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.

7 files changed, 1062 insertions, 420 deletions

diff --git a/quantum/split_common/matrix.c b/quantum/split_common/matrix.c
index 2cf7b70582..56d91b07fe 100644
--- a/quantum/split_common/matrix.c
+++ b/quantum/split_common/matrix.c
@@ -23,9 +23,11 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include "quantum.h"
 #include "split_util.h"
 #include "config.h"
-#include "transport.h"
+#include "transactions.h"
 
-#define ERROR_DISCONNECT_COUNT 5
+#ifndef ERROR_DISCONNECT_COUNT
+#    define ERROR_DISCONNECT_COUNT 5
+#endif  // ERROR_DISCONNECT_COUNT
 
 #define ROWS_PER_HAND (MATRIX_ROWS / 2)
 
diff --git a/quantum/split_common/post_config.h b/quantum/split_common/post_config.h
index 4ae1d52732..a4c0a1956b 100644
--- a/quantum/split_common/post_config.h
+++ b/quantum/split_common/post_config.h
@@ -7,13 +7,4 @@
 #    ifndef F_SCL
 #        define F_SCL 100000UL  // SCL frequency
 #    endif
-
-#else  // use serial
-// When using serial, the user must define RGBLIGHT_SPLIT explicitly
-//  in config.h as needed.
-//      see quantum/rgblight_post_config.h
-#    if defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
-// When using serial and RGBLIGHT_SPLIT need separate transaction
-#        define SERIAL_USE_MULTI_TRANSACTION
-#    endif
 #endif
diff --git a/quantum/split_common/transaction_id_define.h b/quantum/split_common/transaction_id_define.h
new file mode 100644
index 0000000000..464c73478a
--- /dev/null
+++ b/quantum/split_common/transaction_id_define.h
@@ -0,0 +1,94 @@
+/* Copyright 2021 QMK
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+enum serial_transaction_id {
+#ifdef USE_I2C
+    I2C_EXECUTE_CALLBACK,
+#endif  // USE_I2C
+
+    GET_SLAVE_MATRIX_CHECKSUM,
+    GET_SLAVE_MATRIX_DATA,
+
+#ifdef SPLIT_TRANSPORT_MIRROR
+    PUT_MASTER_MATRIX,
+#endif  // SPLIT_TRANSPORT_MIRROR
+
+#ifdef ENCODER_ENABLE
+    GET_ENCODERS_CHECKSUM,
+    GET_ENCODERS_DATA,
+#endif  // ENCODER_ENABLE
+
+#ifndef DISABLE_SYNC_TIMER
+    PUT_SYNC_TIMER,
+#endif  // DISABLE_SYNC_TIMER
+
+#if !defined(NO_ACTION_LAYER) && defined(SPLIT_LAYER_STATE_ENABLE)
+    PUT_LAYER_STATE,
+    PUT_DEFAULT_LAYER_STATE,
+#endif  // !defined(NO_ACTION_LAYER) && defined(SPLIT_LAYER_STATE_ENABLE)
+
+#ifdef SPLIT_LED_STATE_ENABLE
+    PUT_LED_STATE,
+#endif  // SPLIT_LED_STATE_ENABLE
+
+#ifdef SPLIT_MODS_ENABLE
+    PUT_MODS,
+#endif  // SPLIT_MODS_ENABLE
+
+#ifdef BACKLIGHT_ENABLE
+    PUT_BACKLIGHT,
+#endif  // BACKLIGHT_ENABLE
+
+#if defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
+    PUT_RGBLIGHT,
+#endif  // defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
+
+#if defined(LED_MATRIX_ENABLE) && defined(LED_MATRIX_SPLIT)
+    PUT_LED_MATRIX,
+#endif  // defined(LED_MATRIX_ENABLE) && defined(LED_MATRIX_SPLIT)
+
+#if defined(RGB_MATRIX_ENABLE) && defined(RGB_MATRIX_SPLIT)
+    PUT_RGB_MATRIX,
+#endif  // defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
+
+#if defined(WPM_ENABLE) && defined(SPLIT_WPM_ENABLE)
+    PUT_WPM,
+#endif  // defined(WPM_ENABLE) && defined(SPLIT_WPM_ENABLE)
+
+#if defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
+    PUT_RPC_INFO,
+    PUT_RPC_REQ_DATA,
+    EXECUTE_RPC,
+    GET_RPC_RESP_DATA,
+#endif  // defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
+
+// keyboard-specific
+#ifdef SPLIT_TRANSACTION_IDS_KB
+    SPLIT_TRANSACTION_IDS_KB,
+#endif  // SPLIT_TRANSACTION_IDS_KB
+
+// user/keymap-specific
+#ifdef SPLIT_TRANSACTION_IDS_USER
+    SPLIT_TRANSACTION_IDS_USER,
+#endif  // SPLIT_TRANSACTION_IDS_USER
+
+    NUM_TOTAL_TRANSACTIONS
+};
+
+// Ensure we only use 5 bits for transaction
+_Static_assert(NUM_TOTAL_TRANSACTIONS <= (1 << 5), "Max number of usable transactions exceeded");
diff --git a/quantum/split_common/transactions.c b/quantum/split_common/transactions.c
new file mode 100644
index 0000000000..99a8623b3c
--- /dev/null
+++ b/quantum/split_common/transactions.c
@@ -0,0 +1,670 @@
+/* Copyright 2021 QMK
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <string.h>
+#include <stddef.h>
+
+#include "debug.h"
+#include "matrix.h"
+#include "quantum.h"
+#include "transactions.h"
+#include "transport.h"
