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Date:   Sun, 18 Oct 2020 12:23:14 +0000
From:   Barnabás Pőcze <pobrn@...tonmail.com>
To:     Coiby Xu <coiby.xu@...il.com>
Cc:     "linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
        Helmut Stult <helmut.stult@...info.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] HID: i2c-hid: add polling mode based on connected GPIO chip's pin status

> [...]
> > > > > > +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.


> 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

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