extmod/machine_i2c: Change C-level API to allow split I2C transactions.

API is:

    int transfer(
        mp_obj_base_t *obj,
        uint16_t addr,
        size_t n,
        mp_machine_i2c_buf_t *bufs,
        unsigned int flags
    )

extmod/machine_i2c.c

@@ -180,9 +180,9 @@ STATIC int mp_hal_i2c_read_byte(machine_i2c_obj_t *self, uint8_t *val, int nack)
 }
 
 // return value:
-//  >=0 - number of acks received
+//  >=0 - success; for read it's 0, for write it's number of acks received
 //   <0 - error, with errno being the negative of the return value
-int mp_machine_soft_i2c_writeto(mp_obj_base_t *self_in, uint16_t addr, const uint8_t *src, size_t len, bool stop) {
+int mp_machine_soft_i2c_transfer(mp_obj_base_t *self_in, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags) {
     machine_i2c_obj_t *self = (machine_i2c_obj_t*)self_in;
 
     // start the I2C transaction
@@ -192,7 +192,7 @@ int mp_machine_soft_i2c_writeto(mp_obj_base_t *self_in, uint16_t addr, const uin
     }
 
     // write the slave address
-    ret = mp_hal_i2c_write_byte(self, addr << 1);
+    ret = mp_hal_i2c_write_byte(self, (addr << 1) | (flags & MP_MACHINE_I2C_FLAG_READ));
     if (ret < 0) {
         return ret;
     } else if (ret != 0) {
@@ -201,69 +201,102 @@ int mp_machine_soft_i2c_writeto(mp_obj_base_t *self_in, uint16_t addr, const uin
         return -MP_ENODEV;
     }
 
-    // write the buffer to the I2C memory
-    int num_acks = 0;
-    while (len--) {
-        ret = mp_hal_i2c_write_byte(self, *src++);
-        if (ret < 0) {
-            return ret;
-        } else if (ret != 0) {
-            // nack received, stop sending
-            break;
+    int transfer_ret = 0;
+    for (; n--; ++bufs) {
+        size_t len = bufs->len;
+        uint8_t *buf = bufs->buf;
+        if (flags & MP_MACHINE_I2C_FLAG_READ) {
+            // read bytes from the slave into the given buffer(s)
+            while (len--) {
+                ret = mp_hal_i2c_read_byte(self, buf++, (n | len) == 0);
+                if (ret != 0) {
+                    return ret;
+                }
+            }
+        } else {
+            // write bytes from the given buffer(s) to the slave
+            while (len--) {
+                ret = mp_hal_i2c_write_byte(self, *buf++);
+                if (ret < 0) {
+                    return ret;
+                } else if (ret != 0) {
+                    // nack received, stop sending
+                    n = 0;
+                    break;
+                }
+                ++transfer_ret; // count the number of acks
+            }
         }
-        ++num_acks;
     }
 
     // finish the I2C transaction
-    if (stop) {
+    if (flags & MP_MACHINE_I2C_FLAG_STOP) {
         ret = mp_hal_i2c_stop(self);
         if (ret != 0) {
             return ret;
         }
     }
 