+#include "transaction_id_define.h"
+
+#define SYNC_TIMER_OFFSET 2
+
+#ifndef FORCED_SYNC_THROTTLE_MS
+#    define FORCED_SYNC_THROTTLE_MS 100
+#endif  // FORCED_SYNC_THROTTLE_MS
+
+#define sizeof_member(type, member) sizeof(((type *)NULL)->member)
+
+#define trans_initiator2target_initializer_cb(member, cb) \
+    { &dummy, sizeof_member(split_shared_memory_t, member), offsetof(split_shared_memory_t, member), 0, 0, cb }
+#define trans_initiator2target_initializer(member) trans_initiator2target_initializer_cb(member, NULL)
+
+#define trans_target2initiator_initializer_cb(member, cb) \
+    { &dummy, 0, 0, sizeof_member(split_shared_memory_t, member), offsetof(split_shared_memory_t, member), cb }
+#define trans_target2initiator_initializer(member) trans_target2initiator_initializer_cb(member, NULL)
+
+#define transport_write(id, data, length) transport_execute_transaction(id, data, length, NULL, 0)
+#define transport_read(id, data, length) transport_execute_transaction(id, NULL, 0, data, length)
+
+static uint8_t crc8(const void *data, size_t len) {
+    const uint8_t *p   = (const uint8_t *)data;
+    uint8_t        crc = 0xff;
+    size_t         i, j;
+    for (i = 0; i < len; i++) {
+        crc ^= p[i];
+        for (j = 0; j < 8; j++) {
+            if ((crc & 0x80) != 0)
+                crc = (uint8_t)((crc << 1) ^ 0x31);
+            else
+                crc <<= 1;
+        }
+    }
+    return crc;
+}
+
+#if defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
+// Forward-declare the RPC callback handlers
+void slave_rpc_info_callback(uint8_t initiator2target_buffer_size, const void *initiator2target_buffer, uint8_t target2initiator_buffer_size, void *target2initiator_buffer);
+void slave_rpc_exec_callback(uint8_t initiator2target_buffer_size, const void *initiator2target_buffer, uint8_t target2initiator_buffer_size, void *target2initiator_buffer);
+#endif  // defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
+
+////////////////////////////////////////////////////
+// Helpers
+
+bool transaction_handler_master(bool okay, matrix_row_t master_matrix[], matrix_row_t slave_matrix[], const char *prefix, bool (*handler)(matrix_row_t master_matrix[], matrix_row_t slave_matrix[])) {
+    if (okay) {
+        bool this_okay = true;
+        for (int iter = 1; iter <= 10; ++iter) {
+            if (!this_okay) {
+                for (int i = 0; i < iter * iter; ++i) {
+                    wait_us(10);
+                }
+            }
+            ATOMIC_BLOCK_FORCEON { this_okay = handler(master_matrix, slave_matrix); };
+            if (this_okay) break;
+        }
+        okay &= this_okay;
+        if (!okay) {
+            dprintf("Failed to execute %s\n", prefix);
+        }
+    }
+    return okay;
+}
+
+#define TRANSACTION_HANDLER_MASTER(prefix)                                                                \
+    do {                                                                                                  \
+        okay &= transaction_handler_master(okay, master_matrix, slave_matrix, #prefix, &prefix##_master); \
+    } while (0)
+
+#define TRANSACTION_HANDLER_SLAVE(prefix)                                      \
+    do {                                                                       \
+        ATOMIC_BLOCK_FORCEON { prefix##_slave(master_matrix, slave_matrix); }; \
+    } while (0)
+
+inline static bool read_if_checksum_mismatch(int8_t trans_id_checksum, int8_t trans_id_retrieve, uint32_t *last_update, void *destination, const void *equiv_shmem, size_t length) {
+    uint8_t curr_checksum;
+    bool    okay = transport_read(trans_id_checksum, &curr_checksum, sizeof(curr_checksum));
+    if (okay && (timer_elapsed32(*last_update) >= FORCED_SYNC_THROTTLE_MS || curr_checksum != crc8(equiv_shmem, length))) {
+        okay &= transport_read(trans_id_retrieve, destination, length);
+        okay &= curr_checksum == crc8(equiv_shmem, length);
+        if (okay) {
+            *last_update = timer_read32();
+        }
+    } else {
+        memcpy(destination, equiv_shmem, length);
+    }
+    return okay;
+}
+
+inline static bool send_if_condition(int8_t trans_id, uint32_t *last_update, bool condition, void *source, size_t length) {
