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

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Message-ID: <CAL_JsqK877WtcFDEVc-fqRu0nSJwpgGBNjuxH0yiN7uy1BBRbw@mail.gmail.com>
Date:   Tue, 11 Dec 2018 09:29:04 -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, 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.

Rob