Re: [PATCH v2 3/4] touchscreen: colibri-vf50-ts: Add touchscreen support for Colibri VF50

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Hi Stefan,

On Tue, Jul 21, 2015 at 04:43:36PM +0200, Stefan Agner wrote:
> Hi Dmitry,
> 
> As the original author of the driver I have some remarks to your review
> 
> On 2015-07-18 01:42, Dmitry Torokhov wrote:
> >> +		/*
> >> +		 * If touch pressure is too low, stop measuring and reenable
> >> +		 * touch detection
> >> +		 */
> >> +		if (val_p < min_pressure || val_p > 2000)
> >> +			break;
> 
> This is where the modules touch pressure is used to stop the measurement
> process and switch back to interrupt mode. See my remarks at the end.
> 
> >> +
> >> +		/*
> >> +		 * The pressure may not be enough for the first x and the
> >> +		 * second y measurement, but, the pressure is ok when the
> >> +		 * driver is doing the third and fourth measurement. To
> >> +		 * take care of this, we drop the first measurement always.
> >> +		 */
> >> +		if (discard_val_on_start) {
> >> +			discard_val_on_start = false;
> >> +		} else {
> >> +			/*
> >> +			 * Report touch position and sleep for
> >> +			 * next measurement
> >> +			 */
> >> +			input_report_abs(vf50_ts->ts_input,
> >> +					ABS_X, VF_ADC_MAX - val_x);
> >> +			input_report_abs(vf50_ts->ts_input,
> >> +					ABS_Y, VF_ADC_MAX - val_y);
> >> +			input_report_abs(vf50_ts->ts_input,
> >> +					ABS_PRESSURE, val_p);
> >> +			input_report_key(vf50_ts->ts_input, BTN_TOUCH, 1);
> >> +			input_sync(vf50_ts->ts_input);
> >> +		}
> >> +
> >> +		msleep(10);
> >> +	}
> >> +
> >> +	/* Report no more touch, reenable touch detection */
> >> +	input_report_abs(vf50_ts->ts_input, ABS_PRESSURE, 0);
> >> +	input_report_key(vf50_ts->ts_input, BTN_TOUCH, 0);
> >> +	input_sync(vf50_ts->ts_input);
> >> +
> >> +	vf50_ts_enable_touch_detection(vf50_ts);
> >> +
> >> +	/* Wait for the pull-up to be stable on high */
> >> +	msleep(10);
> >> +
> >> +	/* Reenable IRQ to detect touch */
> >> +	enable_irq(vf50_ts->pen_irq);
> >> +
> >> +	dev_dbg(dev, "Reenabled touch detection interrupt\n");
> >> +}
> >> +
> >> +static irqreturn_t vf50_ts_touched(int irq, void *dev_id)
> >> +{
> >> +	struct vf50_touch_device *vf50_ts = (struct vf50_touch_device *)dev_id;
> >> +	struct device *dev = &vf50_ts->pdev->dev;
> >> +
> >> +	dev_dbg(dev, "Touch detected, start worker thread\n");
> >> +
> >> +	disable_irq_nosync(irq);
> >> +
> >> +	/* Disable the touch detection plates */
> >> +	gpiod_set_value(vf50_ts->gpio_ym, 0);
> >> +
> >> +	/* Let the platform mux to default state in order to mux as ADC */
> >> +	pinctrl_pm_select_default_state(dev);
> >> +
> >> +	queue_work(vf50_ts->ts_workqueue, &vf50_ts->ts_work);
> > 
> > If you convert this to a threaded interrupt you won't need to
> > disable/reenable interrupt or queue work. You should also be able to use
> > gpiod_set_value_cansleep() extending the range of ways the controller
> > could be connected to systems.
> > 
> 
> I'm not sure if a threaded interrupt is the right thing here. While the
> pen is on the touchscreen (which can be for several seconds)
> measurements have to be made in a continuous loop. Is it ok for a
> threaded interrupt to run that long?

Yes, why not? Threaded interrupt is simply a kernel thread that is woken
when hard interrupt handler tells it to wake up. Very similar to
interrupt + work queue, except that the kernel manages interactions
properly for you. There are several drivers in kernel that do that, for
example auo-pixcir-ts.c or tsc2007.c

> 
> I'm also not sure if it is really safe to _not_ disable the pen down
> GPIO interrupt. If we get a interrupt while measuring, we should ignore
> that interrupt.

The interrupt management core (you'll have to annotate it as
IRQF_ONESHOT) will make sure it stays masked properly until the threaded
handler completes so you do not need to disable it explicitly.

> 
> On resistive touch screens the pen down works by relying on the high
> resistance between the two plates while not being touched. The X-Plate
> will be pulled high, the Y-Plate is strong GND. We measure on the
> X-Plate (XM) which is high too. As soon as the plate is touched, XM will
> be GND (since the resistance over the two plates is way lower then the
> pull-up resistance). An interrupt on falling edge will trigger.
> 
> Now the measuring takes place, X, Y and pressure by using different
> measuring methods.
> 
> While Y-Plate measurement the same GPIO interupt pin is used for ADC
> measurement! The voltage on that pin will at that point depend on the
> Y-Position of the pen position... Is it guaranteed that the GPIO
> interrupt is not fired? I guess because we muxed to ADC at that point,
> it won't lead to a second (spurious) interrupt... However this is a
> thing which needs to be checked before removing interrupt enable/disable
> calls.
> 
> >> +};
> >> +
> >> +module_platform_driver(vf50_touch_driver);
> >> +
> >> +module_param(min_pressure, int, 0600);
> >> +MODULE_PARM_DESC(min_pressure, "Minimum pressure for touch detection");
> > 
> > I'd rather let userspace figure out what it recognizes as valid touch.
> > 
> 
> This is value is used as the termination condition for the measurement
> loop. It essentially defines at which pressure level we stop measuring
> X/Y values. Depending on the size and resistance of the plates. However,
> it is not safe to measure even at low/no pressure, since then we would
> only get maximum X/Y values. Hence it is crucial that this value is
> choosen properly, otherwise the driver will report "wrong" X/Y values.
> 
> Since we use the value for measurement termination, we need it in kernel
> space. As far as I know we do not get such a value from user space.

If this value is device model-specific and not user preference then we
probably should pass it via device tree data instead of module parameter
then.

Thanks.

-- 
Dmitry