+    bool okay = true;
+    if (timer_elapsed32(*last_update) >= FORCED_SYNC_THROTTLE_MS || condition) {
+        okay &= transport_write(trans_id, source, length);
+        if (okay) {
+            *last_update = timer_read32();
+        }
+    }
+    return okay;
+}
+
+inline static bool send_if_data_mismatch(int8_t trans_id, uint32_t *last_update, void *source, const void *equiv_shmem, size_t length) {
+    // Just run a memcmp to compare the source and equivalent shmem location
+    return send_if_condition(trans_id, last_update, (memcmp(source, equiv_shmem, length) != 0), source, length);
+}
+
+////////////////////////////////////////////////////
+// Slave matrix
+
+static bool slave_matrix_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t     last_update                    = 0;
+    static matrix_row_t last_matrix[(MATRIX_ROWS) / 2] = {0};  // last successfully-read matrix, so we can replicate if there are checksum errors
+    matrix_row_t        temp_matrix[(MATRIX_ROWS) / 2];        // holding area while we test whether or not checksum is correct
+
+    bool okay = read_if_checksum_mismatch(GET_SLAVE_MATRIX_CHECKSUM, GET_SLAVE_MATRIX_DATA, &last_update, temp_matrix, split_shmem->smatrix.matrix, sizeof(split_shmem->smatrix.matrix));
+    if (okay) {
+        // Checksum matches the received data, save as the last matrix state
+        memcpy(last_matrix, temp_matrix, sizeof(temp_matrix));
+    }
+    // Copy out the last-known-good matrix state to the slave matrix
+    memcpy(slave_matrix, last_matrix, sizeof(last_matrix));
+    return okay;
+}
+
+static void slave_matrix_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    memcpy(split_shmem->smatrix.matrix, slave_matrix, sizeof(split_shmem->smatrix.matrix));
+    split_shmem->smatrix.checksum = crc8(split_shmem->smatrix.matrix, sizeof(split_shmem->smatrix.matrix));
+}
+
+// clang-format off
+#define TRANSACTIONS_SLAVE_MATRIX_MASTER() TRANSACTION_HANDLER_MASTER(slave_matrix_handlers)
+#define TRANSACTIONS_SLAVE_MATRIX_SLAVE() TRANSACTION_HANDLER_SLAVE(slave_matrix_handlers)
+#define TRANSACTIONS_SLAVE_MATRIX_REGISTRATIONS \
+    [GET_SLAVE_MATRIX_CHECKSUM] = trans_target2initiator_initializer(smatrix.checksum), \
+    [GET_SLAVE_MATRIX_DATA]     = trans_target2initiator_initializer(smatrix.matrix),
+// clang-format on
+
+////////////////////////////////////////////////////
+// Master matrix
+
+#ifdef SPLIT_TRANSPORT_MIRROR
+
+static bool master_matrix_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_update = 0;
+    return send_if_data_mismatch(PUT_MASTER_MATRIX, &last_update, master_matrix, split_shmem->mmatrix.matrix, sizeof(split_shmem->mmatrix.matrix));
+}
+
+static void master_matrix_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    // Always copy to the master matrix
+    memcpy(master_matrix, split_shmem->mmatrix.matrix, sizeof(split_shmem->mmatrix.matrix));
+}
+
+#    define TRANSACTIONS_MASTER_MATRIX_MASTER() TRANSACTION_HANDLER_MASTER(master_matrix_handlers)
+#    define TRANSACTIONS_MASTER_MATRIX_SLAVE() TRANSACTION_HANDLER_SLAVE(master_matrix_handlers)
+#    define TRANSACTIONS_MASTER_MATRIX_REGISTRATIONS [PUT_MASTER_MATRIX] = trans_initiator2target_initializer(mmatrix.matrix),
+
+#else  // SPLIT_TRANSPORT_MIRROR
+
+#    define TRANSACTIONS_MASTER_MATRIX_MASTER()
+#    define TRANSACTIONS_MASTER_MATRIX_SLAVE()
+#    define TRANSACTIONS_MASTER_MATRIX_REGISTRATIONS
+
+#endif  // SPLIT_TRANSPORT_MIRROR
+
+////////////////////////////////////////////////////
+// Encoders
+
+#ifdef ENCODER_ENABLE
+
+static bool encoder_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_update = 0;
+    uint8_t         temp_state[NUMBER_OF_ENCODERS];
+
+    bool okay = read_if_checksum_mismatch(GET_ENCODERS_CHECKSUM, GET_ENCODERS_DATA, &last_update, temp_state, split_shmem->encoders.state, sizeof(temp_state));
+    if (okay) encoder_update_raw(temp_state);
+    return okay;
+}
+
+static void encoder_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    uint8_t encoder_state[NUMBER_OF_ENCODERS];
+    encoder_state_raw(encoder_state);
+    // Always prepare the encoder state for read.
