extmod/machine_i2c: Add optional support for write-then-read transfers.

This option is useful for ports where it's more efficient to do a full I2C
transfer in one go.
Signed-off-by: Damien George <damien@micropython.org>

extmod/machine_i2c.c

@@ -514,6 +514,21 @@ STATIC int read_mem(mp_obj_t self_in, uint16_t addr, uint32_t memaddr, uint8_t a
     uint8_t memaddr_buf[4];
     size_t memaddr_len = fill_memaddr_buf(&memaddr_buf[0], memaddr, addrsize);
 
+    #if MICROPY_PY_MACHINE_I2C_TRANSFER_WRITE1
+
+    // Create partial write and read buffers
+    mp_machine_i2c_buf_t bufs[2] = {
+        {.len = memaddr_len, .buf = memaddr_buf},
+        {.len = len, .buf = buf},
+    };
+
+    // Do write+read I2C transfer
+    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t *)self->type->protocol;
+    return i2c_p->transfer(self, addr, 2, bufs,
+        MP_MACHINE_I2C_FLAG_WRITE1 | MP_MACHINE_I2C_FLAG_READ | MP_MACHINE_I2C_FLAG_STOP);
+
+    #else
+
     int ret = mp_machine_i2c_writeto(self, addr, memaddr_buf, memaddr_len, false);
     if (ret != memaddr_len) {
         // must generate STOP
@@ -521,6 +536,8 @@ STATIC int read_mem(mp_obj_t self_in, uint16_t addr, uint32_t memaddr, uint8_t a
         return ret;
     }
     return mp_machine_i2c_readfrom(self, addr, buf, len, true);
+
+    #endif
 }
 
 STATIC int write_mem(mp_obj_t self_in, uint16_t addr, uint32_t memaddr, uint8_t addrsize, const uint8_t *buf, size_t len) {

extmod/machine_i2c.h

@@ -45,6 +45,11 @@
 #define MP_MACHINE_I2C_FLAG_READ (0x01) // if not set then it's a write
 #define MP_MACHINE_I2C_FLAG_STOP (0x02)
 
+#if MICROPY_PY_MACHINE_I2C_TRANSFER_WRITE1
+// If set, the first mp_machine_i2c_buf_t in a transfer is a write.
+#define MP_MACHINE_I2C_FLAG_WRITE1 (0x04)
+#endif
+
 typedef struct _mp_machine_i2c_buf_t {
     size_t len;
     uint8_t *buf;

py/mpconfig.h

@@ -1585,6 +1585,11 @@ typedef double mp_float_t;
 #define MICROPY_PY_MACHINE_I2C (0)
 #endif
 
+// Whether the low-level I2C transfer function supports a separate write as the first transfer
+#ifndef MICROPY_PY_MACHINE_I2C_TRANSFER_WRITE1
+#define MICROPY_PY_MACHINE_I2C_TRANSFER_WRITE1 (0)
+#endif
+
 // Whether to provide the "machine.SoftI2C" class
 #ifndef MICROPY_PY_MACHINE_SOFTI2C
 #define MICROPY_PY_MACHINE_SOFTI2C (0)