Re: [PATCH v4 1/5] dt-bindings: misc: Add mikrobus-connector

["Michael Walle" <mwalle@kernel.org>]

Hi,

> >>> Is that because the current software support is too limited? Are there
> >>> manufactures who want to create more complex designed, but are limited
> >>> by what can be described in the manifest?
> >>>
> >>
> >> most mikroBUS add-on boards in production lies in the category of
> >> sensors, displays, connectivity, mixed signal (ADC/DAC .etc) and if you
> >> look at the existing bindings under bindings/iio/ , most devices need
> >> only simple descriptions and the properties are mainly standard bus
> >> properties (SPI/I2C properties), IRQ, named-gpios, named properties,
> >> regulators, clocks the extension to manifest was made taking this into
> >> account and the named property description interface just maps the
> >> manifest entries to the unified device property interface under
> >> include/linux/property.h
> > 
> > How will the ethernet boards ([1], [2]) work? Where do they get
> > their MAC address from, for example. The DT has some nice properties
> > for that, but I doubt that will be possible with the manifest files.
> > I've looked at the manifest file for the w5500 board [3] and to me
> > it looks like that board will come up with a random MAC address on
> > each start. Thus, even today, you have some boards which require
> > a more complex description.
> > 
>
> Agreed, this is a limitation, unless the corresponding 
> drivers/subsystems use device_property_read_* helper to fetch 
> properties, it will not work and net/core/of_net.c only implements 
> of_get_* helpers even though the underlying functions can be implemented 
> with equivalent device_property_read_* equivalent as well.

And I don't think this is a good idea given that the alternative is
just working.

> > Apart from the discussion whether the manifest is a suitable or
> > sufficient mechanism to describe the hardware, I think the main
> > problem with the proposed binding, is that it doesn't follow the
> > binding Rob was proposing for a socket. If I want to use DT
> > overlays, how would you describe an add-on board?
> > 
> > The proposal was that the base board has something like
> > 
> > mikrobus: socket {
> > 	compatible = "mikrobus-socket";
> > 	i2c-parent = <&i2c0>;
> > 	spi-parent = <&spi0>;
> > 
> > 	i2c {};
> > 	spi {};
> > };
> > 
> > an add-on board can then have a DT snippet/overlay like the
> > following:
> > 
> > &mikrobus {
> > 	i2c {
> > 		eeprom@52: {
> > 			reg = <52>;
> > 			compatible = <atmel,at24..>;
> > 		}
> > 	};
> > 
> > 	spi {
> > 		sensor@0: {
> > 			reg = <0>;
> > 			compatible = <foobar>;
> > 		};
> > 	};
> > };
> > 
> > That should be possible with a binding for the mikrobus, which
> > in fact it is just a pin header with a standard pinout. Also as
> > Russell pointed out in v3, the EEPROM/manifest is not part of the
> > mikrobus standard. So maybe that deserves an own compatible, like
> > 
> >     compatible = "mikroe,click", "mikrobus-socket";
> > 
> > Or maybe click-eeprom? Although click seems to be the brand name of
> > MikroElektronika.
>
> Agreed, there is nothing preventing us to convert the binding and update 
> the driver to follow the above proposed format and will be done in next 
> revision. Click is brand name of MikroElektronika and they don't allow 
> custom boards to use that branding, however clickid can be used in the 
> case where EEPROM is present/enable the socket to be probeable.

Thinking about this again. I'm not sure this additional compatible
really helps the discovery. It's a chicken egg problem. Only the
modules knows if it has an EEPROM, but then, we already have to
know it's in the socket. So while it might help for a static
configuration, it does not for the discovery.

-michael

> > [1] https://www.mikroe.com/eth-3-click
> > [2] https://www.mikroe.com/eth-wiz-click
> > [3] https://github.com/MikroElektronika/click_id/blob/main/manifests/ETH-WIZ-CLICK.mnfs