stm32/can: Add "list" param to CAN.recv() to receive data inplace.

This API matches (as close as possible) how other pyb classes allow inplace
operations, such as pyb.SPI.recv(buf).

docs/library/pyb.CAN.rst

@@ -187,11 +187,12 @@ Methods
 
    Return ``True`` if any message waiting on the FIFO, else ``False``.
 
-.. method:: CAN.recv(fifo, \*, timeout=5000)
+.. method:: CAN.recv(fifo, list=None, \*, timeout=5000)
 
    Receive data on the bus:
 
      - *fifo* is an integer, which is the FIFO to receive on
+     - *list* is an optional list object to be used as the return value
      - *timeout* is the timeout in milliseconds to wait for the receive.
 
    Return value: A tuple containing four values.
@@ -201,6 +202,24 @@ Methods
      - The FMI (Filter Match Index) value.
      - An array containing the data.
 
+   If *list* is ``None`` then a new tuple will be allocated, as well as a new
+   bytes object to contain the data (as the fourth element in the tuple).
+
+   If *list* is not ``None`` then it should be a list object with a least four
+   elements.  The fourth element should be a memoryview object which is created
+   from either a bytearray or an array of type 'B' or 'b', and this array must
+   have enough room for at least 8 bytes.  The list object will then be
+   populated with the first three return values above, and the memoryview object
+   will be resized inplace to the size of the data and filled in with that data.
+   The same list and memoryview objects can be reused in subsequent calls to
+   this method, providing a way of receiving data without using the heap.
+   For example::
+
+        buf = bytearray(8)
+        lst = [0, 0, 0, memoryview(buf)]
+        # No heap memory is allocated in the following call
+        can.recv(0, lst)
+
 .. method:: CAN.send(data, id, \*, timeout=0, rtr=False)
 
    Send a message on the bus:

ports/stm32/can.c

@@ -29,8 +29,10 @@
 #include <stdarg.h>
 
 #include "py/objtuple.h"
+#include "py/objarray.h"
 #include "py/runtime.h"
 #include "py/gc.h"
+#include "py/binary.h"
 #include "py/stream.h"
 #include "py/mperrno.h"
 #include "py/mphal.h"
@@ -645,18 +647,20 @@ STATIC mp_obj_t pyb_can_send(size_t n_args, const mp_obj_t *pos_args, mp_map_t *
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_KW(pyb_can_send_obj, 1, pyb_can_send);
 
-/// \method recv(fifo, *, timeout=5000)
+/// \method recv(fifo, list=None, *, timeout=5000)
 ///
 /// Receive data on the bus:
 ///
 ///   - `fifo` is an integer, which is the FIFO to receive on
+///   - `list` if not None is a list with at least 4 elements
 ///   - `timeout` is the timeout in milliseconds to wait for the receive.
 ///
 /// Return value: buffer of data bytes.
 STATIC mp_obj_t pyb_can_recv(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
-    enum { ARG_fifo, ARG_timeout };
+    enum { ARG_fifo, ARG_list, ARG_timeout };
     static const mp_arg_t allowed_args[] = {
         { MP_QSTR_fifo,    MP_ARG_REQUIRED | MP_ARG_INT, {.u_int = 0} },
+        { MP_QSTR_list,    MP_ARG_OBJ, {.u_obj = mp_const_none} },
         { MP_QSTR_timeout, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 5000} },
     };
 
@@ -700,23 +704,49 @@ STATIC mp_obj_t pyb_can_recv(size_t n_args, const mp_obj_t *pos_args, mp_map_t *
         }
     }
 
-    // return the received data
-    // TODO use a namedtuple (when namedtuple types can be stored in ROM)
-    mp_obj_tuple_t *tuple = mp_obj_new_tuple(4, NULL);
-    if (rx_msg.IDE == CAN_ID_STD) {
-        tuple->items[0] = MP_OBJ_NEW_SMALL_INT(rx_msg.StdId);
+    // Create the tuple, or get the list, that will hold the return values
+    // Also populate the fourth element, either a new bytes or reuse existing memoryview
+    mp_obj_t ret_obj = args[ARG_list].u_obj;
+    mp_obj_t *items;
+    if (ret_obj == mp_const_none) {
+        ret_obj = mp_obj_new_tuple(4, NULL);
+        items = ((mp_obj_tuple_t*)MP_OBJ_TO_PTR(ret_obj))->items;
+        items[3] = mp_obj_new_bytes(&rx_msg.Data[0], rx_msg.DLC);
     } else {
-        tuple->items[0] = MP_OBJ_NEW_SMALL_INT(rx_msg.ExtId);
+        // User should provide a list of length at least 4 to hold the values
+        if (!MP_OBJ_IS_TYPE(ret_obj, &mp_type_list)) {
+            mp_raise_TypeError(NULL);
+        }
+        mp_obj_list_t *list = MP_OBJ_TO_PTR(ret_obj);
+        if (list->len < 4) {
+            mp_raise_ValueError(NULL);
+        }
+        items = list->items;
+        // Fourth element must be a memoryview which we assume points to a
+        // byte-like array which is large enough, and then we resize it inplace
+        if (!MP_OBJ_IS_TYPE(items[3], &mp_type_memoryview)) {
+            mp_raise_TypeError(NULL);
+        }
+        mp_obj_array_t *mv = MP_OBJ_TO_PTR(items[3]);
+        if (!(mv->typecode == (0x80 | BYTEARRAY_TYPECODE)
+            || (mv->typecode | 0x20) == (0x80 | 'b'))) {
+            mp_raise_ValueError(NULL);
+        }
+        mv->len = rx_msg.DLC;
+        memcpy(mv->items, &rx_msg.Data[0], rx_msg.DLC);
     }
-    tuple->items[1] = rx_msg.RTR == CAN_RTR_REMOTE ? mp_const_true : mp_const_false;
-    tuple->items[2] = MP_OBJ_NEW_SMALL_INT(rx_msg.FMI);
-    vstr_t vstr;
-    vstr_init_len(&vstr, rx_msg.DLC);
-    for (mp_uint_t i = 0; i < rx_msg.DLC; i++) {
-        vstr.buf[i] = rx_msg.Data[i]; // Data is uint32_t but holds only 1 byte
+
+    // Populate the first 3 values of the tuple/list
+    if (rx_msg.IDE == CAN_ID_STD) {
+        items[0] = MP_OBJ_NEW_SMALL_INT(rx_msg.StdId);
+    } else {
+        items[0] = MP_OBJ_NEW_SMALL_INT(rx_msg.ExtId);
     }
-    tuple->items[3] = mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr);
-    return tuple;
+    items[1] = rx_msg.RTR == CAN_RTR_REMOTE ? mp_const_true : mp_const_false;
+    items[2] = MP_OBJ_NEW_SMALL_INT(rx_msg.FMI);
+
+    // Return the result
+    return ret_obj;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_KW(pyb_can_recv_obj, 1, pyb_can_recv);