Date:   Mon, 26 Nov 2018 13:39:12 -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
Subject: Re: [PATCH v7 1/4] dt-bindings: pps: descriptor-based gpio,
 capture-clear addition

On Sat, Nov 17, 2018 at 6:35 PM tom burkart <tom@...sec.com> wrote:
>
> Quoting Rob Herring <robh@...nel.org>:
>
> > On Sat, Nov 17, 2018 at 4:35 AM tom burkart <tom@...sec.com> wrote:
> >>
> >> Quoting Rob Herring <robh@...nel.org>:
> >>
> >> > On Wed, Nov 14, 2018 at 11:54:29PM +1100, Tom Burkart wrote:
> >> >> This patch changes the devicetree bindings for the pps-gpio driver
> >> >> from the integer based ABI to the descriptor based ABI.
> >> > ? That has nothing to do with DT.
> >>
> >> I believe it does, as the change in ABI forces a rename in the DT
> >> naming convention.
> >> This is due to the descriptor based ABI appending "-gpio" or "-gpios" (see
> >> Documentation/gpio/base.txt.)
> >> Admittedly, I may have called it by the wrong name due to ignorance,
> >> my apologies.
> >
> > If what you say is correct, then you can't change this driver. You'll
> > break compatibility with any existing DT.
> >
> > Changing the binding reasoning should purely be that this is the
> > preferred form. Bindings must be independent from changing kernel
> > APIs.
>
> See comments from Philip Zabel.  I misread the documentation and this
> has now been corrected in v8 of the patch.  I hope that eliminates all
> comments made above.
>
> >> >>  It also adds
> >> >> documentation for the device tree capture-clear option.  The legacy
> >> >> device tree entry for the GPIO pin is supported.
> >> >>
> >> >> Signed-off-by: Tom Burkart <tom@...sec.com>
> >> >> ---
> >> >>  Documentation/devicetree/bindings/pps/pps-gpio.txt | 8 ++++++--
> >> >>  1 file changed, 6 insertions(+), 2 deletions(-)
> >> >>
> >> >> diff --git a/Documentation/devicetree/bindings/pps/pps-gpio.txt
> >> >> b/Documentation/devicetree/bindings/pps/pps-gpio.txt
> >> >> index 3683874832ae..6c9fc0998d94 100644
> >> >> --- a/Documentation/devicetree/bindings/pps/pps-gpio.txt
> >> >> +++ b/Documentation/devicetree/bindings/pps/pps-gpio.txt
> >> >> @@ -5,19 +5,23 @@ a GPIO pin.
> >> >>
> >> >>  Required properties:
> >> >>  - compatible: should be "pps-gpio"
> >> >> -- gpios: one PPS GPIO in the format described by ../gpio/gpio.txt
> >> >> +- pps-gpios: one PPS GPIO in the format described by ../gpio/gpio.txt
> >> >> +Alternatively (DEPRECATED), instead of pps-gpios above, it may have:
> >> >> +- gpios: one PPS GPIO as above
> >> >>
> >> >>  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
> >> >
> >> > Is this a h/w thing? or driver configuration?
> >>
> >> Driver configuration.  Most of the code was present in the driver, yet
> >> it was not documented, or usable due to a two line (code) omission
> >> (the value was not being fetched from DT).
> >
> > So what determines how you want to configure this? If the user will
> > want to change it, then it should be a sysfs attr and exposed to
> > userspace. If it depends on h/w config for a board then it can be in
> > DT.
>
> Sorry, I misled you somewhat.  If the PPS pulse active transition from
> the hardware is on the falling edge, this flag is required to get the
> OS to use that as the active transition.  This would not change at the
> user's whim but rather it is dependent on connected hardware.

This description sounds more like 'assert-falling-edge' than 'capture-clear'.

I'm still not clear on what 'capture-clear' is.

Rob

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