Defined IS_(HOST_)LED_ON/OFF() and improved LED documentation (#4853)

* Defined IS_(HOST_)LED_ON/OFF() and improved LED documentation

* Update docs/custom_quantum_functions.md

Co-Authored-By: DidierLoiseau <didierloiseau+github@gmail.com>

* Update docs/custom_quantum_functions.md

Co-Authored-By: DidierLoiseau <didierloiseau+github@gmail.com>

* Integrated @drashna and @fauxpark's PR comments

- changed all plurals of "LED" to "LEDs" in the file
- rewording of the note about host_keyboard_leds() vs. led_set_user()

* Update docs/custom_quantum_functions.md

Co-Authored-By: DidierLoiseau <didierloiseau+github@gmail.com>

2 files changed, 52 insertions, 8 deletions

diff --git a/docs/custom_quantum_functions.md b/docs/custom_quantum_functions.md
index 5b95450f26..3397bf2d4f 100644
--- a/docs/custom_quantum_functions.md
+++ b/docs/custom_quantum_functions.md
@@ -90,7 +90,7 @@ keyrecord_t record {
 
 # LED Control
 
-This allows you to control the 5 LED's defined as part of the USB Keyboard spec. It will be called when the state of one of those 5 LEDs changes.
+QMK provides methods to read the 5 LEDs defined as part of the HID spec:
 
 * `USB_LED_NUM_LOCK`
 * `USB_LED_CAPS_LOCK`
@@ -98,31 +98,46 @@ This allows you to control the 5 LED's defined as part of the USB Keyboard spec.
 * `USB_LED_COMPOSE`
 * `USB_LED_KANA`
 
+These five constants correspond to the positional bits of the host LED state.
+There are two ways to get the host LED state:
+
+* by implementing `led_set_user()`
+* by calling `host_keyboard_leds()`
+
+## `led_set_user()`
+
+This function will be called when the state of one of those 5 LEDs changes.
+It receives the LED state as parameter.
+Use the `IS_LED_ON(USB_LED, LED_NAME)` and `IS_LED_OFF(USB_LED, LED_NAME)`
+macros to check the LED status.
+
+!> `host_keyboard_leds()` may already reflect a new value before `led_set_user()` is called.
+
 ### Example `led_set_user()` Implementation
 
 ```c
 void led_set_user(uint8_t usb_led) {
-    if (usb_led & (1<<USB_LED_NUM_LOCK)) {
+    if (IS_LED_ON(usb_led, USB_LED_NUM_LOCK)) {
         PORTB |= (1<<0);
     } else {
         PORTB &= ~(1<<0);
     }
-    if (usb_led & (1<<USB_LED_CAPS_LOCK)) {
+    if (IS_LED_ON(usb_led, USB_LED_CAPS_LOCK)) {
         PORTB |= (1<<1);
     } else {
         PORTB &= ~(1<<1);
     }
-    if (usb_led & (1<<USB_LED_SCROLL_LOCK)) {
+    if (IS_LED_ON(usb_led, USB_LED_SCROLL_LOCK)) {
         PORTB |= (1<<2);
     } else {
         PORTB &= ~(1<<2);
     }
-    if (usb_led & (1<<USB_LED_COMPOSE)) {
+    if (IS_LED_ON(usb_led, USB_LED_COMPOSE)) {
         PORTB |= (1<<3);
     } else {
         PORTB &= ~(1<<3);
     }
-    if (usb_led & (1<<USB_LED_KANA)) {
+    if (IS_LED_ON(usb_led, USB_LED_KANA)) {
         PORTB |= (1<<4);
     } else {
         PORTB &= ~(1<<4);
@@ -135,10 +150,33 @@ void led_set_user(uint8_t usb_led) {
 * Keyboard/Revision: `void led_set_kb(uint8_t usb_led)`
 * Keymap: `void led_set_user(uint8_t usb_led)`
 
+## `host_keyboard_leds()`
+
+Call this function to get the last received LED state.
+This is useful for reading the LED state outside `led_set_*`, e.g. in [`matrix_scan_user()`](#matrix-scanning-code).
+
+For convenience, you can use the `IS_HOST_LED_ON(LED_NAME)` and `IS_HOST_LED_OFF(LED_NAME)` macros instead of calling `host_keyboard_leds()` directly.
+
+## Setting physical LED state
+
+Some keyboard implementations provide convenience methods for setting the state of the physical LEDs.
+
+### Ergodox and Ergodox EZ
+
+The Ergodox EZ implementation provides `ergodox_right_led_``1`/`2`/`3_on`/`off()`
+to turn individual LEDs on and off, as well as
+`ergodox_right_led_on`/`off(uint8_t led)`
+to turn them on and off by their number.
+
+In addition, it is possible to specify the brightness level with `ergodox_led_all_set(uint8_t n)`,
+for individual LEDs with `ergodox_right_led_1`/`2`/`3_set(uint8_t n)`
+or by their number using `ergodox_right_led_set(uint8_t led, uint8_t n)`.
+
+It defines `LED_BRIGHTNESS_LO` for the lowest brightness and `LED_BRIGHTNESS_HI` for the highest brightness, which is also the default.
 
 # Matrix Initialization Code
 
-Before a keyboard can be used the hardware must be initialized. QMK handles initialization of the keyboard matrix itself, but if you have other hardware like LED's or i&#xb2;c controllers you will need to set up that hardware before it can be used.
+Before a keyboard can be used the hardware must be initialized. QMK handles initialization of the keyboard matrix itself, but if you have other hardware like LEDs or i&#xb2;c controllers you will need to set up that hardware before it can be used.
 
 
 ### Example `matrix_init_user()` Implementation
@@ -176,7 +214,7 @@ This example has been deliberately omitted. You should understand enough about Q
 
 This function gets called at every matrix scan, which is basically as often as the MCU can handle. Be careful what you put here, as it will get run a lot.
 
-You should use this function if you need custom matrix scanning code. It can also be used for custom status output (such as LED's or a display) or other functionality that you want to trigger regularly even when the user isn't typing.
+You should use this function if you need custom matrix scanning code. It can also be used for custom status output (such as LEDs or a display) or other functionality that you want to trigger regularly even when the user isn't typing.
 
 
 # Keyboard Idling/Wake Code
diff --git a/tmk_core/common/host.h b/tmk_core/common/host.h
index aeabba7107..e70bb68533 100644
--- a/tmk_core/common/host.h
+++ b/tmk_core/common/host.h
@@ -46,6 +46,12 @@ void host_consumer_send(uint16_t data);
 uint16_t host_last_system_report(void);
 uint16_t host_last_consumer_report(void);
 
+#define IS_LED_ON(USB_LED, LED_NAME)    ((USB_LED) & (1 << (LED_NAME)))
+#define IS_LED_OFF(USB_LED, LED_NAME)   (~(USB_LED) & (1 << (LED_NAME)))
+
+#define IS_HOST_LED_ON(LED_NAME)     IS_LED_ON(host_keyboard_leds(), (LED_NAME))
+#define IS_HOST_LED_OFF(LED_NAME)    IS_LED_OFF(host_keyboard_leds(), (LED_NAME))
+
 #ifdef __cplusplus
 }
 #endif