Extensible split data sync (#11930)

* Extensible split data sync capability through transactions.

- Split common transport has been split up between the transport layer
  and data layer.
- Split "transactions" model used, with convergence between I2C and
  serial data definitions.
- Slave matrix "generation count" is used to determine if the full slave
  matrix needs to be retrieved.
- Encoders get the same "generation count" treatment.
- All other blocks of data are synchronised when a change is detected.
- All transmissions have a globally-configurable deadline before a
  transmission is forced (`FORCED_SYNC_THROTTLE_MS`, default 100ms).
- Added atomicity for all core-synced data, preventing partial updates
- Added retries to AVR i2c_master's i2c_start, to minimise the number of
  failed transactions when interrupts are disabled on the slave due to
  atomicity checks.
- Some keyboards have had slight modifications made in order to ensure
  that they still build due to firmware size restrictions.

* Fixup LED_MATRIX compile.

* Parameterise ERROR_DISCONNECT_COUNT.

1 files changed, 24 insertions, 2 deletions

diff --git a/docs/config_options.md b/docs/config_options.md
index 26fe8cea55..980195ac68 100644
--- a/docs/config_options.md
+++ b/docs/config_options.md
@@ -274,7 +274,7 @@ There are a few different ways to set handedness for split keyboards (listed in
 ### Other Options
 
 * `#define USE_I2C`
-  * For using I2C instead of Serial (defaults to serial)
+  * For using I2C instead of Serial (default is serial; serial transport is supported on ARM -- I2C is AVR-only)
 
 * `#define SOFT_SERIAL_PIN D0`
   * When using serial, define this. `D0` or `D1`,`D2`,`D3`,`E6`.
@@ -303,7 +303,7 @@ There are a few different ways to set handedness for split keyboards (listed in
 * `#define SPLIT_USB_DETECT`
   * Detect (with timeout) USB connection when delegating master/slave
   * Default behavior for ARM
-  * Required for AVR Teensy
+  * Required for AVR Teensy (without hardware mods)
 
 * `#define SPLIT_USB_TIMEOUT 2000`
   * Maximum timeout when detecting master/slave when using `SPLIT_USB_DETECT`
@@ -311,6 +311,28 @@ There are a few different ways to set handedness for split keyboards (listed in
 * `#define SPLIT_USB_TIMEOUT_POLL 10`
   * Poll frequency when detecting master/slave when using `SPLIT_USB_DETECT`
 
+* `#define FORCED_SYNC_THROTTLE_MS 100`
+  * Deadline for synchronizing data from master to slave when using the QMK-provided split transport.
+
+* `#define SPLIT_TRANSPORT_MIRROR`
+  * Mirrors the master-side matrix on the slave when using the QMK-provided split transport.
+
+* `#define SPLIT_LAYER_STATE_ENABLE`
+  * Ensures the current layer state is available on the slave when using the QMK-provided split transport.
+
+* `#define SPLIT_LED_STATE_ENABLE`
+  * Ensures the current host indicator state (caps/num/scroll) is available on the slave when using the QMK-provided split transport.
+
+* `#define SPLIT_MODS_ENABLE`
+  * Ensures the current modifier state (normal, weak, and oneshot) is available on the slave when using the QMK-provided split transport.
+
+* `#define SPLIT_WPM_ENABLE`
+  * Ensures the current WPM is available on the slave when using the QMK-provided split transport.
+
+* `#define SPLIT_TRANSACTION_IDS_KB .....`
+* `#define SPLIT_TRANSACTION_IDS_USER .....`
+  * Allows for custom data sync with the slave when using the QMK-provided split transport. See [custom data sync between sides](feature_split_keyboard.md#custom-data-sync) for more information.
+
 # The `rules.mk` File
 
 This is a [make](https://www.gnu.org/software/make/manual/make.html) file that is included by the top-level `Makefile`. It is used to set some information about the MCU that we will be compiling for as well as enabling and disabling certain features.