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

[tom burkart]

Quoting Rob Herring <robh@xxxxxxxxxx>:

> On Mon, Dec 10, 2018 at 8:17 PM tom burkart <tom@xxxxxxxxxx> wrote:
> > Quoting Rob Herring <robh@xxxxxxxxxx>:
> >
> > > On Mon, Nov 26, 2018 at 10:06 PM tom burkart <tom@xxxxxxxxxx> wrote:
> > >>
> > >> Quoting Rob Herring <robh@xxxxxxxxxx>:
> > >>
> > >> > On Thu, Nov 22, 2018 at 3:49 AM Tom Burkart <tom@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > >> >> Signed-off-by: Tom Burkart <tom@xxxxxxxxxx>
> > >> >> ---
> > >> >>  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