linux-kernel - Re: [Q] Frequency & duty cycle measurement?

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Message-ID: <dffc0f18-799a-4fc7-a6b5-2fa270e1fc58@prolan.hu>
Date: Thu, 30 Jan 2025 14:59:20 +0100
From: Csókás Bence <csokas.bence@...lan.hu>
To: William Breathitt Gray <wbg@...nel.org>
CC: <linux-iio@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<timestamp@...ts.linux.dev>, Jonathan Cameron <jic23@...nel.org>, "Lars-Peter
 Clausen" <lars@...afoo.de>, Daniel Lezcano <daniel.lezcano@...aro.org>,
	"Thomas Gleixner" <tglx@...utronix.de>, Dipen Patel <dipenp@...dia.com>,
	<dlechner@...libre.com>
Subject: Re: [Q] Frequency & duty cycle measurement?

Hi William,

On 2025. 01. 27. 16:00, William Breathitt Gray wrote:
> In the userspace application, you would setup a Counter "watch" to
> collect each desired timer value on the respective Counter events; I
> assume RA and RB are Count 0 and Count 1 respectively, but if they
> represent something else please let me know:
> 
>      static struct counter_watch watches[2] = {
>              {
>                      /* Component data: Count 0 count */
>                      .component.type = COUNTER_COMPONENT_COUNT,
>                      .component.scope = COUNTER_SCOPE_COUNT,
>                      .component.parent = 0,
>                      /* Event type: Capture */
>                      .event = COUNTER_EVENT_CAPTURE,
>                      /* Device event channel 0 */
>                      .channel = 0,
>              },
>              {
>                      /* Component data: Count 1 count */
>                      .component.type = COUNTER_COMPONENT_COUNT,
>                      .component.scope = COUNTER_SCOPE_COUNT,
>                      .component.parent = 1,
>                      /* Event type: Capture */
>                      .event = COUNTER_EVENT_CAPTURE,
>                      /* Device event channel 0 */
>                      .channel = 0,
>              },
>      };
>      ...
>      int main(void)
>      {
>              int fd;
>              int i;
> 	    unsigned long long delta_ts, delta_ra, delta_rb;
> 	    double ra_frequency, rb_frequency, rb_ra;
>              struct counter_event first_capture[2], second_capture[2];
>              
>              /* Open Counter chrdev */
>              fd = open("/dev/counter0", O_RDWR);
>              
>              for (i = 0; i < 2; i++) {
> 	            /* Register all Counter watches */
>                      ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches + i);
>              }
> 	    /* Start collecting Counter events */
>              ioctl(fd, COUNTER_ENABLE_EVENTS_IOCTL);
>              
>              for (;;) {
> 	        /* Read first Counter event capture */
>              	read(fd, first_capture, sizeof(first_capture));
> 	        /* Read second Counter event capture */
>              	read(fd, second_capture, sizeof(second_capture));
>                  
> 		/* Within each capture, timestamp is the same so only
> 		 * first element of each capture needs to be compared */
> 		delta_ts = second_capture[0].timestamp - first_capture[0].timestamp;
> 		/* Compute deltas of timer register pair RA and RB.
> 		delta_ra = second_capture[0].value - first_capture[0].value;
> 		delta_rb = second_capture[1].value - first_capture[1].value;
>                  
> 		ra_frequency = (double)delta_ra / delta_ts;
> 		rb_frequency = (double)delta_rb / delta_ts;
> 		rb_ra = (double)delta_rb / delta_ra;
>                  
>              	printf("RA frequency: %ld\n"
> 		       "RB frequency: %ld\n"
> 		       "RB per RA: %ld\n"
>              	       ra_frequency, rb_frequency, rb_ra);
>              }
>              
>              return 0;
>      }
> 
> If RA and RB are provided as a memory buffer on your device, you can
> instead expose them via DEFINE_COUNTER_ARRAY_CAPTURE() such as the
> ti-ecap-capture driver does, then perform your userspace computations
> by utilizing those respective "capture" array attribute values (via
> chrdev like the example above or alternatively via sysfs).

Thanks for your extensive explanation! With 
DEFINE_COUNTER_ARRAY_CAPTURE() I was able to expose RA and RB as 
`/sys/bus/counter/devices/counter0/count0/capture{0,1}`, and could 
verify that by replacing `devmem` calls with read()-reopen(), our PoC 
code still works. Now I want to use the chardev interface, but I 
couldn't find how to set up the watches appropriately. So far I have:

	{
		.component.type = COUNTER_COMPONENT_EXTENSION,
		// also tried COUNTER_COMPONENT_COUNT
		.component.scope = COUNTER_SCOPE_COUNT,
		.component.parent = 0,
		.component.id = X, // also tried this instead:
		// .channel = X,
		.event = COUNTER_EVENT_CAPTURE,
	},

However, with this, the first read() never comes back.

Bence