+    memcpy(split_shmem->encoders.state, encoder_state, sizeof(encoder_state));
+    // Now update the checksum given that the encoders has been written to
+    split_shmem->encoders.checksum = crc8(encoder_state, sizeof(encoder_state));
+}
+
+// clang-format off
+#    define TRANSACTIONS_ENCODERS_MASTER() TRANSACTION_HANDLER_MASTER(encoder_handlers)
+#    define TRANSACTIONS_ENCODERS_SLAVE() TRANSACTION_HANDLER_SLAVE(encoder_handlers)
+#    define TRANSACTIONS_ENCODERS_REGISTRATIONS \
+    [GET_ENCODERS_CHECKSUM] = trans_target2initiator_initializer(encoders.checksum), \
+    [GET_ENCODERS_DATA]     = trans_target2initiator_initializer(encoders.state),
+// clang-format on
+
+#else  // ENCODER_ENABLE
+
+#    define TRANSACTIONS_ENCODERS_MASTER()
+#    define TRANSACTIONS_ENCODERS_SLAVE()
+#    define TRANSACTIONS_ENCODERS_REGISTRATIONS
+
+#endif  // ENCODER_ENABLE
+
+////////////////////////////////////////////////////
+// Sync timer
+
+#ifndef DISABLE_SYNC_TIMER
+
+static bool sync_timer_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_update = 0;
+
+    bool okay = true;
+    if (timer_elapsed32(last_update) >= FORCED_SYNC_THROTTLE_MS) {
+        uint32_t sync_timer = sync_timer_read32() + SYNC_TIMER_OFFSET;
+        okay &= transport_write(PUT_SYNC_TIMER, &sync_timer, sizeof(sync_timer));
+        if (okay) {
+            last_update = timer_read32();
+        }
+    }
+    return okay;
+}
+
+static void sync_timer_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_sync_timer = 0;
+    if (last_sync_timer != split_shmem->sync_timer) {
+        last_sync_timer = split_shmem->sync_timer;
+        sync_timer_update(last_sync_timer);
+    }
+}
+
+#    define TRANSACTIONS_SYNC_TIMER_MASTER() TRANSACTION_HANDLER_MASTER(sync_timer_handlers)
+#    define TRANSACTIONS_SYNC_TIMER_SLAVE() TRANSACTION_HANDLER_SLAVE(sync_timer_handlers)
+#    define TRANSACTIONS_SYNC_TIMER_REGISTRATIONS [PUT_SYNC_TIMER] = trans_initiator2target_initializer(sync_timer),
+
+#else  // DISABLE_SYNC_TIMER
+
+#    define TRANSACTIONS_SYNC_TIMER_MASTER()
+#    define TRANSACTIONS_SYNC_TIMER_SLAVE()
+#    define TRANSACTIONS_SYNC_TIMER_REGISTRATIONS
+
+#endif  // DISABLE_SYNC_TIMER
+
+////////////////////////////////////////////////////
+// Layer state
+
+#if !defined(NO_ACTION_LAYER) && defined(SPLIT_LAYER_STATE_ENABLE)
+
+static bool layer_state_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_layer_state_update         = 0;
+    static uint32_t last_default_layer_state_update = 0;
+
+    bool okay = send_if_condition(PUT_LAYER_STATE, &last_layer_state_update, (layer_state != split_shmem->layers.layer_state), &layer_state, sizeof(layer_state));
+    if (okay) {
+        okay &= send_if_condition(PUT_DEFAULT_LAYER_STATE, &last_default_layer_state_update, (default_layer_state != split_shmem->layers.default_layer_state), &default_layer_state, sizeof(default_layer_state));
+    }
+    return okay;
+}
+
+static void layer_state_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    layer_state         = split_shmem->layers.layer_state;
+    default_layer_state = split_shmem->layers.default_layer_state;
+}
+
+// clang-format off
+#    define TRANSACTIONS_LAYER_STATE_MASTER() TRANSACTION_HANDLER_MASTER(layer_state_handlers)
+#    define TRANSACTIONS_LAYER_STATE_SLAVE() TRANSACTION_HANDLER_SLAVE(layer_state_handlers)
+#    define TRANSACTIONS_LAYER_STATE_REGISTRATIONS \
+    [PUT_LAYER_STATE]         = trans_initiator2target_initializer(layers.layer_state), \
+    [PUT_DEFAULT_LAYER_STATE] = trans_initiator2target_initializer(layers.default_layer_state),
+// clang-format on
+
+#else  // !defined(NO_ACTION_LAYER) && defined(SPLIT_LAYER_STATE_ENABLE)
+
+#    define TRANSACTIONS_LAYER_STATE_MASTER()
+#    define TRANSACTIONS_LAYER_STATE_SLAVE()
+#    define TRANSACTIONS_LAYER_STATE_REGISTRATIONS
+
+#endif  // !defined(NO_ACTION_LAYER) && defined(SPLIT_LAYER_STATE_ENABLE)
+
+////////////////////////////////////////////////////
+// LED state
+
+#ifdef SPLIT_LED_STATE_ENABLE
+
