tests/multi_espnow: Add channel setting test, add some docs.

Test currently passes. It was added so it can be used to check for
regressions when fixing channel selection for AP mode in a follow-up
commit.

Also add some docs about how channel setting is observed to work for
ESP-NOW.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

tests/multi_espnow/70_channel.py

@@ -0,0 +1,89 @@
+# Test that ESP-NOW picks up the channel configuration for STA
+# mode on ESP32.
+#
+# Note that setting the channel on a peer in ESP-NOW on modern ESP-IDF only
+# checks it against the configured channel, it doesn't ever change the radio
+# channel
+import sys
+import time
+
+try:
+    import network
+    import espnow
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+# ESP8266 doesn't support config('channel') on the STA interface,
+# and the channel parameter to add_peer doesn't appear to set the
+# channel either.
+if sys.platform == "esp8266":
+    print("SKIP")
+    raise SystemExit
+
+
+timeout_ms = 1000
+default_pmk = b"MicroPyth0nRules"
+
+CHANNEL = 3
+WRONG_CHANNEL = 8
+
+
+def init_sta():
+    sta = network.WLAN(network.WLAN.IF_STA)
+    e = espnow.ESPNow()
+    e.active(True)
+    sta.active(True)
+    sta.disconnect()  # Force AP disconnect for any saved config, important so the channel doesn't change
+    sta.config(channel=CHANNEL)
+    e.set_pmk(default_pmk)
+    return sta, e
+
+
+# Receiver
+def instance0():
+    sta, e = init_sta()
+    multitest.globals(PEER=sta.config("mac"))
+    multitest.next()
+    print(sta.config("channel"))
+    while True:
+        peer, msg = e.recv(timeout_ms)
+        if peer is None:
+            print("Timeout")
+            break
+        print(msg)
+    e.active(False)
+
+
+# Sender
+def instance1():
+    sta, e = init_sta()
+    multitest.next()
+    peer = PEER
+
+    # both instances set channel via sta.config(), above
+    msg = b"sent to right channel 1"
+    e.add_peer(peer, channel=CHANNEL)
+    for _ in range(3):
+        e.send(peer, msg)
+    e.del_peer(peer)
+    print(sta.config("channel"))
+
+    sta.config(channel=WRONG_CHANNEL)
+    msg = b"sent to wrong channel"
+    e.add_peer(peer, channel=WRONG_CHANNEL)
+    for _ in range(3):
+        e.send(peer, msg)
+    e.del_peer(peer)
+    print(sta.config("channel"))
+
+    # switching back to the correct channel should also work
+    sta.config(channel=CHANNEL)
+    msg = b"sent to right channel 2"
+    e.add_peer(peer, channel=CHANNEL)
+    for _ in range(3):
+        e.send(peer, msg)
+    e.del_peer(peer)
+    print(sta.config("channel"))
+
+    e.active(False)

tests/multi_espnow/70_channel.py.exp

@@ -0,0 +1,13 @@
+--- instance0 ---
+3
+b'sent to right channel 1'
+b'sent to right channel 1'
+b'sent to right channel 1'
+b'sent to right channel 2'
+b'sent to right channel 2'
+b'sent to right channel 2'
+Timeout
+--- instance1 ---
+3
+8
+3