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

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Date:   Fri, 7 Dec 2018 11:06:13 -0600
From:   Rob Herring <robh@...nel.org>
To:     Tom Burkart <tom@...sec.com>
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

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.

Rob