Re: hid-sensor-custom: async notification for input-report sysfs attributes? (human presence / attention sensors)

Hi,

On Fri, 2026-07-24 at 12:42 +0530, Daniel Paul Perinchery wrote:
> Hi,
> 
> I've been investigating why userspace can't efficiently watch a
> hid-sensor-custom input-report value (specifically a human-presence
> sensor, HID usage 2000e1, on an AMD Sensor Fusion Hub / amd_sfh
> platform) for changes without polling, and wanted to check whether
> there's interest in closing this gap before I try to write anything.
> 
> Setup: Lenovo IdeaPad Pro 5 14AKP10, AMD Ryzen AI 7 350, kernel
> 7.0.13
> (Debian/Parrot, but this is stock hid-sensor-custom.c). Presence and
> gaze/attention are exposed as separate logical sensors under the same
> physical hub (HID-SENSOR-2000e1.x.auto), each with the usual
> auto-generated input-N-<usage>-value sysfs attribute.
> 
> What I found tracing show_value() in hid-sensor-custom.c: for input
> reports, it calls sensor_hub_input_attr_get_raw_value(..., report_id,
> SENSOR_HUB_SYNC, false) -- i.e. every read of the -value file issues
> a
> synchronous GET_REPORT to the hub and returns the live answer.
> There's
> no cached field backing that attribute, so there's nothing for a
> notify-on-change mechanism to attach to on that path.
> 
> Separately, hid_sensor_capture_sample() *is* a genuine async callback
> fired on hub-pushed input reports, but as far as I can tell it only
> feeds the raw kfifo behind the driver's misc-device streaming
> interface (custom_dev), gated on test_bit(0,
> &sensor_inst->misc_opened). It doesn't appear to update any per-field
> cached value, so it isn't reachable from the sysfs show_value() path
> either.
> 
> Net effect: there's no way for a userspace consumer to poll()/
> select() on the -value file and be woken only on real hardware
> events.


I have a test program for poll(). Attached poll part.

#define DEVICE_PATH "/dev/HID-SENSOR-2000e1.2.auto"
#define BUFFER_SIZE 256

int main(int argc, char *argv[])
{
	int fd;
	struct pollfd pfd;
	char buffer[BUFFER_SIZE];
	int ret;
	struct sigaction sa;

	if (argc < 2) {
		printf("syntax %s sample_interval\n", argv[0]);
		exit(0);
	}
	

	// 1. Set up Ctrl+C signal handler
	sa.sa_handler = handle_sigint;
	sigemptyset(&sa.sa_mask);
	sa.sa_flags = 0; // Don't use SA_RESTART so poll() unblocks on
signal
	if (sigaction(SIGINT, &sa, NULL) < 0) {
		perror("Error setting up signal handler");
		return -1;
	}

	enable_sensor(1);

	// 2. Open the misc device
	fd = open(DEVICE_PATH, O_RDONLY | O_NONBLOCK);
	if (fd < 0) {
		perror("Failed to open device");
		return -1;
	}

	// 3. Configure poll structure
	pfd.fd = fd;
	pfd.events = POLLIN;

	printf("Polling loop started. Press Ctrl+C to exit
safely.\n");

	// 4. Main event loop
	while (keep_running) {
		printf("Waiting for data...\n");
	
		ret = poll(&pfd, 1, -1); 

		if (ret < 0) {
			// Signal interrupted poll, loop will exit if
it was SIGINT
			continue; 
		}

		if (ret == 0) {
			// Timeout reached with no data
			continue; 
		}

		// 5. Handle readable data event
		if (pfd.revents & POLLIN) {
			rx_tsc = read_rdtsc();
			ssize_t bytes_read = read(fd, buffer,
sizeof(buffer) - 1);
			if (bytes_read > 0) {
				time_t tm;

				buffer[bytes_read] = '\0'; // Null-
terminate string
				printf("tm:%ld Received %ld bytes:
%s\n", time(&tm), (long)bytes_read, buffer);
				read_time_stamp(buffer, bytes_read);
			} else if (bytes_read < 0) {
				perror("Read error");
			}
		}
	}

	// 6. Cleanup
	printf("\nExiting program cleanly...\n");
	close(fd);

	enable_sensor(0);

	return 0;
}

Does it help.

Thanks,
Srinivas


> The hub itself is already configured for threshold-based reporting
> (property-reporting-state = 2, i.e. "Threshold Events") in my case,
> so
> the inefficiency is entirely on the kernel/sysfs side -- userspace
> has
> to fall back to timed re-reads of a synchronous, firmware-round-trip
> attribute, which is more expensive per-read than a typical cached
> sysfs value and awkward to size a safe polling interval for.
> 
> This seems like a real gap for the presence/proximity use case
> specifically (walk-away lock, auto-dim, etc.), which is inherently
> event-oriented and is exactly the kind of feature vendors implement
> natively on Windows via push notifications from the same class of
> sensor.
> 
> Before I put together a patch I wanted to ask: is there interest in
> adding async caching + sysfs_notify() (or sysfs_notify_dirent(), if
> attrs end up carrying a stored kernfs_node) for input-report values
> specifically, sourced from hid_sensor_capture_sample()'s existing
> async path? Rough shape as I see it:
> 
>   - add a cached "last input value" (+ a bool "valid") per relevant
> field in struct hid_sensor_custom_field
>   - in hid_sensor_capture_sample(), in addition to (or instead of,
> behind a config/attr) feeding the misc-device fifo, decode and store
> the value for the matching field, then sysfs_notify() its -value
> attribute
>   - leave show_value()'s SENSOR_HUB_SYNC path untouched for feature
> reports and for any input attrs that never get an async report, so
> nothing regresses for existing consumers
> 
> Happy to be told this already exists via a different interface I've
> missed (I did look at whether this should just go through the IIO
> buffered/trigger interface instead, given some other hid-sensor-*
> drivers are IIO-backed -- but hid-sensor-custom exposes vendor/custom
> usages directly via this sysfs shape rather than IIO channels, so I
> wasn't sure that's the right layer to add it at). Also open to being
> told the misc-device/fifo interface is meant to be the async
> consumption path already and userspace tools should be using that
> instead of polling -value -- if so, is there existing tooling/docs
> for
> that path I should be pointing presence-detection use cases at?
> 
> Not attaching a patch yet since I'd rather get a read on the right
> approach (and whether it's wanted at all) before writing one.
> 
> Thanks,
> Daniel Paul