-    return num_acks;
+    return transfer_ret;
 }
 
-// return value:
-//    0 - success
-//   <0 - error, with errno being the negative of the return value
-int mp_machine_soft_i2c_readfrom(mp_obj_base_t *self_in, uint16_t addr, uint8_t *dest, size_t len, bool stop) {
-    machine_i2c_obj_t *self = (machine_i2c_obj_t*)self_in;
-
-    // start the I2C transaction
-    int ret = mp_hal_i2c_start(self);
-    if (ret != 0) {
-        return ret;
-    }
-
-    // write the slave address
-    ret = mp_hal_i2c_write_byte(self, (addr << 1) | 1);
-    if (ret < 0) {
-        return ret;
-    } else if (ret != 0) {
-        // nack received, release the bus cleanly
-        mp_hal_i2c_stop(self);
-        return -MP_ENODEV;
-    }
-
-    // read the bytes from the slave
-    while (len--) {
-        ret = mp_hal_i2c_read_byte(self, dest++, len == 0);
-        if (ret != 0) {
-            return ret;
+/******************************************************************************/
+// Generic helper functions
+
+// For use by ports that require a single buffer of data for a read/write transfer
+int mp_machine_i2c_transfer_adaptor(mp_obj_base_t *self, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags) {
+    size_t len;
+    uint8_t *buf;
+    if (n == 1) {
+        // Use given single buffer
+        len = bufs[0].len;
+        buf = bufs[0].buf;
+    } else {
+        // Combine buffers into a single one
+        len = 0;
+        for (size_t i = 0; i < n; ++i) {
+            len += bufs[i].len;
+        }
+        buf = m_new(uint8_t, len);
+        if (!(flags & MP_MACHINE_I2C_FLAG_READ)) {
+            len = 0;
+            for (size_t i = 0; i < n; ++i) {
+                memcpy(buf + len, bufs[i].buf, bufs[i].len);
+                len += bufs[i].len;
+            }
         }
     }
 
-    // finish the I2C transaction
-    if (stop) {
-        ret = mp_hal_i2c_stop(self);
-        if (ret != 0) {
-            return ret;
+    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
+    int ret = i2c_p->transfer_single(self, addr, len, buf, flags);
+
+    if (n > 1) {
+        if (flags & MP_MACHINE_I2C_FLAG_READ) {
+            // Copy data from single buffer to individual ones
+            len = 0;
+            for (size_t i = 0; i < n; ++i) {
+                memcpy(bufs[i].buf, buf + len, bufs[i].len);
+                len += bufs[i].len;
+            }
         }
+        m_del(uint8_t, buf, len);
     }
 
-    return 0; // success
+    return ret;
+}
+
+STATIC int mp_machine_i2c_readfrom(mp_obj_base_t *self, uint16_t addr, uint8_t *dest, size_t len, bool stop) {
+    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
+    mp_machine_i2c_buf_t buf = {.len = len, .buf = dest};
+    unsigned int flags = MP_MACHINE_I2C_FLAG_READ | (stop ? MP_MACHINE_I2C_FLAG_STOP : 0);
+    return i2c_p->transfer(self, addr, 1, &buf, flags);
+}
+
+STATIC int mp_machine_i2c_writeto(mp_obj_base_t *self, uint16_t addr, const uint8_t *src, size_t len, bool stop) {
+    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
+    mp_machine_i2c_buf_t buf = {.len = len, .buf = (uint8_t*)src};
+    unsigned int flags = stop ? MP_MACHINE_I2C_FLAG_STOP : 0;
+    return i2c_p->transfer(self, addr, 1, &buf, flags);
 }
 
 /******************************************************************************/
@@ -318,11 +351,10 @@ MP_DEFINE_CONST_FUN_OBJ_KW(machine_i2c_init_obj, 1, machine_i2c_obj_init);
 
 STATIC mp_obj_t machine_i2c_scan(mp_obj_t self_in) {
     mp_obj_base_t *self = MP_OBJ_TO_PTR(self_in);
-    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
     mp_obj_t list = mp_obj_new_list(0, NULL);
     // 7-bit addresses 0b0000xxx and 0b1111xxx are reserved
     for (int addr = 0x08; addr < 0x78; ++addr) {
-        int ret = i2c_p->writeto(self, addr, NULL, 0, true);
+        int ret = mp_machine_i2c_writeto(self, addr, NULL, 0, true);
         if (ret == 0) {
             mp_obj_list_append(list, MP_OBJ_NEW_SMALL_INT(addr));
         }
@@ -407,12 +439,11 @@ MP_DEFINE_CONST_FUN_OBJ_2(machine_i2c_write_obj, machine_i2c_write);
 
