Readbytesb -- return bytes object for efficiency

README.md

@@ -50,7 +50,11 @@ Connects to the specified SPI device, opening `/dev/spidev<bus>.<device>`
 
     readbytes(n)
 
-Read n bytes from SPI device.
+Read n bytes from SPI device, returning a list of integers.
+
+    readbytesb(n)
+
+Read n bytes from SPI device, returning an (immutable) bytes object (more efficient for larger transfers).
 
     writebytes(list of values)
 

spidev_module.c

@@ -223,27 +223,41 @@ SpiDev_writebytes(SpiDevObject *self, PyObject *args)
 	return Py_None;
 }
 
-PyDoc_STRVAR(SpiDev_read_doc,
-	"read(len) -> [values]\n\n"
-	"Read len bytes from SPI device.\n");
 
 static PyObject *
-SpiDev_readbytes(SpiDevObject *self, PyObject *args)
+SpiDev_readbytes_generic(SpiDevObject *self, PyObject *args, int resultType)
 {
-	uint8_t	rxbuf[SPIDEV_MAXPATH];
-	int		status, len, ii;
-	PyObject	*list;
-
-	if (!PyArg_ParseTuple(args, "i:read", &len))
-		return NULL;
-
-	/* read at least 1 byte, no more than SPIDEV_MAXPATH */
-	if (len < 1)
-		len = 1;
-	else if ((unsigned)len > sizeof(rxbuf))
-		len = sizeof(rxbuf);
+    static uint8_t stackbuf[SPIDEV_MAXPATH];
+	int		status, len;
+	Py_buffer pybuff;
+	uint8_t* rxbuf = stackbuf;
+
+    if (resultType == 2) { //existing bytearray
+        if (!PyArg_ParseTuple(args, "y*", &pybuff))
+            return NULL;
+        if (!PyBuffer_IsContiguous(&pybuff, 'A')) {
+            PyErr_SetString(PyExc_RuntimeError, "SpiDev.readbuffer: buffer must be contiguous");
+            return NULL;
+        }
+        len = pybuff.len;
+        rxbuf = pybuff.buf;
+        if (len < 1) {
+            PyErr_SetString(PyExc_RuntimeError, "SpiDev.readbuffer: buffer must not be empty");
+            return NULL;
+        }
+    }
+    else {
+    	if (!PyArg_ParseTuple(args, "i:read", &len))
+	    	return NULL;
+
+        /* read at least 1 byte, no more than SPIDEV_MAXPATH */
+        if (len < 1)
+            len = 1;
+        else if ((unsigned)len > sizeof(stackbuf))
+            len = sizeof(stackbuf);
+    	memset(rxbuf, 0, sizeof(stackbuf));
+    }
 
-	memset(rxbuf, 0, sizeof rxbuf);
 	status = read(self->fd, &rxbuf[0], len);
 
 	if (status < 0) {
@@ -252,20 +266,69 @@ SpiDev_readbytes(SpiDevObject *self, PyObject *args)
 	}
 
 	if (status != len) {
-		perror("short read");
+		printf("Short Read: %d/%d bytes\n", status, len);
+        PyErr_SetString(PyExc_IOError, "SpiDev.readbuffer: I/O error: short read.");
 		return NULL;
 	}
 
-	list = PyList_New(len);
+	if (resultType == 0) { // list
+    	int ii;
+	    PyObject	*list;
+	    list = PyList_New(len);
 
-	for (ii = 0; ii < len; ii++) {
-		PyObject *val = Py_BuildValue("l", (long)rxbuf[ii]);
-		PyList_SET_ITEM(list, ii, val);
-	}
+        for (ii = 0; ii < len; ii++) {
+            PyObject *val = Py_BuildValue("l", (long)rxbuf[ii]);
+            PyList_SET_ITEM(list, ii, val);
+        }
+
+        return list;
+    }
+    else if (resultType == 1) { // bytes
+        PyObject	*bytes;
+        bytes = Py_BuildValue("y#", rxbuf, len);
+        return bytes;
+    }
+    else if (resultType == 2) { // bytearray
+        Py_RETURN_NONE;
+    }
+    else { // unknown type
+        PyErr_SetString(PyExc_RuntimeError, "SpiDev.readbuffer: internal problem.  Type was not 0, 1 or 2.  Weird.");
+        return NULL;
+    }
+}
+
+
+PyDoc_STRVAR(SpiDev_read_doc,
+	"read(len) -> [values]\n\n"
+	"Read len bytes from SPI device, returning a list.\n");
+
+static PyObject *
+SpiDev_readbytes(SpiDevObject *self, PyObject *args)
+{
+    return SpiDev_readbytes_generic(self, args, 0);
+}
 
