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SubjectRe: [PATCH v3 4/7] dt-bindings: gpio: Add gpio-repeater bindings
On Fri, Dec 6, 2019 at 3:17 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Rob,
>
> 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?

> > > 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.

> 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.

Rob

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