Re: [PATCHv3 3/5] dt-bindings: document the CEC GPIO bindings

On 31/08/17 11:20, Linus Walleij wrote:

On Wed, Aug 30, 2017 at 6:10 PM, Hans Verkuil [off-list ref] wrote:

quoted

From: Hans Verkuil <redacted>

Document the bindings for the cec-gpio module for hardware where the
CEC pin and optionally the HPD pin are connected to GPIO pins.

Signed-off-by: Hans Verkuil <redacted>

I usually refer to GPIO "lines" rather than "pins" for clarity.
It's because some systems also have pin control, and then it becomes
a bit muddy what is a pin.

I'll change the terminology.

quoted

+* HDMI CEC GPIO driver
+
+The HDMI CEC GPIO module supports CEC implementations where the CEC pin
+is hooked up to a pull-up GPIO pin and - optionally - the HPD pin is
+hooked up to a pull-down GPIO pin.
+
+Required properties:
+  - compatible: value must be "cec-gpio"
+  - cec-gpio: gpio that the CEC line is connected to
+
+Optional property:
+  - hpd-gpio: gpio that the HPD line is connected to
+
+Example for the Raspberry Pi 3 where the CEC line is connected to
+pin 26 aka BCM7 aka CE1 on the GPIO pin header and the HPD line is
+connected to pin 11 aka BCM17:
+
+cec-gpio@7 {
+       compatible = "cec-gpio";
+       cec-gpio = <&gpio 7 GPIO_ACTIVE_HIGH>;
+       hpd-gpio = <&gpio 17 GPIO_ACTIVE_HIGH>;
+};

So what I understood from the driver is that the cec-gpio is maybe actually
an open drain output line, so in that case it should be stated in the docs and
cec-gpio  = <&gpio 7 GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN>
or GPIO_LINE_OPEN_DRAIN if it is not also single-ended.

Yes, I agree, it's open drain. I'm not sure whether or not it is single-ended.
I'm not sure what the difference is, and I have no electronics background.

Looking at fwnode_get_named_gpiod() it seems that it has to be single-ended
for GPIO_OPEN_DRAIN to be set, so I am going with that. It works, so it is
probably right :-)

Regards,

	Hans
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