On Sun, Oct 18, 2020 at 12:23:14PM +0000, Barnabás Pőcze wrote:
[...]
> > > > > +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?
> > >
> > > >
> >
> > 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.
>
> >
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!
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
