - No "old-school" GPIO/I2C/SPI functions used -- must use QMK abstractions unless justifiable (and laziness is not valid justification)
+    - Timing abstractions should be followed too:
+        - `wait_ms()` instead of `_delay_ms()` (remove `#include <util/delay.h>` too)
+        - `timer_read()` and `timer_read32()` etc. -- see [timer.h](https://github.com/qmk/qmk_firmware/blob/master/tmk_core/common/timer.h) for the timing APIs
+    - If you think a new abstraction is useful, you're encouraged to:
+        - prototype it in your own keyboard until it's feature-complete
+        - discuss it with QMK Collaborators on Discord
+        - refactor it as a separate core change
+        - remove your specific copy in your board
+
+## Core PRs
+
+- Must now target `develop` branch, which will subsequently be merged back to `master` on the breaking changes timeline
+- Other notes TBD
+    - Core is a lot more subjective given the breadth of posted changes
+
+## Keyboard PRs
+
+Closed PRs (for inspiration, previous sets of review comments will help you eliminate ping-pong of your own reviews):
+https://github.com/qmk/qmk_firmware/pulls?q=is%3Apr+is%3Aclosed+label%3Akeyboard
+
+- `info.json`
+    - valid URL
+    - valid maintainer
+    - displays correctly in Configurator (press Ctrl+Shift+I to preview local file, turn on fast input to verify ordering)
+- `readme.md`
+    - standard template should be present
+    - flash command has `:flash` at end
+    - valid hardware availability link (unless handwired) -- private groupbuys are okay, but one-off prototypes will be questioned. If open-source, a link to files should be provided.
+- `rules.mk`
+    - removed `MIDI_ENABLE`, `FAUXCLICKY_ENABLE` and `HD44780_ENABLE`
+    - modified `# Enable Bluetooth with the Adafruit EZ-Key HID` -> `# Enable Bluetooth`
+    - No `(-/+size)` comments related to enabling features
+    - Remove the list of alternate bootloaders if one has been specified
+    - No re-definitions of the default MCU parameters if same value, when compared to the equivalent MCU in [mcu_selection.mk](https://github.com/qmk/qmk_firmware/blob/master/quantum/mcu_selection.mk)
+- keyboard `config.h`
+    - don't repeat `MANUFACTURER` in the `PRODUCT` value
+    - no `#define DESCRIPTION`
+    - no Magic Key Options, MIDI Options or HD44780 configuration
+    - user preference configurable `#define`s need to be moved to keymap `config.h`
+    - "`DEBOUNCE`" instead of "`DEBOUNCING_DELAY`"
+    - bare minimum required code for a board to boot into QMK should be present
+        - initialisation code for the matrix and critical devices
+        - mirroring existing functionality of a commercial board (like custom keycodes and special animations etc.) should be handled through non-`default` keymaps
+- `keyboard.c`
+    - empty `xxxx_xxxx_kb()` or other weak-defined default implemented functions removed
+    - commented-out functions removed too
+    - `matrix_init_board()` etc. migrated to `keyboard_pre_init_kb()`, see: [keyboard_pre_init*](https://docs.qmk.fm/#/custom_quantum_functions?id=keyboard_pre_init_-function-documentation)
+    - prefer `CUSTOM_MATRIX = lite` if custom matrix used, allows for standard debounce, see [custom matrix 'lite'](https://docs.qmk.fm/#/custom_matrix?id=lite)
+- `keyboard.h`
+    - `#include "quantum.h"` appears at the top
+    - `LAYOUT` macros should use standard definitions if applicable
+        - Use the Community Layout macro names where they apply (preferred above `LAYOUT`/`LAYOUT_all`)
+- keymap `config.h`
+    - no duplication of `rules.mk` or `config.h` from keyboard
+- `keymaps/default/keymap.c`
+    - `QMKBEST`/`QMKURL` removed (sheesh)
+    - If using `MO(_LOWER)` and `MO(_RAISE)` keycodes or equivalent, and the keymap has an adjust layer when holding both keys -- if the keymap has no "direct-to-adjust" keycode (such as `MO(_ADJUST)`) then you should prefer to write...
+        ```
+        layer_state_t layer_state_set_user(layer_state_t state) {
+          return update_tri_layer_state(state, _LOWER, _RAISE, _ADJUST);
+        }
+        ```
+        ...instead of manually handling `layer_on()`, `update_tri_layer()` inside the keymap's `process_record_user()`.
+- default (and via) keymaps should be "pristine"
+    - bare minimum to be used as a "clean slate" for another user to develop their own user-specific keymap
+    - standard layouts preferred in these keymaps, if possible
+- submitters can have a personal (or bells-and-whistles) keymap showcasing capabilities in the same PR but it shouldn't be embedded in the 'default' keymap
+- submitters can also have a "manufacturer-matching" keymap that mirrors existing functionality of the commercial product, if porting an existing board
+
+Also, specific to ChibiOS:
+- **Strong** preference to using existing ChibiOS board definitions.
+    - A lot of the time, an equivalent Nucleo board can be used with a different flash size or slightly different model in the same family
+        - Example: For an STM32L082KZ, given the similarity to an STM32L073RZ, you can use `BOARD = ST_NUCLEO64_L073RZ` in rules.mk
+    - QMK is migrating to not having custom board definitions if at all possible, due to the ongoing maintenance burden when upgrading ChibiOS
+- If a board definition is unavoidable, `board.c` must have a standard `__early_init()` (as per normal ChibiOS board defs) and an empty `boardInit()`:
+    - see Arm/ChibiOS [early initialization](https://docs.qmk.fm/#/platformdev_chibios_earlyinit?id=board-init)
+    - `__early_init()` should be replaced by either `early_hardware_init_pre()` or `early_hardware_init_post()` as appropriate
+    - `boardInit()` should be migrated to `board_init()`
+
+## Keymap PRs
+
+- `#include QMK_KEYBOARD_H` preferred to including specific board files
+- Prefer layer `enum`s to `#define`s
+- Require custom keycode `enum`s to `#define`s, first entry must have ` = SAFE_RANGE`
+- Terminating backslash (`\`) in lines of LAYOUT macro parameters is superfluous
+- Some care with spacing (e.g., alignment on commas or first char of keycodes) makes for a much nicer-looking keymap
+
+---
+
+## Notes
+
+For when people use their own `master` branch, post this after merge:
+```
+For future reference, we recommend against committing to your `master` branch as you've done here, because pull requests from modified `master` branches can make it more difficult to keep your QMK fork updated. It is highly recommended for QMK development – regardless of what is being done or where – to keep your master updated, but **NEVER** commit to it. Instead, do all your changes in a branch (branches are basically free in Git) and issue PRs from your branches when you're developing.
+
+There are instructions on how to keep your fork updated here:
+
+[**Best Practices: Your Fork's Master: Update Often, Commit Never**](https://docs.qmk.fm/#/newbs_git_using_your_master_branch)
+
+[Fixing Your Branch](https://docs.qmk.fm/#/newbs_git_resynchronize_a_branch) will walk you through fixing up your `master` branch moving forward. If you need any help with this just ask.
+
+Thanks for contributing!
+```