 STATIC mp_obj_t machine_i2c_readfrom(size_t n_args, const mp_obj_t *args) {
     mp_obj_base_t *self = (mp_obj_base_t*)MP_OBJ_TO_PTR(args[0]);
-    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
     mp_int_t addr = mp_obj_get_int(args[1]);
     vstr_t vstr;
     vstr_init_len(&vstr, mp_obj_get_int(args[2]));
     bool stop = (n_args == 3) ? true : mp_obj_is_true(args[3]);
-    int ret = i2c_p->readfrom(self, addr, (uint8_t*)vstr.buf, vstr.len, stop);
+    int ret = mp_machine_i2c_readfrom(self, addr, (uint8_t*)vstr.buf, vstr.len, stop);
     if (ret < 0) {
         mp_raise_OSError(-ret);
     }
@@ -422,12 +453,11 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_i2c_readfrom_obj, 3, 4, machine_i2c_
 
 STATIC mp_obj_t machine_i2c_readfrom_into(size_t n_args, const mp_obj_t *args) {
     mp_obj_base_t *self = (mp_obj_base_t*)MP_OBJ_TO_PTR(args[0]);
-    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
     mp_int_t addr = mp_obj_get_int(args[1]);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[2], &bufinfo, MP_BUFFER_WRITE);
     bool stop = (n_args == 3) ? true : mp_obj_is_true(args[3]);
-    int ret = i2c_p->readfrom(self, addr, bufinfo.buf, bufinfo.len, stop);
+    int ret = mp_machine_i2c_readfrom(self, addr, bufinfo.buf, bufinfo.len, stop);
     if (ret < 0) {
         mp_raise_OSError(-ret);
     }
@@ -437,12 +467,11 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_i2c_readfrom_into_obj, 3, 4, machine
 
 STATIC mp_obj_t machine_i2c_writeto(size_t n_args, const mp_obj_t *args) {
     mp_obj_base_t *self = (mp_obj_base_t*)MP_OBJ_TO_PTR(args[0]);
-    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
     mp_int_t addr = mp_obj_get_int(args[1]);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[2], &bufinfo, MP_BUFFER_READ);
     bool stop = (n_args == 3) ? true : mp_obj_is_true(args[3]);
-    int ret = i2c_p->writeto(self, addr,  bufinfo.buf, bufinfo.len, stop);
+    int ret = mp_machine_i2c_writeto(self, addr, bufinfo.buf, bufinfo.len, stop);
     if (ret < 0) {
         mp_raise_OSError(-ret);
     }
@@ -453,19 +482,18 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_i2c_writeto_obj, 3, 4, machin
 
 STATIC int read_mem(mp_obj_t self_in, uint16_t addr, uint32_t memaddr, uint8_t addrsize, uint8_t *buf, size_t len) {
     mp_obj_base_t *self = (mp_obj_base_t*)MP_OBJ_TO_PTR(self_in);
-    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
     uint8_t memaddr_buf[4];
     size_t memaddr_len = 0;
     for (int16_t i = addrsize - 8; i >= 0; i -= 8) {
         memaddr_buf[memaddr_len++] = memaddr >> i;
     }
-    int ret = i2c_p->writeto(self, addr, memaddr_buf, memaddr_len, false);
+    int ret = mp_machine_i2c_writeto(self, addr, memaddr_buf, memaddr_len, false);
     if (ret != memaddr_len) {
         // must generate STOP
-        i2c_p->writeto(self, addr, NULL, 0, true);
+        mp_machine_i2c_writeto(self, addr, NULL, 0, true);
         return ret;
     }
-    return i2c_p->readfrom(self, addr, buf, len, true);
+    return mp_machine_i2c_readfrom(self, addr, buf, len, true);
 }
 
 #define MAX_MEMADDR_SIZE (4)
@@ -473,7 +501,6 @@ STATIC int read_mem(mp_obj_t self_in, uint16_t addr, uint32_t memaddr, uint8_t a
 