-	return list;
+PyDoc_STRVAR(SpiDev_readb_doc,
+             "read(len) -> [values]\n\n"
+             "Read len bytes from SPI device, returning a bytes object.\n");
+
+static PyObject *
+SpiDev_readbytesb(SpiDevObject *self, PyObject *args)
+{
+    return SpiDev_readbytes_generic(self, args, 1);
+}
+
+PyDoc_STRVAR(SpiDev_readbuffer_doc,
+             "read(bytearray) -> [values]\n\n"
+             "Read bytes from SPI device, filling the supplied byte array.\n");
+
+static PyObject *
+SpiDev_readbuffer(SpiDevObject *self, PyObject *args)
+{
+    return SpiDev_readbytes_generic(self, args, 2);
 }
 
+
 static PyObject *
 SpiDev_writebytes2_buffer(SpiDevObject *self, Py_buffer *buffer)
 {
@@ -861,6 +924,7 @@ SpiDev_xfer3(SpiDevObject *self, PyObject *args)
 	return rx_tuple;
 }
 
+
 static int __spidev_set_mode( int fd, __u8 mode) {
 	__u8 test;
 	if (ioctl(fd, SPI_IOC_WR_MODE, &mode) == -1) {
@@ -1365,6 +1429,8 @@ static PyMethodDef SpiDev_methods[] = {
 		SpiDev_fileno_doc},
 	{"readbytes", (PyCFunction)SpiDev_readbytes, METH_VARARGS,
 		SpiDev_read_doc},
+	{"readbytesb", (PyCFunction)SpiDev_readbytesb, METH_VARARGS,
+		SpiDev_readb_doc},
 	{"writebytes", (PyCFunction)SpiDev_writebytes, METH_VARARGS,
 		SpiDev_write_doc},
 	{"writebytes2", (PyCFunction)SpiDev_writebytes2, METH_VARARGS,
@@ -1375,6 +1441,7 @@ static PyMethodDef SpiDev_methods[] = {
 		SpiDev_xfer2_doc},
 	{"xfer3", (PyCFunction)SpiDev_xfer3, METH_VARARGS,
 		SpiDev_xfer3_doc},
+	{"readbuffer", (PyCFunction)SpiDev_readbuffer, METH_VARARGS, SpiDev_readbuffer_doc},
 	{"__enter__", (PyCFunction)SpiDev_enter, METH_VARARGS,
 		NULL},
 	{"__exit__", (PyCFunction)SpiDev_exit, METH_VARARGS,

test.py

@@ -0,0 +1,31 @@
+import sys
+sys.path = ['.'] + sys.path
+
+# https://pypi.org/project/spidev/
+import spidev
+import numpy
+import timeit
+class testspi (object):
+    def __init__(self):
+        spi=spidev.SpiDev()
+        spi.open(0, 0)
+        print("devspi readbytes", spi.readbytes(50))
+        spibytes = spi.readbytesb(50)
+        npbytes = numpy.frombuffer(spibytes, dtype=numpy.int16)
+        print("devspi readbytesb", npbytes)
+        bytes = 1000
+        buffer = bytearray(1000)
+        count = 1000
+        def read(): spi.readbytes(bytes)
+        def readb(): spi.readbytesb(bytes)
+        def readbuffer(): spi.readbuffer(buffer)
+        duration = timeit.timeit(read, number=count)
+        print('list: total dur %f; secs/byte=%f; bytes/sec=%f' % (duration, duration/(bytes*count), (bytes*count)/duration))
+        duration = timeit.timeit(readb, number=count)
+        print('bytes: total dur %f; secs/byte=%f; bytes/sec=%f' % (duration, duration/(bytes*count), (bytes*count)/duration))
+        duration = timeit.timeit(readbuffer, number=count)
+        print('bytes: total dur %f; secs/byte=%f; bytes/sec=%f' % (duration, duration/(bytes*count), (bytes*count)/duration))
+        spi.close()
+
+
+testspi()