Re: [PATCH v3 4/7] dt-bindings: gpio: Add gpio-repeater bindings

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Rob,

On Fri, Dec 6, 2019 at 4:04 PM Rob Herring <robh@kernel.org> wrote:
> On Fri, Dec 6, 2019 at 3:17 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Thu, Dec 5, 2019 at 10:06 PM Rob Herring <robh@kernel.org> wrote:
> > > On Wed, Nov 27, 2019 at 09:42:50AM +0100, Geert Uytterhoeven wrote:
> > > > Add Device Tree bindings for a GPIO repeater, with optional translation
> > > > of physical signal properties.  This is useful for describing explicitly
> > > > the presence of e.g. an inverter on a GPIO line, and was inspired by the
> > > > non-YAML gpio-inverter bindings by Harish Jenny K N
> > > > <harish_kandiga@mentor.com>[1].
> > > >
> > > > Note that this is different from a GPIO Nexus Node[2], which cannot do
> > > > physical signal property translation.
> > >
> > > It can't? Why not? The point of the passthru mask is to not do
> > > translation of flags, but without it you are always doing translation of
> > > cells.
> >
> > Thanks for pushing me deeper into nexuses!
> > You're right, you can map from one type to another.
> > However, you cannot handle the "double inversion" of an ACTIVE_LOW
> > signal with a physical inverter added:
> >
> >         nexus: led-nexus {
> >                 #gpio-cells = <2>;
> >                 gpio-map = <0 0 &gpio2 19 GPIO_ACTIVE_LOW>,     // inverted
> >                            <1 0 &gpio2 20 GPIO_ACTIVE_HIGH>,    // noninverted
> >                            <2 0 &gpio2 21 GPIO_ACTIVE_LOW>;     // inverted
> >                 gpio-map-mask = <3 0>;
> >                 // default gpio-map-pass-thru = <0 0>;
> >         };
> >
> >         leds {
> >                 compatible = "gpio-leds";
> >                 led6-inverted {
> >                         gpios = <&nexus 0 GPIO_ACTIVE_HIGH>;
> >                 };
> >                 led7-noninverted {
> >                         gpios = <&nexus 1 GPIO_ACTIVE_HIGH>;
> >                 };
> >                 led8-double-inverted {  // FAILS: still inverted
> >                         gpios = <&nexus 2 GPIO_ACTIVE_LOW>;
> >                 };
> >         };
> >
> > It "works" if the last entry in gpio-map is changed to GPIO_ACTIVE_HIGH.
> > Still, the consumer would see the final translated polarity, and not the
> > actual one it needs to program the consumer for.
>
> I'm not really following. Why isn't a double inversion just the same
> as no inversion?

Because the nexus can only mask and/or substitute bits.
It cannot do a XOR operation on the GPIO flags.

> > > > While an inverter can be described implicitly by exchanging the
> > > > GPIO_ACTIVE_HIGH and GPIO_ACTIVE_LOW flags, this has its limitations.
> > > > Each GPIO line has only a single GPIO_ACTIVE_* flag, but applies to both
> > > > th provider and consumer sides:
> > > >   1. The GPIO provider (controller) looks at the flags to know the
> > > >      polarity, so it can translate between logical (active/not active)
> > > >      and physical (high/low) signal levels.
> > > >   2. While the signal polarity is usually fixed on the GPIO consumer
> > > >      side (e.g. an LED is tied to either the supply voltage or GND),
> > > >      it may be configurable on some devices, and both sides need to
> > > >      agree.  Hence the GPIO_ACTIVE_* flag as seen by the consumer must
> > > >      match the actual polarity.
> > > >      There exists a similar issue with interrupt flags, where both the
> > > >      interrupt controller and the device generating the interrupt need
> > > >      to agree, which breaks in the presence of a physical inverter not
> > > >      described in DT (see e.g. [3]).
> > >
> > > Adding an inverted flag as I've suggested would also solve this issue.
> >
> > As per your suggestion in "Re: [PATCH V4 2/2] gpio: inverter: document
> > the inverter bindings"?
> > https://lore.kernel.org/linux-devicetree/CAL_JsqLp___2O-naU+2PPQy0QmJX6+aN3hByz-OB9+qFvWgN9Q@mail.gmail.com/
> >
> > Oh, now I understand. I was misguided by Harish' interpretation
> > https://lore.kernel.org/linux-devicetree/dde73334-a26d-b53f-6b97-4101c1cdc185@mentor.com/
> > which assumed an "inverted" property, e.g.
> >
> >     inverted = /bits/ 8 <0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0>;
> >
> > But you actually meant a new GPIO_INVERTED flag, to be ORed into the 2nd
> > cell of a GPIO specifier? I.e. add to include/dt-bindings/gpio/gpio.h"
> >
> >     /* Bit 6 expresses the presence of a physical inverter */
> >     #define GPIO_INVERTED 64
>
> Exactly.

OK, makes sense.

> > We need to be very careful in defining to which side the GPIO_ACTIVE_*
> > applies to (consumer?), and which side the GPIO_INVERTED flag (provider?).
> > Still, this doesn't help if e.g. a FET is used instead of a push-pull
> > inverter, as the former needs translation of other flags (which the
> > nexus can do, the caveats above still applies, though).
>
> Yes. Historically the cells values are meaningful to the provider and
> opaque to the consumer. Standardized cell values changes that
> somewhat. I think we want the active flag to be from the provider's
> prospective because the provider always needs to know. The consumer
> often doesn't need to know. That also means things work without the
> GPIO_INVERTED flag if the consumer doesn't care which is what we have
> today already and we can't go back in time.
>
>
> > Same for adding IRQ_TYPE_INVERTED.
>
> I suppose so, yes.
>
> > Related issue: how to handle physical inverters on SPI chip select lines,
> > if the SPI slave can be configured for both polarities?
>
> Good question. Perhaps in a different way because we have to handle
> both h/w controlled and gpio chip selects.
>
> However, how would one configure the polarity in the device in the
> first place? You have to assert the CS first to give a command to
> reprogram it.

That's indeed true for a simple SPI slave.
But if it is a smarter device (e.g. a generic micro controller), it may use the
system's DTB to configure itself.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds