linux-kernel - Re: [PATCH v9 3/4] dt-bindings: pps: pps-gpio PPS ECHO implementation

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Date:   Wed, 12 Dec 2018 13:25:51 +1100
From:   tom burkart <tom@...sec.com>
To:     Rob Herring <robh@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org, lukas@...dolin.com
Subject: Re: [PATCH v9 3/4] dt-bindings: pps: pps-gpio PPS ECHO
 implementation

Quoting Rob Herring <robh@...nel.org>:

> On Mon, Dec 10, 2018 at 8:17 PM tom burkart <tom@...sec.com> wrote:
>>
>> Quoting Rob Herring <robh@...nel.org>:
>>
>> > On Mon, Nov 26, 2018 at 10:06 PM tom burkart <tom@...sec.com> wrote:
>> >>
>> >> Quoting Rob Herring <robh@...nel.org>:
>> >>
>> >> > On Thu, Nov 22, 2018 at 3:49 AM Tom Burkart <tom@...sec.com> wrote:
>> >> >>
>> >> >> This patch implements the device tree changes required for the pps
>> >> >> echo functionality for pps-gpio, that sysfs claims is available
>> >> >> already.
>> >> >>
>> >> >> This patch was originally written by Lukas Senger as part of a masters
>> >> >> thesis project and modified for inclusion into the linux kernel by Tom
>> >> >> Burkart.
>> >> >>
>> >> >> Signed-off-by: Lukas Senger <lukas@...dolin.com>
>> >> >> Signed-off-by: Tom Burkart <tom@...sec.com>
>> >> >> ---
>> >> >>  Documentation/devicetree/bindings/pps/pps-gpio.txt | 9 +++++++++
>> >> >>  1 file changed, 9 insertions(+)
>> >> >>
>> >> >> diff --git a/Documentation/devicetree/bindings/pps/pps-gpio.txt
>> >> >> b/Documentation/devicetree/bindings/pps/pps-gpio.txt
>> >> >> index 1155d49c2699..e09f6f2405c5 100644
>> >> >> --- a/Documentation/devicetree/bindings/pps/pps-gpio.txt
>> >> >> +++ b/Documentation/devicetree/bindings/pps/pps-gpio.txt
>> >> >> @@ -7,10 +7,15 @@ Required properties:
>> >> >>  - compatible: should be "pps-gpio"
>> >> >>  - gpios: one PPS GPIO in the format described by ../gpio/gpio.txt
>> >> >>
>> >> >> +Additional required properties for the PPS ECHO functionality:
>> >> >> +- echo-gpios: one PPS ECHO GPIO in the format described by
>> >> ../gpio/gpio.txt
>> >> >> +- echo-active-ms: duration in ms of the active portion of  
>> the echo pulse
>> >> >> +
>> >> >>  Optional properties:
>> >> >>  - assert-falling-edge: when present, assert is indicated by a
>> >> falling edge
>> >> >>                         (instead of by a rising edge)
>> >> >>  - capture-clear: when present, also capture the PPS clear event
>> >> >> +- invert-pps-echo: when present, invert the PPS ECHO pulse
>> >> >
>> >> > Why do you need this? Can't you just make the echo gpio  
>> GPIO_ACTIVE_LOW?
>> >> >
>> >> > BTW, using the flag probably should have been done for
>> >> > 'assert-falling-edge' as well.
>> >>
>> >> The hardware I use expects a positive-going echo pulse, however, it
>> >> was really easy to give users the option to have it inverted in case
>> >> they use different hardware that expects a negative-going edge.
>> >
>> > It will be even easier to implement if you use GPIO_ACTIVE_LOW or
>> > GPIO_ACTIVE_HIGH as appropriate. If the flag is set appropriately,
>> > then gpiod_set_value(gpio, 1) asserts the pulse and
>> > gpiod_set_value(gpio, 0) deasserts it no matter which way the h/w is
>> > wired. You can then get rid of invert_pps_echo in the driver.
>>
>> Hi Rob,
>> I have looked at the appropriate changes to my code to implement the
>> above.  However, there is no GPIO_ACTIVE_* as part of the gpiod_flags
>> enum (include/linux/gpio/consumer.h).
>>
>> What am I missing?
>
> You don't need to know in the driver. You set the gpio state to 1 for
> active, 0 for inactive and that does the right thing based on the flag
> in the DT. IOW, if the DT defines the GPIO as active low, then setting
> the gpio state to 1 will result in the GPIO being 0V. It's a bit
> annoying and confusing at first until you realize the driver can just
> handle either polarity transparently.

Hi Rob,
thanks a lot!  The penny finally dropped.

My patch v12 will have the changes implemented (no invert-pps-echo).

Tom