Re: [PATCH v12 12/17] tools/counter: Create Counter tools

[David Lechner]

On 7/11/21 6:28 AM, William Breathitt Gray wrote:
> On Sat, Jul 10, 2021 at 11:53:35AM -0500, David Lechner wrote:
> > On 7/5/21 3:19 AM, William Breathitt Gray wrote:

> > > +	{
> > > +		/* Component data: Count 0 count */
> > > +		.component.type = COUNTER_COMPONENT_COUNT,
> > > +		.component.scope = COUNTER_SCOPE_COUNT,
> > > +		.component.parent = 0,
> > > +		/* Event type: Index */
> > > +		.event = COUNTER_EVENT_INDEX,
> > > +		/* 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: Index */
> > > +		.event = COUNTER_EVENT_INDEX,
> > > +		/* Device event channel 0 */
> > > +		.channel = 0,
> > > +	},
> > > +};
> > > +
> > > +int main(void)
> > > +{
> > > +	int fd;
> > > +	int ret;
> > > +	struct counter_event event_data[2];
> > > +
> > > +	fd = open("/dev/counter0", O_RDWR);
> > > +	if (fd == -1) {
> > > +		perror("Unable to open /dev/counter0");
> > > +		return -errno;
> >
> > errno is no longer valid after calling perror(). Since this
> > is example code, we can just return 1 instead (exit codes
> > positive number between 0 and 255 so -1 would be 255).
>
> Ack.
>
> > > +	}
> > > +
> > > +	ret = ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches);
> > > +	if (ret == -1) {
> > > +		perror("Error adding watches[0]");
> > > +		return -errno;
> > > +	}
> > > +	ret = ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches + 1);
> > > +	if (ret == -1) {
> > > +		perror("Error adding watches[1]");
> > > +		return -errno;
> > > +	}
> > > +	ret = ioctl(fd, COUNTER_ENABLE_EVENTS_IOCTL);
> > > +	if (ret == -1) {
> > > +		perror("Error enabling events");
> > > +		return -errno;
> > > +	}
> > > +
> > > +	for (;;) {
> > > +		ret = read(fd, event_data, sizeof(event_data));
> > > +		if (ret == -1) {
> > > +			perror("Failed to read event data");
> > > +			return -errno;
> > > +		}
> > > +
> > > +		if (ret != sizeof(event_data)) {
> > > +			fprintf(stderr, "Failed to read event data\n");
> > > +			return -EIO;
> > > +		}
> > > +
> > > +		printf("Timestamp 0: %llu\tCount 0: %llu\n"
> > > +		       "Error Message 0: %s\n"
> > > +		       "Timestamp 1: %llu\tCount 1: %llu\n"
> > > +		       "Error Message 1: %s\n",
> > > +		       (unsigned long long)event_data[0].timestamp,
> > > +		       (unsigned long long)event_data[0].value,
> > > +		       strerror(event_data[0].status),
> > > +		       (unsigned long long)event_data[1].timestamp,
> > > +		       (unsigned long long)event_data[1].value,
> > > +		       strerror(event_data[1].status));
> > > +	}
> >
> > Aren't the Count 0 and Count 1 events independent? Why should we expect to
> > always get both events at the same time in the same order?
>
> Watch 0 and Watch 1 are both triggered by the same event: a
> COUNTER_EVENT_INDEX event on device event channel 0. If we had set
> channel to 1 for Watch 1, then we would have two independent events, but
> in this case both Watches have their respective channel set to 0.

The thing that jumped out to me is that they have different parents.
But I guess I forgot that the event itself always has a scope of
device and that the component just says what value to record and
is otherwise independent of the event.