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

[Conor Dooley <conor@kernel.org>]

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On Tue, Mar 19, 2024 at 11:05:37PM +0530, Vaishnav Achath wrote:
> Hi Andrew,
> 
> On 19/03/24 17:55, Andrew Lunn wrote:
> > > The device tree defines the SPI controller associated with mikroBUS SPI
> > > pins. The driver on match queries and takes a reference to the SPI
> > > controller but does nothing with it. Once a mikroBUS add-on board is
> > > detected (by passing manifest using sysfs or reading from 1-wire EEPROM),
> > > the driver parses the manifest, and if it detects an SPI device in manifest,
> > > it registers SPI device along with setting properties such as `chip_select`,
> > > `max_speed_hz`, `mode`, etc.,
> > 
> > How complex can the description of the hardware be in the manifest?
> > 
> > Could i describe an SPI to I2C converter? And then a few temperature
> > sensors, a fan controller, and a GPIO controller on that I2C bus? And
> > the GPIO controller is then used for LEDs and a push button? DT
> > overlays could describe that. Can the manifest?
> 
> No, it cannot describe such complex hardware, it can only describe simple
> devices (sensors/displays .etc) on a standard mikroBUS add-on board, we did
> a analysis on what mikroBUS add-on boards have driver support in Linux and
> then noticed that most devices does not need this kind of complex
> description to work:
> https://elinux.org/MikroEClicks_with_Linux_Support

What happens to the devices that fall outside of the "do not need a
complex description" category? Do you expect that those would be
described by a dt overlay?

> The greybus manifest already is being used in the greybus susbystem for
> describing an interface and there are already greybus controllers (SPI/I2C
> .etc) being created according to the manifest contents, all this driver does
> is to extend that format to be able to instantiate devices on these buses.
> The primary goals for introducing the driver for mikroBUS add-on boards are:
> 
> 1) A way to isolate platform specific information from add-on board specific
> information - so that each permutation of connecting the add-on board on
> different ports on different board does not require a new overlay.
> 2) A way to instantiate add-on boards on greybus created virtual mikroBUS
> ports.
> 3) Both 1 and 2 should use the same add-on board description format.
> 
> Standard device tree overlays did not help to achieve this and that is why
> the standard interface discovery mechanism in greybus, the manifest was
> extended even though it is not the most optimal way to describe hardware.
> 
> The greybus manifest extensions were made with the following things in mind
> and three new descriptor were introduced:
> 1) mikrobus descriptor - pinmux/port state
> 2) device descriptor - contains information which is a superset of struct
> i2c_board_info , struct spi_board_info .etc
> 3) property descriptor - to describe named properties of the types defined
> under https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/property.h#n22
> 
> With these we were able to test around 150 add-on boards with corresponding
> drivers in Linux :
> https://github.com/MikroElektronika/click_id/tree/main/manifests
> 
> The mechanism is not as robust a device tree and should not be compared, the

Why not? You're suggesting this as a method for describing devices and you
seem to have extended the manifest to support more complex properties, why
shouldn't someone question make that comparison?

> intent was not to create a new hardware description format, but extend the
> existing greybus manifest format to be able to instantiate devices on the
> greybus SPI/I2C/GPIO/ (mikroBUS)


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