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

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Message-ID: <20181209140759.18342zz66oeo3767@www.aussec.com>
Date:   Sun, 09 Dec 2018 14:07:59 +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, Nov 26, 2018 at 10:06 PM tom burkart <tom@...sec.com> wrote:
>>
>> Hi Rob,
>>
>> 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,
sure we can do that, however, a hardware change will need a recompile  
of the kernel, rather than setting a flag.
You still need something to help decide whether to use GPIO_ACTIVE_LOW  
or GPIO_ACTIVE_HIGH depending on what hardware is currently connected  
to the system.

I am happy to delete that code as well, if it makes you happy.

Tom