Enabling trace resource in the message_handler_openamp application

The message_handler_openamp application provides support for logging to a trace buffer via the trace entry in the resource table. The following instructions briefly describe one way of enabling the trace resource in the firmware's resource table:

1. Enable building message_handler_openamp with trace buffer support by setting the CMake BOOL cache entry ENABLE_REMOTEPROC_TRACE_BUFFER to ON in the target's CMakeLists.txt

2. Update the target's linker scripts to include an ethosu_core_trace_buffer section that is placed in the DRAM as shown in the examples below. The linker-defined symbols for the trace buffer's address and size are later used by the message_handler_openamp to setup the trace buffer in the resource table.

linker-script.ld:

SECTIONS
{
  .dram ():
  {
    /* Trace buffer */
    __ethosu_core_trace_buffer_start__ = .;
    *(ethosu_core_trace_buffer)
    __ethosu_core_trace_buffer_size__ = ABSOLUTE(. - __ethosu_core_trace_buffer_start__);
  } > DRAM_REGION

scatter-file.scatter:

LOAD_REGION_DRAM DRAM_ADDRESS DRAM_SIZE
{
    /* Trace buffer */
    trace_buffer (DRAM_ADDRESS + 0x20000) FIXED
    {
        * (ethosu_core_trace_buffer)
    }
}

3. Allocate the trace buffer and place it in the section defined above using the section attribute. Finally, implement the fputc function in the retarget file so that writes to stderr and stdout are redirected to the trace buffer. Example:

retarget.c:

#define TRACE_BUFFER_SIZE 0xXXXX
__attribute__((section("ethosu_core_trace_buffer"))) char trace_buffer[TRACE_BUFFER_SIZE] = {0};
int fputc(int ch, FILE *f) {
    (void)f;
    uint8_t c = (uint8_t)ch;

    static uint32_t write = 0;
    trace_buffer[write % TRACE_BUFFER_SIZE] = c;
    write++;

    return c;
}