On Sat, Oct 17, 2020 at 02:58:13PM +0000, Barnabás Pőcze wrote:

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+static int get_gpio_pin_state(struct irq_desc *irq_desc)
+{
+	struct gpio_chip *gc = irq_data_get_irq_chip_data(&irq_desc->irq_data);
+
+	return gc->get(gc, irq_desc->irq_data.hwirq);
+}
+
+static bool interrupt_line_active(struct i2c_client *client)
+{
+	unsigned long trigger_type = irq_get_trigger_type(client->irq);
+	struct irq_desc *irq_desc = irq_to_desc(client->irq);
+
+	/*
+	 * According to Windows Precsiontion Touchpad's specs
+	 * https://docs.microsoft.com/en-us/windows-hardware/design/component-guidelines/windows-precision-touchpad-device-bus-connectivity,
+	 * GPIO Interrupt Assertion Leve could be either ActiveLow or
+	 * ActiveHigh.
+	 */
+	if (trigger_type & IRQF_TRIGGER_LOW)
+		return !get_gpio_pin_state(irq_desc);
+
+	return get_gpio_pin_state(irq_desc);
+}

Excuse my ignorance, but I think some kind of error handling regarding the return
value of `get_gpio_pin_state()` should be present here.

What kind of errors would you expect? It seems (struct gpio_chip *)->get
only return 0 or 1.

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I read the code of a couple gpio chips and - I may be wrong, but - it seems they
can return an arbitrary errno.

I thought all GPIO chip return 0 or 1 since !!val is returned. I find
an example which could return negative value,

Yes, when a function returns `int`, there is a very high chance that the return
value may be an errno.

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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"]);

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

Thank you for suggesting this approach! It seems it would add a bit of
complexity to detect this situation which could introduce other bugs.

I did a experiment of creating a kthread that will loop forever and found
the rebooting process wasn't stalled. I don't expect user to load&unload
this module. So the end user could not notice this problem so  my rationale
is invalid.

Thus I would be prefer to check `kthread_should_stop()` in the inner
while loop instead.

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+			i2c_hid_get_input(ihid);
+			usleep_range(polling_interval_active_us,
+				     polling_interval_active_us + 100);
+		}
+
+		usleep_range(polling_interval_idle,
+			     polling_interval_idle + 1000);
+	}
+
+	do_exit(0);
+	return 0;
+}

[...]
Thank you for offering your understandings on this patch. When I'm going
to submit next version, I will add a "Signed-off-by" tag with your name
and email, does it look good to you?
[...]

I'm not sure if that follows proper procedures.

"The sign-off is a simple line at the end of the explanation for the patch, which
 certifies that you wrote it or otherwise have the right to pass it on as an
 open-source patch."[1]

I'm not the author, nor co-author, nor am I going to pass this patch on, so I don't
think that's appropriate.

Furthermore, please note that

"[...] you may optionally add a Cc: tag to the patch. **This is the only tag which
 might be added without an explicit action by the person it names** - but it should
 indicate that this person was copied on the patch."[2]
 (emphasis mine)

You have been directly contributing to this patch because several
improvements of next version are from you. I experienced a similar
situation when submitting patches for QEMU. The only difference is
that the feedbacks on the QEMU patches were also given in the format
of patch. Or do you think a reviewed-by tag from you after you think
the next version is of production quality is a better way to credit
you?

Regards,
Barnabás Pőcze


[1]: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin
[2]: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by

--
Best regards,
Coiby