[rtl872x] fixes DMA enabled USART flush

[technobly]

Problem

• Serial2.flush() on P2/Photon2 does not wait for bytes to be emptied from USART buffer on D4 (Serial2 TX), but instead when DMA transfer is complete.

Solution

• Enable the IRQ used to signal TX buffer FIFO emptied for DMA enabled USARTs (Serial2 on P2/Photon2), along with a new busy_ flag, to assist in waiting long enough for all bytes to be transmitted when busy waiting with Serial2.flush()

Steps to Test

• Run integration tests TEST=wiring/no_fixture
• Run fixture test TEST=wiring/serial_loopback2 (with TX jumpered to RX)
• Run example test app while watching the TX pin for each Serial1/2/3 (TX/D4/MOSI) interface and also the signalPin (A0) on Photon2.
• Run example test app with Serial1 on M-SoM ensuring the Cellular connection works properly.

Example App

#include "Particle.h"

SYSTEM_MODE(SEMI_AUTOMATIC);
SYSTEM_THREAD(ENABLED);

SerialLogHandler logHandler(LOG_LEVEL_INFO);

// Try them all, signalPin should not go LOW before all bytes are flush()'d
// #define MY_SERIAL Serial1
#define MY_SERIAL Serial2
// #define MY_SERIAL Serial3

// P2/Photon2
// TX = Serial1 TX
// D4 = Serial2 TX
// MOSI = Serial3 TX

const pin_t signalPin = A0;
const int baudRate = 600;

auto TEST_PERIOD_MS = 5000;
auto lastTest = 0;

void runTest();

void setup() {
    pinMode(signalPin, OUTPUT);
    digitalWrite(signalPin, LOW);

    Particle.connect();
    waitFor(Particle.connected, 60000);

    MY_SERIAL.begin(baudRate);
}

void loop() {
    if (millis() - lastTest >= TEST_PERIOD_MS) {
        lastTest = millis();
        runTest();
    }
}

void runTest() {
    digitalWrite(signalPin, HIGH);
    int bufferSize = MY_SERIAL.availableForWrite();
    auto start = millis();

    for (int ii = 0; ii < 64; ii++) {
        char c = 0x20 + ii;
        MY_SERIAL.write(c);
    }
    auto writeComplete = millis();
    MY_SERIAL.flush();
    auto empty = millis();
    digitalWrite(signalPin, LOW);
    Log.info("bufferSize=%d writeComplete=%lu empty=%lu", bufferSize, writeComplete - start, empty - start);
}

References

• https://community.particle.io/t/issues-with-serial2-in-a-photon2-p2-and-modbusclient-library/67177/4

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• (N/A) Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)