tmc_uart: Limit to only one active uart at a time on an mcu

The tmcuart_send command increases cpu usage on the micro-controller.
Should multiple tmcuart_send commands be issued at the same time to a
single AVR micro-controller, it could increase the load to the point
that it introduces a failure. It could also lead to tmcuart_send
transmission errors, which would cause retransmission requests, which
further increase the load.

Track and share mutexes so that only one tmcuart_send command can be
active on a single mcu at a time.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/tmc_uart.py

@@ -53,13 +53,30 @@ class MCU_analog_mux:
 # TMC uart communication
 ######################################################################
 
+# Share mutexes so only one active tmc_uart command on a single mcu at
+# a time. This helps limit cpu usage on slower micro-controllers.
+class PrinterTMCUartMutexes:
+    def __init__(self):
+        self.mcu_to_mutex = {}
+def lookup_tmc_uart_mutex(mcu):
+    printer = mcu.get_printer()
+    pmutexes = printer.lookup_object('tmc_uart', None)
+    if pmutexes is None:
+        pmutexes = PrinterTMCUartMutexes()
+        printer.add_object('tmc_uart', pmutexes)
+    mutex = pmutexes.mcu_to_mutex.get(mcu)
+    if mutex is None:
+        mutex = printer.get_reactor().mutex()
+        pmutexes.mcu_to_mutex[mcu] = mutex
+    return mutex
+
 TMC_BAUD_RATE = 9000
 
 # Code for sending messages on a TMC uart
 class MCU_TMC_uart_bitbang:
     def __init__(self, rx_pin_params, tx_pin_params, select_pins_desc):
         self.mcu = rx_pin_params['chip']
-        self.mutex = self.mcu.get_printer().get_reactor().mutex()
+        self.mutex = lookup_tmc_uart_mutex(self.mcu)
         self.pullup = rx_pin_params['pullup']
         self.rx_pin = rx_pin_params['pin']
         self.tx_pin = tx_pin_params['pin']