linux-kernel - Re: [PATCH v2 1/2] dt-bindings: i2c: add binding for i2c-hotplug-gpio

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Message-ID: <ZJDiiZLfRO62NFPx@qmqm.qmqm.pl>
Date:   Tue, 20 Jun 2023 01:19:37 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Svyatoslav Ryhel <clamor95@...il.com>,
        Andi Shyti <andi.shyti@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Wolfram Sang <wsa@...nel.org>, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: i2c: add binding for i2c-hotplug-gpio

On Mon, Jun 19, 2023 at 05:53:24PM +0200, Krzysztof Kozlowski wrote:
> On 19/06/2023 17:37, Svyatoslav Ryhel wrote:
> > Document device tree schema which describes hot-pluggable via GPIO
> > i2c bus.
> > 
> > Signed-off-by: Svyatoslav Ryhel <clamor95@...il.com>
[...]
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/i2c/i2c-hotplug-gpio.yaml
> > @@ -0,0 +1,65 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/i2c/i2c-hotplug-gpio.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: GPIO detected hot-plugged I2C bus
> > +
> > +maintainers:
> > +  - Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > +
> > +description:
> > +  Driver for hot-plugged I2C busses, where some devices on a bus
> 
> "Driver" so SW? Bindings are for hardware, not for drivers.
[...]
> > +  detect-gpios:
> > +    maxItems: 1
> > +
> > +  i2c-parent:
> > +    maxItems: 1
> 
> Discussion from v1 stands - this is a software construct, not a real device.
[...]
> Anyway, don't send v3, before the discussion about the entire concept
> finishes. You create a software/virtual device, instead of adding these
> properties to bindings for a real hardware.

Hi,

In this case it's hard for me to tell the difference if this is
real or virtual hardware.

The Transformers have a connector that's used for USB, charging or
for attaching a keyboard (called a dock; it also has a battery and
a touchpad). This connector probably (I don't have the means to verify
that) has an I2C bus lines and a "detect" line (pulled low on the dock
side) among the pins. I guess there is either no additional chip or
a transparent bridge/buffer chip, but nothing that could be controlled
by software. For DT this setup could be modelled like an I2C gate or
2-port mux with enable joining two I2C busses (one "closer" to the
CPU -- parent).

> > +
> > +examples:
> > +  - |
> > +    /*
> > +     * Asus Transformers use I2C hotplug for attachable dock keyboard
> > +     */
> > +    #include <dt-bindings/gpio/gpio.h>
> > +    #include <dt-bindings/interrupt-controller/irq.h>
> > +
> > +    i2c-dock {
> > +        compatible = "i2c-hotplug-gpio";
> > +
> > +        #address-cells = <1>;
> > +        #size-cells = <0>;
> > +
> > +        interrupts-extended = <&gpio 164 IRQ_TYPE_EDGE_BOTH>;
> > +        detect-gpios = <&gpio 164 GPIO_ACTIVE_LOW>;
> 
> I don't think you can have both interrupt and GPIO on the same line.

This actually works as expected. There are multiple devices (and
drivers) that depend on this, e.g. matrix-keypad and gpio-keys.

Best Regards
Michał Mirosław