+static bool led_state_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_update = 0;
+    uint8_t         led_state   = host_keyboard_leds();
+    return send_if_data_mismatch(PUT_LED_STATE, &last_update, &led_state, &split_shmem->led_state, sizeof(led_state));
+}
+
+static void led_state_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    void set_split_host_keyboard_leds(uint8_t led_state);
+    set_split_host_keyboard_leds(split_shmem->led_state);
+}
+
+#    define TRANSACTIONS_LED_STATE_MASTER() TRANSACTION_HANDLER_MASTER(led_state_handlers)
+#    define TRANSACTIONS_LED_STATE_SLAVE() TRANSACTION_HANDLER_SLAVE(led_state_handlers)
+#    define TRANSACTIONS_LED_STATE_REGISTRATIONS [PUT_LED_STATE] = trans_initiator2target_initializer(led_state),
+
+#else  // SPLIT_LED_STATE_ENABLE
+
+#    define TRANSACTIONS_LED_STATE_MASTER()
+#    define TRANSACTIONS_LED_STATE_SLAVE()
+#    define TRANSACTIONS_LED_STATE_REGISTRATIONS
+
+#endif  // SPLIT_LED_STATE_ENABLE
+
+////////////////////////////////////////////////////
+// Mods
+
+#ifdef SPLIT_MODS_ENABLE
+
+static bool mods_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t   last_update    = 0;
+    bool              mods_need_sync = timer_elapsed32(last_update) >= FORCED_SYNC_THROTTLE_MS;
+    split_mods_sync_t new_mods;
+    new_mods.real_mods = get_mods();
+    if (!mods_need_sync && new_mods.real_mods != split_shmem->mods.real_mods) {
+        mods_need_sync = true;
+    }
+
+    new_mods.weak_mods = get_weak_mods();
+    if (!mods_need_sync && new_mods.weak_mods != split_shmem->mods.weak_mods) {
+        mods_need_sync = true;
+    }
+
+#    ifndef NO_ACTION_ONESHOT
+    new_mods.oneshot_mods = get_oneshot_mods();
+    if (!mods_need_sync && new_mods.oneshot_mods != split_shmem->mods.oneshot_mods) {
+        mods_need_sync = true;
+    }
+#    endif  // NO_ACTION_ONESHOT
+
+    bool okay = true;
+    if (mods_need_sync) {
+        okay &= transport_write(PUT_MODS, &new_mods, sizeof(new_mods));
+        if (okay) {
+            last_update = timer_read32();
+        }
+    }
+
+    return okay;
+}
+
+static void mods_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    set_mods(split_shmem->mods.real_mods);
+    set_weak_mods(split_shmem->mods.weak_mods);
+#    ifndef NO_ACTION_ONESHOT
+    set_oneshot_mods(split_shmem->mods.oneshot_mods);
+#    endif
+}
+
+#    define TRANSACTIONS_MODS_MASTER() TRANSACTION_HANDLER_MASTER(mods_handlers)
+#    define TRANSACTIONS_MODS_SLAVE() TRANSACTION_HANDLER_SLAVE(mods_handlers)
+#    define TRANSACTIONS_MODS_REGISTRATIONS [PUT_MODS] = trans_initiator2target_initializer(mods),
+
+#else  // SPLIT_MODS_ENABLE
+
+#    define TRANSACTIONS_MODS_MASTER()
+#    define TRANSACTIONS_MODS_SLAVE()
+#    define TRANSACTIONS_MODS_REGISTRATIONS
+
+#endif  // SPLIT_MODS_ENABLE
+
+////////////////////////////////////////////////////
+// Backlight
+
+#ifdef BACKLIGHT_ENABLE
+
+static bool backlight_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_update = 0;
+    uint8_t         level       = is_backlight_enabled() ? get_backlight_level() : 0;
+    return send_if_condition(PUT_BACKLIGHT, &last_update, (level != split_shmem->backlight_level), &level, sizeof(level));
+}
+
+static void backlight_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) { backlight_set(split_shmem->backlight_level); }
+
+#    define TRANSACTIONS_BACKLIGHT_MASTER() TRANSACTION_HANDLER_MASTER(backlight_handlers)
+#    define TRANSACTIONS_BACKLIGHT_SLAVE() TRANSACTION_HANDLER_SLAVE(backlight_handlers)
+#    define TRANSACTIONS_BACKLIGHT_REGISTRATIONS [PUT_BACKLIGHT] = trans_initiator2target_initializer(backlight_level),
+
+#else  // BACKLIGHT_ENABLE
+
+#    define TRANSACTIONS_BACKLIGHT_MASTER()
+#    define TRANSACTIONS_BACKLIGHT_SLAVE()
+#    define TRANSACTIONS_BACKLIGHT_REGISTRATIONS
+
+#endif  // BACKLIGHT_ENABLE
+
+////////////////////////////////////////////////////
+// RGBLIGHT
+
+#if defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
+
+static bool rgblight_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t     last_update = 0;
