On Sun, Oct 18, 2020 at 12:23:14PM +0000, Barnabás Pőcze wrote:

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> +static int i2c_hid_polling_thread(void *i2c_hid) > +{ > > - struct i2c_hid *ihid = i2c_hid; > - struct i2c_client *client = ihid->client; > - unsigned int polling_interval_idle; > - > - while (1) { > - /* > > > - * re-calculate polling_interval_idle > > > - * so the module parameters polling_interval_idle_ms can be > > > - * changed dynamically through sysfs as polling_interval_active_us > > > - */ > > > - polling_interval_idle = polling_interval_idle_ms * 1000; > > > - if (test_bit(I2C_HID_READ_PENDING, &ihid->flags)) > > > - usleep_range(50000, 100000); > > > - > - if (kthread_should_stop()) > > > - break; > > > - > - while (interrupt_line_active(client)) { > >

I realize it's quite unlikely, but can't this be a endless loop if data is coming in at a high enough rate? Maybe the maximum number of iterations could be limited here?

If we find HID reports are constantly read and send to front-end application like libinput, won't it help expose the problem of the I2C HiD device?

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I'm not sure I completely understand your point. The reason why I wrote what I wrote is that this kthread could potentially could go on forever (since `kthread_should_stop()` is not checked in the inner while loop) if the data is supplied at a high enough rate. That's why I said, to avoid this problem, only allow a certain number of iterations for the inner loop, to guarantee that the kthread can stop in any case.

I mean if "data is supplied at a high enough rate" does happen, this is an abnormal case and indicates a bug. So we shouldn't cover it up. We expect the user to report it to us.

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I agree in principle, but if this abnormal case ever occurs, that'll prevent this module from being unloaded since `kthread_stop()` will hang because the thread is "stuck" in the inner loop, never checking `kthread_should_stop()`. That's why I think it makes sense to only allow a certain number of operations for the inner loop, and maybe show a warning if that's exceeded:

for (i = 0; i < max_iter && interrupt_line_active(...); i++) { .... }

WARN_ON[CE](i == max_iter[, "data is coming in at an unreasonably high rate"]);

I now realize that WARN_ON[CE] is probably not the best fit here, `hid_warn()` is possibly better.

Thank you for the suggestion!

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or something like this, where `max_iter` could possibly be some value dependent on `polling_interval_active_us`, or even just a constant. [...]

Regards, Barnabás Pőcze

-- Best regards, Coiby