 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) {
     mp_obj_base_t *self = (mp_obj_base_t*)MP_OBJ_TO_PTR(self_in);
-    mp_machine_i2c_p_t *i2c_p = (mp_machine_i2c_p_t*)self->type->protocol;
 
     // need some memory to create the buffer to send; try to use stack if possible
     uint8_t buf2_stack[MAX_MEMADDR_SIZE + BUF_STACK_SIZE];
@@ -493,7 +520,7 @@ STATIC int write_mem(mp_obj_t self_in, uint16_t addr, uint32_t memaddr, uint8_t
     }
     memcpy(buf2 + memaddr_len, buf, len);
 
-    int ret = i2c_p->writeto(self, addr, buf2, memaddr_len + len, true);
+    int ret = mp_machine_i2c_writeto(self, addr, buf2, memaddr_len + len, true);
     if (buf2_alloc != 0) {
         m_del(uint8_t, buf2, buf2_alloc);
     }
@@ -625,8 +652,7 @@ STATIC const mp_machine_i2c_p_t mp_machine_soft_i2c_p = {
     .stop = (int(*)(mp_obj_base_t*))mp_hal_i2c_stop,
     .read = mp_machine_soft_i2c_read,
     .write = mp_machine_soft_i2c_write,
-    .readfrom = mp_machine_soft_i2c_readfrom,
-    .writeto = mp_machine_soft_i2c_writeto,
+    .transfer = mp_machine_soft_i2c_transfer,
 };
 
 const mp_obj_type_t machine_i2c_type = {

extmod/machine_i2c.h

@@ -28,15 +28,24 @@
 
 #include "py/obj.h"
 
+#define MP_MACHINE_I2C_FLAG_READ (0x01) // if not set then it's a write
+#define MP_MACHINE_I2C_FLAG_STOP (0x02)
+
+typedef struct _mp_machine_i2c_buf_t {
+    size_t len;
+    uint8_t *buf;
+} mp_machine_i2c_buf_t;
+
 // I2C protocol
 // the first 4 methods can be NULL, meaning operation is not supported
+// transfer_single only needs to be set if transfer=mp_machine_i2c_transfer_adaptor
 typedef struct _mp_machine_i2c_p_t {
     int (*start)(mp_obj_base_t *obj);
     int (*stop)(mp_obj_base_t *obj);
     int (*read)(mp_obj_base_t *obj, uint8_t *dest, size_t len, bool nack);
     int (*write)(mp_obj_base_t *obj, const uint8_t *src, size_t len);
-    int (*readfrom)(mp_obj_base_t *obj, uint16_t addr, uint8_t *dest, size_t len, bool stop);
-    int (*writeto)(mp_obj_base_t *obj, uint16_t addr, const uint8_t *src, size_t len, bool stop);
+    int (*transfer)(mp_obj_base_t *obj, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags);
+    int (*transfer_single)(mp_obj_base_t *obj, uint16_t addr, size_t len, uint8_t *buf, unsigned int flags);
 } mp_machine_i2c_p_t;
 
 typedef struct _mp_machine_soft_i2c_obj_t {
@@ -50,7 +59,7 @@ typedef struct _mp_machine_soft_i2c_obj_t {
 extern const mp_obj_type_t machine_i2c_type;
 extern const mp_obj_dict_t mp_machine_soft_i2c_locals_dict;
 
-int mp_machine_soft_i2c_readfrom(mp_obj_base_t *self_in, uint16_t addr, uint8_t *dest, size_t len, bool stop);
-int mp_machine_soft_i2c_writeto(mp_obj_base_t *self_in, uint16_t addr, const uint8_t *src, size_t len, bool stop);
+int mp_machine_i2c_transfer_adaptor(mp_obj_base_t *self, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags);
+int mp_machine_soft_i2c_transfer(mp_obj_base_t *self, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags);
 
 #endif // MICROPY_INCLUDED_EXTMOD_MACHINE_I2C_H