It should be possible to write out a serial port asynchronously without DMA (using `DMA_USAGE_NEVER`)

[Patater]

Description

• Type: Bug
• Priority: Major

---

Bug

Target
Observed on K64F
Should be reproducible on any other target supporting async serial and DMA

Toolchain:
GCC_ARM

Toolchain version:
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 5.4.1 20160919 (release) [ARM/embedded-5-branch revision 240496]

mbed-cli version:
0.9.10

meed-os sha:
e7361eb with additional patch applied from NXP, "K64F: Add support for SERIAL ASYNCH API"
Use PR #3438
or use my branch at https://github.com/Patater/mbed-os/tree/k64f-serial-dma-fun)

I'd expect the bug would be reproducible any target that supports the serial async API with DMA, however.

Expected behavior
It should be possible to write out a serial port asynchronously without DMA.

Actual behavior
The write goes out the serial port, but we never receive a TX completion event (SERIAL_EVENT_TX_COMPLETE or anything else matching SERIAL_EVENT_TX_ALL)

If no DMA channels are available or if the DMA usage hint DMA_USAGE_NEVER is provided, the serial driver is supposed to fall back to using interrupts. However, it looks like that fallback behavior isn't working. After a quick inspection of the code, I couldn't see where the UART's interrupt is enabled: EnableIRQ is called, but K64F's UART_EnableInterrupts or the HAL's serial_irq_set appears not to be called, so the UART appears to never trigger the IRQ and we never get a TX completion event at the application level.

Steps to reproduce
I was able to reproduce the issue on K64F with the following steps. However, one should be able to reproduce the issue on any other target with DEVICE_SERIAL_ASYNCH

1. Make a new project (mbed new serial_dma_fun)
2. Get mbed-os from PR K64F: Add support for SERIAL ASYNCH API #3438 (or use my branch at https://github.com/Patater/mbed-os/tree/k64f-serial-dma-fun)
3. Create main.cpp as below
4. Compile and run (mbed compile -m K64F -t GCC_ARM)

/* main.cpp */
#include <stdint.h>

#include <mbed.h>
#include <rtos.h>

#if !DEVICE_SERIAL_ASYNCH
#error "This test suite requires the async serial API to be supported on this target"
#endif

/* This semaphore keeps track of whether or not the serial callback has
 * finished. This semaphore allows the main thread to wait for the serial
 * callback to finish. */
Semaphore serial_semaphore(0);

Timeout watchdog;

static void serial_write_callback(int events)
{
    serial_semaphore.release();
}

static void fail() {
    puts("\r\n\tSorry, we failed something.\r\n");
    for(;;);
}

static void dma_serial_write_async(const DMAUsage usage)
{
    int result;
    Serial serial(USBTX, USBRX);
    event_callback_t event;
    const uint8_t buffer[] = "Howdy\r\n";

    event.attach(serial_write_callback);

    /* Configure DMA */
    result = serial.set_dma_usage_tx(usage);
    if (result) {
        fail();
    }
    result = serial.set_dma_usage_rx(usage);
    if (result) {
        fail();
    }

    serial.write(buffer, sizeof(buffer) - 1, event, SERIAL_EVENT_TX_ALL);

    /* Wait for the write to finish by waiting for the serial callback to
     * finish. */
    /* NOTE: Completion event (callback) never happens, even though we expect
     * such an event, but the serial data does still go out the serial port. */
    serial_semaphore.wait();
}

static void dma_serial_write_async_without_dma(void)
{
    printf("-------- %s --------", __func__);
    fflush(stdout);
    dma_serial_write_async(DMA_USAGE_NEVER);
    puts("\r\n");
    fflush(stdout);
}

static void timed_out() {
    /* We've timed out. */
    puts("\r\n\tSorry, looks like we hung.\r\n");
}

static void setup_watchdog(void)
{
    /* Set up a watchdog timer to kill things that hang. This will call our
     * callback in ISR mode after 2 seconds. */
    watchdog.attach(timed_out, 2.0);
}

int main(void)
{
    puts("\r\nHello and welcome to serial DMA fun!\r\n");

    setup_watchdog();

    /* This will hang. */
    dma_serial_write_async_without_dma();

    puts("Congratulations! You've made it through the gauntlet!\r\n");
}

The output of running the program is as follows. The "Howdy" text is what we attempted to write out the serial port with asynchronously with the DMA hint DMA_USAGE_NEVER.

Hello and welcome to serial DMA fun!

-------- dma_serial_write_async_without_dma -------Howdy

	Sorry, looks like we hung.

[bridadan]

cc @0xc0170 @mmahadevan108

[mmahadevan108]

Thanks for the test, I have submitted a PR with the fix.

[Patater]

Tested working with PR #3558 on both K64F and EFM32GG_STK3700 (which actually didn't have this problem, despite the original suspicion that it might).

Thanks for the fix!

[theotherjimmy]

So, can we close this? @Patater

[Patater]

PR #3558 was merged.