Re: [PATCH v3] HID: i2c-hid: add polling mode based on connected GPIO chip's pin status

[Coiby Xu]

Hi,

On Thu, Oct 22, 2020 at 02:22:51PM +0000, Barnabás Pőcze wrote:
> Hi,
>
> I think this looks a lot better than the first version, the issues around
> suspend/resume are sorted out as far as I can see. However, I still have a couple
> comments, mainly minor ones.
Thank you for reviewing this patch!
> > [...]
> > +/* polling mode */
> > +#define I2C_HID_POLLING_DISABLED 0
> > +#define I2C_HID_POLLING_GPIO_PIN 1
> > +#define I2C_HID_POLLING_INTERVAL_ACTIVE_US 4000
> > +#define I2C_HID_POLLING_INTERVAL_IDLE_MS 10
> > +
> > +static u8 polling_mode;
> > +module_param(polling_mode, byte, 0444);
> > +MODULE_PARM_DESC(polling_mode, "How to poll - 0 disabled; 1 based on GPIO pin's status");
> > +
>
> Minor thing, but maybe the default value should be documented in the parameter
> description?
>
>
> > +static unsigned int polling_interval_active_us = I2C_HID_POLLING_INTERVAL_ACTIVE_US;
> > +module_param(polling_interval_active_us, uint, 0644);
> > +MODULE_PARM_DESC(polling_interval_active_us,
> > +		 "Poll every {polling_interval_active_us} us when the touchpad is active. Default to 4000 us");
> > +
> > +static unsigned int polling_interval_idle_ms = I2C_HID_POLLING_INTERVAL_IDLE_MS;
>
> Since these two parameters are mostly read, I think the `__read_mostly`
> attribute (linux/cache.h) is justified here.
>
>
> > +module_param(polling_interval_idle_ms, uint, 0644);
> > +MODULE_PARM_DESC(polling_interval_idle_ms,
> > +		 "Poll every {polling_interval_idle_ms} ms when the touchpad is idle. Default to 10 ms");
>
> This is minor stylistic thing; as far as I see, the prevalent pattern is to put
> the default value at the end, in parenthesis:
> E.g. "some parameter description (default=X)" or "... (default: X)" or something similar
>
> Maybe __stringify() (linux/stringify.h) could be used here and for the previous
> module parameter?
>
> E.g. "... (default=" __stringify(I2C_HID_POLLING_INTERVAL_IDLE_MS) ")"
Thank you for the above three suggestions! Will be applied in v4.
> > [...]
> > +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);
>
> Can the trigger type change? Because if not, then I think it'd be better to store
> the value somewhere and not query it every time.
The irq trigger type is obtained from ACPI so I don't think it won't
change.
> > +	struct irq_desc *irq_desc = irq_to_desc(client->irq);
>
> Same here.
Thank you for the reminding!
> > +	ssize_t	status = get_gpio_pin_state(irq_desc);
>
> `get_gpio_pin_state()` returns an `int`, so I am not sure why `ssize_t` is used here.

I used `ssize_t` because I found gpiolib-sysfs.c uses `ssize_t`

    // drivers/gpio/gpiolib-sysfs.c
    static ssize_t value_show(struct device *dev,
    		struct device_attribute *attr, char *buf)
    {
    	struct gpiod_data *data = dev_get_drvdata(dev);
    	struct gpio_desc *desc = data->desc;
    	ssize_t			status;

    	mutex_lock(&data->mutex);

    	status = gpiod_get_value_cansleep(desc);
        ...
    	return status;
    }

According to the book Advanced Programming in the UNIX Environment by
W. Richard Stevens,
    With the 1990 POSIX.1 standard, the primitive system data type
    ssize_t was introduced to provide the signed return value...

So ssize_t is fairly common, for example, the read and write syscall
return a value of type ssize_t. But I haven't found out why ssize_t is
better int.
> > +
> > +	if (status < 0) {
> > +		dev_warn(&client->dev,
> > +			 "Failed to get GPIO Interrupt line status for %s",
> > +			 client->name);
>
> I think it's possible that the kernel message buffer is flooded with these
> messages, which is not optimal in my opinion.
Thank you! Replaced with dev_dbg in v4.
> > +		return false;
> > +	}
> > +	/*
> > +	 * 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 !status;
> > +
> > +	return 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) {
> > +		if (kthread_should_stop())
> > +			break;
>
> I think this should be `while (!kthread_should_stop())`.
This simplifies the code. Thank you!
> > +
> > +		while (interrupt_line_active(client) &&
> > +		       !test_bit(I2C_HID_READ_PENDING, &ihid->flags) &&
> > +		       !kthread_should_stop()) {
> > +			i2c_hid_get_input(ihid);
> > +			usleep_range(polling_interval_active_us,
> > +				     polling_interval_active_us + 100);
> > +		}
> > +		/*
> > +		 * 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;
> > +		usleep_range(polling_interval_idle,
> > +			     polling_interval_idle + 1000);
>
> I don't quite understand why you use an extra variable here. I'm assuming
> you want to "save" a multiplication? I believe the compiler will optimize it
> to a single read, and single multiplication regardless whether you use a "temporary"
> variable or not.
>
>
> > +	}
> > +
> > +	do_exit(0);
>
> Looking at other examples, I don't think `do_exit()` is necessary.
According to the doc of kthread_create_on_node,
    @threadfn() can either call do_exit() directly if it is a
    * standalone thread for which no one will call kthread_stop(), or
    * return when 'kthread_should_stop()' is true (which means
    * kthread_stop() has been called).

do_exit is not necessary. Thank you for raising up this issue and
looking at other examples for me!
> > +	return 0;
> > +}
> > +
> > +static int i2c_hid_init_polling(struct i2c_hid *ihid)
> > +{
> > +	struct i2c_client *client = ihid->client;
> > +
> > +	if (!irq_get_trigger_type(client->irq)) {
> > +		dev_warn(&client->dev,
> > +			 "Failed to get GPIO Interrupt Assertion Level, could not enable polling mode for %s",
> > +			 client->name);
> > +		return -EINVAL;
> > +	}
> > +
> > +	ihid->polling_thread = kthread_create(i2c_hid_polling_thread, ihid,
> > +					      "I2C HID polling thread");
> > +
> > +	if (!IS_ERR(ihid->polling_thread)) {
> > +		pr_info("I2C HID polling thread created");
> > +		wake_up_process(ihid->polling_thread);
> > +		return 0;
> > +	}
> > +
> > +	return PTR_ERR(ihid->polling_thread);
>
> I would personally rewrite this parts as
>
> ```
> if (IS_ERR(...)) {
>  dev_err(...);
>  return PTR_ERR(...);
> }
> ....
> return 0;
> ```

Thank you! This style is consistent with other functions in this file.
> > +}
> > [...]
>
>
> Regards,
> Barnabás Pőcze

--
Best regards,
Coiby