So I've based my implementation on two documents, the section on ETF in STMicro STM32 reference manual and ARM's Trace Memory Controller Reference Manual. In the latter in particular there is a sequence to setup the ETF:
Circular Buffer mode
The recommended standard usage model in this mode is as follows:
1. Wait until TMCReady is equal to one.
2. Program the MODE Register for Circular Buffer mode.
3. Program the FFCR Register. ARM recommends that you set the TrigOnTrigIn,
FOnTrigEvt, StopOnFl, EnTI, and EnFt bits. This enables formatting, inserting a trigger
when a trigger is observed on TRIGIN, and following a delay corresponding to the value
of the TRG Register, flushing and then stopping the TMC.
4. Program the TRG Register, to control the amount of buffer to be dedicated to the period
after a trigger is observed.
5. Set the TraceCaptEn bit in the CTL Register. This starts the trace session.
6. Wait until TMCReady is equal to one. This indicates that the trace session is over.
7. Read the contents of the trace buffer by performing successive reads to the RRD Register,
until the value 0xFFFFFFFF is returned.
8. Clear the TraceCaptEn bit in the CTL Register.
The notion of triggers is more or less incomprehensible so I've stupidly written the registers as described without really understanding what's going on.
But actually I can't go very far in my debug: it looks like the ETF registers are not accessible (they're all read to 0 either through the debugger or through my embedded software). There is probably an "enable" bit somewhere to activate or clock the TMC or ETM or ETF or who knows what macrocell.
Here is the piece of quick and dirty code I tried, every register is read to 0 and not modified by this code
HAL_Init();
SystemClock_Config();
MX_GPIO_Init();
MX_USART3_UART_Init();
// enable ETM trace
*ETM_PRGCTRL = 1;
// enable ETF circular mode
*ETF_FFCR = 0x00001123; // ARM recommends to set bits TRIGONTRIGIN FONTRIGEVT STOPONFL ENTI AND ENFT
*ETF_TRG = 16; // number of 32 words to capture
*ETF_CTL = 1; // enable trace
i = 1000;
while(i--)
asm("nop"); // ******** processing just to fill the trace
while ( ((*ETF_STS) & (1<<2)) == 0 );
printf("*ETF_RRD=0x%x\r\n", *ETF_RRD);
printf("*ETF_RRD=0x%x\r\n", *ETF_RRD);
printf("*ETF_RRD=0x%x\r\n", *ETF_RRD);
printf("*ETF_RRD=0x%x\r\n", *ETF_RRD);
*ETF_CTL = 0; // disable trace
