Allow writing buffer of length zero to I2C device; it can be used to poll for existence.

dhalbert · tannewt · commit f738996164fc · 2017-10-30T18:32:36.000-07:00
diff --git a/shared-bindings/busio/I2C.c b/shared-bindings/busio/I2C.c
@@ -171,6 +171,7 @@ MP_DEFINE_CONST_FUN_OBJ_1(busio_i2c_unlock_obj, busio_i2c_obj_unlock);
 //|
 //|      Read into ``buffer`` from the slave specified by ``address``.
 //|      The number of bytes read will be the length of ``buffer``.
+//|      At least one byte must be read.
 //|
 //|      If ``start`` or ``end`` is provided, then the buffer will be sliced
 //|      as if ``buffer[start:end]``. This will not cause an allocation like
@@ -223,6 +224,9 @@ MP_DEFINE_CONST_FUN_OBJ_KW(busio_i2c_readfrom_into_obj, 3, busio_i2c_readfrom_in
 //|      as if ``buffer[start:end]``. This will not cause an allocation like
 //|      ``buffer[start:end]`` will so it saves memory.
 //|
+//|      Writing a buffer or slice of length zero is permitted, as it can be used
+//|      to poll for the existence of a device.
+//|
 //|      :param int address: 7-bit device address
 //|      :param bytearray buffer: buffer containing the bytes to write
 //|      :param int start: Index to start writing from
@@ -254,10 +258,6 @@ STATIC mp_obj_t busio_i2c_writeto(size_t n_args, const mp_obj_t *pos_args, mp_ma
     uint32_t length = bufinfo.len;
     normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
 
-    if (length == 0) {
-        mp_raise_ValueError("Buffer must be at least length 1");
-    }
-
     // do the transfer
     uint8_t status = common_hal_busio_i2c_write(self, args[ARG_address].u_int,
         ((uint8_t*) bufinfo.buf) + start, length, args[ARG_stop].u_bool);
diff --git a/shared-bindings/busio/SPI.c b/shared-bindings/busio/SPI.c
@@ -207,6 +207,7 @@ MP_DEFINE_CONST_FUN_OBJ_1(busio_spi_unlock_obj, busio_spi_obj_unlock);
 //|   .. method:: SPI.write(buffer, \*, start=0, end=len(buffer))
 //|
 //|     Write the data contained in ``buf``. Requires the SPI being locked.
+//|     At least one byte must be written.
 //|
 //|     :param bytearray buffer: buffer containing the bytes to write
 //|     :param int start: Index to start writing from
@@ -247,6 +248,7 @@ MP_DEFINE_CONST_FUN_OBJ_KW(busio_spi_write_obj, 2, busio_spi_write);
 //|   .. method:: SPI.readinto(buffer, \*, start=0, end=len(buffer), write_value=0)
 //|
 //|     Read into the buffer specified by ``buf`` while writing zeroes. Requires the SPI being locked.
+//|     At least one byte must be read.
 //|
 //|     :param bytearray buffer: buffer to write into
 //|     :param int start: Index to start writing at