+    rgblight_syncinfo_t rgblight_sync;
+    rgblight_get_syncinfo(&rgblight_sync);
+    if (send_if_condition(PUT_RGBLIGHT, &last_update, (rgblight_sync.status.change_flags != 0), &rgblight_sync, sizeof(rgblight_sync))) {
+        rgblight_clear_change_flags();
+    } else {
+        return false;
+    }
+    return true;
+}
+
+static void rgblight_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    // Update the RGB with the new data
+    if (split_shmem->rgblight_sync.status.change_flags != 0) {
+        rgblight_update_sync(&split_shmem->rgblight_sync, false);
+        split_shmem->rgblight_sync.status.change_flags = 0;
+    }
+}
+
+#    define TRANSACTIONS_RGBLIGHT_MASTER() TRANSACTION_HANDLER_MASTER(rgblight_handlers)
+#    define TRANSACTIONS_RGBLIGHT_SLAVE() TRANSACTION_HANDLER_SLAVE(rgblight_handlers)
+#    define TRANSACTIONS_RGBLIGHT_REGISTRATIONS [PUT_RGBLIGHT] = trans_initiator2target_initializer(rgblight_sync),
+
+#else  // defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
+
+#    define TRANSACTIONS_RGBLIGHT_MASTER()
+#    define TRANSACTIONS_RGBLIGHT_SLAVE()
+#    define TRANSACTIONS_RGBLIGHT_REGISTRATIONS
+
+#endif  // defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
+
+////////////////////////////////////////////////////
+// LED Matrix
+
+#if defined(LED_MATRIX_ENABLE) && defined(LED_MATRIX_SPLIT)
+
+static bool led_matrix_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t   last_update = 0;
+    led_matrix_sync_t led_matrix_sync;
+    memcpy(&led_matrix_sync.led_matrix, &led_matrix_eeconfig, sizeof(led_eeconfig_t));
+    led_matrix_sync.led_suspend_state = led_matrix_get_suspend_state();
+    return send_if_data_mismatch(PUT_LED_MATRIX, &last_update, &led_matrix_sync, &split_shmem->led_matrix_sync, sizeof(led_matrix_sync));
+}
+
+static void led_matrix_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    memcpy(&led_matrix_eeconfig, &split_shmem->led_matrix_sync.led_matrix, sizeof(led_eeconfig_t));
+    led_matrix_set_suspend_state(split_shmem->led_matrix_sync.led_suspend_state);
+}
+
+#    define TRANSACTIONS_LED_MATRIX_MASTER() TRANSACTION_HANDLER_MASTER(led_matrix_handlers)
+#    define TRANSACTIONS_LED_MATRIX_SLAVE() TRANSACTION_HANDLER_SLAVE(led_matrix_handlers)
+#    define TRANSACTIONS_LED_MATRIX_REGISTRATIONS [PUT_LED_MATRIX] = trans_initiator2target_initializer(led_matrix_sync),
+
+#else  // defined(LED_MATRIX_ENABLE) && defined(LED_MATRIX_SPLIT)
+
+#    define TRANSACTIONS_LED_MATRIX_MASTER()
+#    define TRANSACTIONS_LED_MATRIX_SLAVE()
+#    define TRANSACTIONS_LED_MATRIX_REGISTRATIONS
+
+#endif  // defined(LED_MATRIX_ENABLE) && defined(LED_MATRIX_SPLIT)
+
+////////////////////////////////////////////////////
+// RGB Matrix
+
+#if defined(RGB_MATRIX_ENABLE) && defined(RGB_MATRIX_SPLIT)
+
+static bool rgb_matrix_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t   last_update = 0;
+    rgb_matrix_sync_t rgb_matrix_sync;
+    memcpy(&rgb_matrix_sync.rgb_matrix, &rgb_matrix_config, sizeof(rgb_config_t));
+    rgb_matrix_sync.rgb_suspend_state = rgb_matrix_get_suspend_state();
+    return send_if_data_mismatch(PUT_RGB_MATRIX, &last_update, &rgb_matrix_sync, &split_shmem->rgb_matrix_sync, sizeof(rgb_matrix_sync));
+}
+
+static void rgb_matrix_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    memcpy(&rgb_matrix_config, &split_shmem->rgb_matrix_sync.rgb_matrix, sizeof(rgb_config_t));
+    rgb_matrix_set_suspend_state(split_shmem->rgb_matrix_sync.rgb_suspend_state);
+}
+
+#    define TRANSACTIONS_RGB_MATRIX_MASTER() TRANSACTION_HANDLER_MASTER(rgb_matrix_handlers)
+#    define TRANSACTIONS_RGB_MATRIX_SLAVE() TRANSACTION_HANDLER_SLAVE(rgb_matrix_handlers)
+#    define TRANSACTIONS_RGB_MATRIX_REGISTRATIONS [PUT_RGB_MATRIX] = trans_initiator2target_initializer(rgb_matrix_sync),
+
+#else  // defined(RGB_MATRIX_ENABLE) && defined(RGB_MATRIX_SPLIT)
+
+#    define TRANSACTIONS_RGB_MATRIX_MASTER()
+#    define TRANSACTIONS_RGB_MATRIX_SLAVE()
+#    define TRANSACTIONS_RGB_MATRIX_REGISTRATIONS
+
