esp32/machine_i2c: Implement write-then-read I2C transfers.

I2C transfers are much more efficient if they are combined, instead of
doing separate writes and reads.

Fixes issue #7134.

Signed-off-by: Damien George <damien@micropython.org>

ports/esp32/machine_i2c.c

@@ -79,10 +79,20 @@ int machine_hw_i2c_transfer(mp_obj_base_t *self_in, uint16_t addr, size_t n, mp_
     machine_hw_i2c_obj_t *self = MP_OBJ_TO_PTR(self_in);
 
     i2c_cmd_handle_t cmd = i2c_cmd_link_create();
+    int data_len = 0;
+
+    if (flags & MP_MACHINE_I2C_FLAG_WRITE1) {
+        i2c_master_start(cmd);
+        i2c_master_write_byte(cmd, addr << 1, true);
+        i2c_master_write(cmd, bufs->buf, bufs->len, true);
+        data_len += bufs->len;
+        --n;
+        ++bufs;
+    }
+
     i2c_master_start(cmd);
     i2c_master_write_byte(cmd, addr << 1 | (flags & MP_MACHINE_I2C_FLAG_READ), true);
 
-    int data_len = 0;
     for (; n--; ++bufs) {
         if (flags & MP_MACHINE_I2C_FLAG_READ) {
             i2c_master_read(cmd, bufs->buf, bufs->len, n == 0 ? I2C_MASTER_LAST_NACK : I2C_MASTER_ACK);

ports/esp32/mpconfigport.h

@@ -87,6 +87,7 @@
 #define MICROPY_PY_MACHINE_PWM_DUTY_U16_NS  (1)
 #define MICROPY_PY_MACHINE_PWM_INCLUDEFILE  "ports/esp32/machine_pwm.c"
 #define MICROPY_PY_MACHINE_I2C              (1)
+#define MICROPY_PY_MACHINE_I2C_TRANSFER_WRITE1 (1)
 #define MICROPY_PY_MACHINE_SOFTI2C          (1)
 #define MICROPY_PY_MACHINE_SPI              (1)
 #define MICROPY_PY_MACHINE_SPI_MSB          (0)