Re: [PATCH v3 1/2] dt-bindings: display: bridge: add TI TDP158

[Maxime Ripard <mripard@kernel.org>]

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On Tue, Jul 30, 2024 at 11:46:24AM GMT, Dmitry Baryshkov wrote:
> On Tue, 30 Jul 2024 at 11:27, Maxime Ripard <mripard@kernel.org> wrote:
> >
> > On Wed, Jul 24, 2024 at 07:59:21PM GMT, Marc Gonzalez wrote:
> > > On 15/07/2024 16:40, Maxime Ripard wrote:
> > > > On Thu, Jul 04, 2024 at 07:04:41PM GMT, Marc Gonzalez wrote:
> > > >> On 01/07/2024 15:50, Maxime Ripard wrote:
> > > >>
> > > >>> The i2c register access (and the whole behaviour of the device) is
> > > >>> constrained on the I2C_EN pin status, and you can't read it from the
> > > >>> device, so it's also something we need to have in the DT.
> > > >>
> > > >> I think the purpose of the I2C_EN pin might have been misunderstood.
> > > >>
> > > >> I2C_EN is not meant to be toggled, ever, by anyone from this planet.
> > > >
> > > > Toggled, probably not. Connected to a GPIO and the kernel has to assert
> > > > a level at boot, I've seen worse hardware design already.
> > > >
> > > >> I2C_EN is a layout-time setting, decided by a board manufacturer:
> > > >>
> > > >> - If the TDP158 is fully configured once-and-for-all at layout-time,
> > > >> then no I2C bus is required, and I2C_EN is pulled down forever.
> > > >>
> > > >> - If the board manufacturer wants to keep open the possibility
> > > >> to adjust some parameters at run-time, then they must connect
> > > >> the device to an I2C bus, and I2C_EN is pulled up forever.
> > > >
> > > > How do you express both cases in your current binding?
> > >
> > > It's not that I'm ignoring your question.
> > >
> > > It's that I don't understand what you're asking.
> >
> > And that's fine, you just need to say so.
> >
> > Generally speaking, you're focusing on the driver. The driver is not the
> > issue here. You can do whatever you want in the driver for all I care,
> > we can change that later on as we wish.
> >
> > The binding however cannot change, so it *has* to ideally cover all
> > possible situations the hardware can be used in, or at a minimum leave
> > the door open to support those without a compatibility breakage.
> >
> > That's why I've been asking those questions, because so far the only
> > thing you've claimed is that "I can't test the driver for anything
> > else", but, again, whether there's a driver or not, or if it's
> > functional, is completely missing the point.
> >
> > > SITUATION 1
> > > tdp158 is pin strapped.
> > > Device node is child of root node.
> > > Properties in proposed binding are valid (regulators and power-on pin)
> > > Can be supported via module_platform_driver.
> > >
> > > SITUATION 2
> > > tdp158 is sitting on I2C bus.
> > > Device node is child of i2c bus node.
> > > (robh said missing reg prop would be flagged by the compiler)
> > > Properties in proposed binding are valid (regulators and power-on pin)
> > > Supported via module_i2c_driver.
> > >
> > > If some settings-specific properties are added later, like skew,
> > > they would only be valid for the I2C programmable mode, obviously.
> >
> > I think there's a couple more combinations:
> >
> >   - The device is connected on an I2C bus, but I2C_EN is tied low
> 
> No, this is not possible. I2C pins are repurposed if I2C_EN is low.
> You can not call that an i2c bus anymore.
> 
> >   - The device is connected on an I2C bus, but I2C_EN is connected to a
> >     GPIO and the kernel needs to assert its state at boot.
> 
> This is a pretty strange configuration.  The I2C_EN pin isn't supposed
> to be toggled dynamically. Anyway, if that happens, I'd use pinctrl /
> hog to control the pin.

ACK. I still believe it would be valuable, but I don't really want to be
part of that conversation anymore. Marc, do whatever you want.

Maxime

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