Hi,

On 05/23/2016 06:48 PM, Uwe Kleine-K?nig wrote:

Hello,

On Mon, May 23, 2016 at 04:48:40PM +0530, R, Vignesh wrote:

quoted

On 5/22/2016 3:56 PM, Uwe Kleine-K?nig wrote:

quoted

Hello,

On Thu, May 19, 2016 at 02:34:00PM +0530, Vignesh R wrote:

quoted

There are rotary-encoders where GPIO lines reflect the actual position
of the rotary encoder dial. For example, if dial points to 9, then four
GPIO lines connected to the rotary encoder will read HLLH(1001b = 9).
Add support for such rotary-encoder.
The driver relies on rotary-encoder,absolute-encoder DT property to
detect such encoders.
Since, GPIO IRQs are not necessary to work with
such encoders, optional polling mode support is added using

I don't understand this. It's necessary in the same way as with the
already supported devices. I.e. you want to trigger an irq when the
encoder is moved and then check for it's position in the handler.

Unlike already supported device, there is no need to count steps or
determine the direction of rotation. Hence, IRQ is not a requirement,
periodically polling the gpio lines is more than sufficient. With
absolute encoder, you are able to determine the current position at any
time just by looking at the gpio inputs.

The timing might not be that critical, but there is no reason to operate
this device without irqs, is there?

No, I am not against the use of IRQs. In fact, my patches add support
for both IRQ based(default) and polling based operation(optional). If
rotary-encoder does not have interrupt capable gpio lines connected to
it (like am335x-ice) then one can make use of the polling mode support.

quoted

Suppose we poll device at t=0ms and see gpio values are LLLH(1), if we
again poll device at t=500ms(which is what input_poll_dev helps to do)
and see that gpio values is LLHH(3), then we know that rotary encoder
has changed to 3. This can be done w/o IRQ for absolute encoders.

quoted

quoted

diff --git a/Documentation/devicetree/bindings/input/rotary-encoder.txt b/Documentation/devicetree/bindings/input/rotary-encoder.txt
index 6c9f0c8a846c..9c928dbd1500 100644
--- a/Documentation/devicetree/bindings/input/rotary-encoder.txt
+++ b/Documentation/devicetree/bindings/input/rotary-encoder.txt

@@ -12,6 +12,10 @@ Optional properties:
 - rotary-encoder,relative-axis: register a relative axis rather than an
   absolute one. Relative axis will only generate +1/-1 events on the input
   device, hence no steps need to be passed.
+- rotary-encoder,absolute-encoder: support encoders where GPIO lines
+  reflect the actual position of the rotary encoder dial. For example,
+  if dial points to 9, then four GPIO lines read HLLH(1001b = 9).
+  In this case, rotary-encoder,steps-per-period needed not be defined.

IMHO this is wrong, I'd formalize this device as:

	{
		compatible = "rotary-encoder";
		gpios = <&gpio 19 1>, <&gpio 20 0>, <...>, <...>;
		rotary-encoder,encoding = "binary";
		rotary-encoder,steps = <16>;
		rotary-encoder,steps-per-period = <16>;

The above bindings essential means quarter_period device. I would not
like to bother with all the logic in rotary_encoder_quarter_period_irq()
when we can know encoder->pos by directly reading state of gpio lines.

OK, we have code that is more complex than it needs to be for your
device. But your device is a special case of the supported devices, so
I'd say don't bother that there is more logic in the driver than you
need and be lucky.

More complexity is just a overhead. Since, encoder can be turned at a
rate faster than handling of IRQs (rotary_encoder_quarter_period_irq()
is threaded IRQ hence, priority is not close to real time), some states
can be missed. rotary_encoder_quarter_period_irq() is not robust in this
case, reading gpios directly is more suitable option. I see similar
views expressed in previously[1]

[1]http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/391196.html

-- 
Regards
Vignesh