+#endif  // defined(RGB_MATRIX_ENABLE) && defined(RGB_MATRIX_SPLIT)
+
+////////////////////////////////////////////////////
+// WPM
+
+#if defined(WPM_ENABLE) && defined(SPLIT_WPM_ENABLE)
+
+static bool wpm_handlers_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    static uint32_t last_update = 0;
+    uint8_t         current_wpm = get_current_wpm();
+    return send_if_condition(PUT_WPM, &last_update, (current_wpm != split_shmem->current_wpm), &current_wpm, sizeof(current_wpm));
+}
+
+static void wpm_handlers_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) { set_current_wpm(split_shmem->current_wpm); }
+
+#    define TRANSACTIONS_WPM_MASTER() TRANSACTION_HANDLER_MASTER(wpm_handlers)
+#    define TRANSACTIONS_WPM_SLAVE() TRANSACTION_HANDLER_SLAVE(wpm_handlers)
+#    define TRANSACTIONS_WPM_REGISTRATIONS [PUT_WPM] = trans_initiator2target_initializer(current_wpm),
+
+#else  // defined(WPM_ENABLE) && defined(SPLIT_WPM_ENABLE)
+
+#    define TRANSACTIONS_WPM_MASTER()
+#    define TRANSACTIONS_WPM_SLAVE()
+#    define TRANSACTIONS_WPM_REGISTRATIONS
+
+#endif  // defined(WPM_ENABLE) && defined(SPLIT_WPM_ENABLE)
+
+////////////////////////////////////////////////////
+
+uint8_t                  dummy;
+split_transaction_desc_t split_transaction_table[NUM_TOTAL_TRANSACTIONS] = {
+    // Set defaults
+    [0 ...(NUM_TOTAL_TRANSACTIONS - 1)] = {NULL, 0, 0, 0, 0, 0},
+
+#ifdef USE_I2C
+    [I2C_EXECUTE_CALLBACK] = trans_initiator2target_initializer(transaction_id),
+#endif  // USE_I2C
+
+    // clang-format off
+    TRANSACTIONS_SLAVE_MATRIX_REGISTRATIONS
+    TRANSACTIONS_MASTER_MATRIX_REGISTRATIONS
+    TRANSACTIONS_ENCODERS_REGISTRATIONS
+    TRANSACTIONS_SYNC_TIMER_REGISTRATIONS
+    TRANSACTIONS_LAYER_STATE_REGISTRATIONS
+    TRANSACTIONS_LED_STATE_REGISTRATIONS
+    TRANSACTIONS_MODS_REGISTRATIONS
+    TRANSACTIONS_BACKLIGHT_REGISTRATIONS
+    TRANSACTIONS_RGBLIGHT_REGISTRATIONS
+    TRANSACTIONS_LED_MATRIX_REGISTRATIONS
+    TRANSACTIONS_RGB_MATRIX_REGISTRATIONS
+    TRANSACTIONS_WPM_REGISTRATIONS
+// clang-format on
+
+#if defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
+        [PUT_RPC_INFO]  = trans_initiator2target_initializer_cb(rpc_info, slave_rpc_info_callback),
+    [PUT_RPC_REQ_DATA]  = trans_initiator2target_initializer(rpc_m2s_buffer),
+    [EXECUTE_RPC]       = trans_initiator2target_initializer_cb(rpc_info.transaction_id, slave_rpc_exec_callback),
+    [GET_RPC_RESP_DATA] = trans_target2initiator_initializer(rpc_s2m_buffer),
+#endif  // defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
+};
+
+bool transactions_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    bool okay = true;
+    TRANSACTIONS_SLAVE_MATRIX_MASTER();
+    TRANSACTIONS_MASTER_MATRIX_MASTER();
+    TRANSACTIONS_ENCODERS_MASTER();
+    TRANSACTIONS_SYNC_TIMER_MASTER();
+    TRANSACTIONS_LAYER_STATE_MASTER();
+    TRANSACTIONS_LED_STATE_MASTER();
+    TRANSACTIONS_MODS_MASTER();
+    TRANSACTIONS_BACKLIGHT_MASTER();
+    TRANSACTIONS_RGBLIGHT_MASTER();
+    TRANSACTIONS_LED_MATRIX_MASTER();
+    TRANSACTIONS_RGB_MATRIX_MASTER();
+    TRANSACTIONS_WPM_MASTER();
+    return okay;
+}
+
+void transactions_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
+    TRANSACTIONS_SLAVE_MATRIX_SLAVE();
+    TRANSACTIONS_MASTER_MATRIX_SLAVE();
+    TRANSACTIONS_ENCODERS_SLAVE();
+    TRANSACTIONS_SYNC_TIMER_SLAVE();
+    TRANSACTIONS_LAYER_STATE_SLAVE();
+    TRANSACTIONS_LED_STATE_SLAVE();
+    TRANSACTIONS_MODS_SLAVE();
+    TRANSACTIONS_BACKLIGHT_SLAVE();
+    TRANSACTIONS_RGBLIGHT_SLAVE();
+    TRANSACTIONS_LED_MATRIX_SLAVE();
+    TRANSACTIONS_RGB_MATRIX_SLAVE();
+    TRANSACTIONS_WPM_SLAVE();
+}
+
+#if defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
+
+void transaction_register_rpc(int8_t transaction_id, slave_callback_t callback) {
+    // Prevent invoking RPC on QMK core sync data
+    if (transaction_id <= GET_RPC_RESP_DATA) return;
+
+    // Set the callback
+    split_transaction_table[transaction_id].slave_callback          = callback;
