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

[tom burkart]

Quoting Rob Herring <robh@xxxxxxxxxx>:

> On Mon, Nov 26, 2018 at 10:06 PM tom burkart <tom@xxxxxxxxxx> wrote:
> > Hi Rob,
> >
> > 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,
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