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From:   Coiby Xu <coiby.xu@gmail.com>
Date:   Mon, 19 Oct 2020 08:54:52 +0800
To:     =?utf-8?Q?Barnab=C3=A1s_P=C5=91cze?= <pobrn@protonmail.com>
Cc:     "linux-input@vger.kernel.org" <linux-input@vger.kernel.org>,
        Helmut Stult <helmut.stult@schinfo.de>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Jiri Kosina <jikos@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] HID: i2c-hid: add polling mode based on connected
 GPIO chip's pin status
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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