+    split_transaction_table[transaction_id].initiator2target_offset = offsetof(split_shared_memory_t, rpc_m2s_buffer);
+    split_transaction_table[transaction_id].target2initiator_offset = offsetof(split_shared_memory_t, rpc_s2m_buffer);
+}
+
+bool transaction_rpc_exec(int8_t transaction_id, uint8_t initiator2target_buffer_size, const void *initiator2target_buffer, uint8_t target2initiator_buffer_size, void *target2initiator_buffer) {
+    // Prevent invoking RPC on QMK core sync data
+    if (transaction_id <= GET_RPC_RESP_DATA) return false;
+    // Prevent sizing issues
+    if (initiator2target_buffer_size > RPC_M2S_BUFFER_SIZE) return false;
+    if (target2initiator_buffer_size > RPC_S2M_BUFFER_SIZE) return false;
+
+    // Prepare the metadata block
+    rpc_sync_info_t info = {.transaction_id = transaction_id, .m2s_length = initiator2target_buffer_size, .s2m_length = target2initiator_buffer_size};
+
+    // Make sure the local side knows that we're not sending the full block of data
+    split_transaction_table[PUT_RPC_REQ_DATA].initiator2target_buffer_size  = initiator2target_buffer_size;
+    split_transaction_table[GET_RPC_RESP_DATA].target2initiator_buffer_size = target2initiator_buffer_size;
+
+    // Run through the sequence:
+    // * set the transaction ID and lengths
+    // * send the request data
+    // * execute RPC callback
+    // * retrieve the response data
+    if (!transport_write(PUT_RPC_INFO, &info, sizeof(info))) {
+        return false;
+    }
+    if (!transport_write(PUT_RPC_REQ_DATA, initiator2target_buffer, initiator2target_buffer_size)) {
+        return false;
+    }
+    if (!transport_write(EXECUTE_RPC, &transaction_id, sizeof(transaction_id))) {
+        return false;
+    }
+    if (!transport_read(GET_RPC_RESP_DATA, target2initiator_buffer, target2initiator_buffer_size)) {
+        return false;
+    }
+    return true;
+}
+
+void slave_rpc_info_callback(uint8_t initiator2target_buffer_size, const void *initiator2target_buffer, uint8_t target2initiator_buffer_size, void *target2initiator_buffer) {
+    // The RPC info block contains the intended transaction ID, as well as the sizes for both inbound and outbound data.
+    // Ignore the args -- the `split_shmem` already has the info, we just need to act upon it.
+    // We must keep the `split_transaction_table` non-const, so that it is able to be modified at runtime.
+
+    split_transaction_table[PUT_RPC_REQ_DATA].initiator2target_buffer_size  = split_shmem->rpc_info.m2s_length;
+    split_transaction_table[GET_RPC_RESP_DATA].target2initiator_buffer_size = split_shmem->rpc_info.s2m_length;
+}
+
+void slave_rpc_exec_callback(uint8_t initiator2target_buffer_size, const void *initiator2target_buffer, uint8_t target2initiator_buffer_size, void *target2initiator_buffer) {
+    // We can assume that the buffer lengths are correctly set, now, given that sequentially the rpc_info callback was already executed.
+    // Go through the rpc_info and execute _that_ transaction's callback, with the scratch buffers as inputs.
+    int8_t transaction_id = split_shmem->rpc_info.transaction_id;
+    if (transaction_id < NUM_TOTAL_TRANSACTIONS) {
+        split_transaction_desc_t *trans = &split_transaction_table[transaction_id];
+        if (trans->slave_callback) {
+            trans->slave_callback(split_shmem->rpc_info.m2s_length, split_shmem->rpc_m2s_buffer, split_shmem->rpc_info.s2m_length, split_shmem->rpc_s2m_buffer);
+        }
+    }
+}
+
+#endif  // defined(SPLIT_TRANSACTION_IDS_KB) || defined(SPLIT_TRANSACTION_IDS_USER)
diff --git a/quantum/split_common/transactions.h b/quantum/split_common/transactions.h
new file mode 100644
index 0000000000..4306ba1d87
--- /dev/null
+++ b/quantum/split_common/transactions.h
@@ -0,0 +1,54 @@
+/* Copyright 2021 QMK
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "stdint.h"
+#include "stdbool.h"
+
+#include "matrix.h"
+#include "transaction_id_define.h"
+#include "transport.h"
+
+typedef void (*slave_callback_t)(uint8_t initiator2target_buffer_size, const v