[rtl872x] hal: fix false TX DMA completion in SPI HAL.

[XuGuohui]

Problem

SPIClass::transfer(const void* tx_buffer, void* rx_buffer, size_t length, wiring_spi_dma_transfercomplete_callback_t user_callback) exits even if the data is not fully clocked out when RX buffer is nullptr. If the CS pin is deasserted following the API, the on-going SPI transaction is ended up unexpectedly.

Solution

Check the busy state of the SPI peripheral, instead of only relying on internal flags.

Steps to Test

Run the attached test app.

Example App

#include "application.h"

SYSTEM_MODE(MANUAL);

#define SPI_TRANSACTION_LEN     (4096 * 3 + 9)

Serial1LogHandler l(115200, LOG_LEVEL_ALL);

uint8_t txBuf[SPI_TRANSACTION_LEN] = {0x00};
uint8_t rxBuf[SPI_TRANSACTION_LEN];

void setup() {
    while (!Serial.isConnected());
    Log.info("Starting...");

    pinMode(SS, OUTPUT);
    SPI.setClockSpeed(1000000);
    SPI.begin(SPI_MODE_MASTER);

    for (uint16_t i = 0; i < SPI_TRANSACTION_LEN; i++) {
        txBuf[i] = i;
    }

    delay(1s);
}

void loop() {
    digitalWrite(SS, LOW);

    // 1. No RX buffer
    SPI.transfer(txBuf, nullptr, SPI_TRANSACTION_LEN, nullptr);

    // 2. With RX buffer
    // SPI.transfer(txBuf, rxBuf, SPI_TRANSACTION_LEN, nullptr);

    // 3. No TX buffer (but there is an internal dummy buffer)
    // SPI.transfer(nullptr, rxBuf, SPI_TRANSACTION_LEN, nullptr);

    digitalWrite(SS, HIGH);

    delay(1s);
    Log.info("loop");
}

References

https://app.shortcut.com/particle/story/125500/fram-read-write-failures-for-jacuzzi-on-msom

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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
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)