Re: [PATCH 0/8] Add generic overlay for MikroBUS addon boards

On Wed, Sep 11, 2024 at 07:57:17PM +0530, Ayush Singh wrote:

Hello all,

This is an attempt to add MikroBUS addon support using the approach
described by Grove connector patch series [0].

The patch series tests out 2 addon boards + pwm and GPIO on the MikroBUS
connector. The boards used are GPS3 Click (for UART) [1] and Weather
Click (for I2C) [2]. Additionally, I have tested relative GPIO numbering
using devicetree nexus nodes [3].

The patch series does not attempt to do any dynamic discovery for 1-wire
eeprom MikroBUS addon boards, nor does it provide any sysfs entry for
board addition/removal. The connector driver might include them after
the basic support is ironed out, but the existing patches for dynamic
overlays work fine.

The patch series has been tested on BeaglePlay [4].

I will now go over individual parts of the patch series, but for a
better picture of the approach, it would be best to checkout [0] first.

MikroBUS connector driver
-------------------------

Just a stub platform driver for now. Will be extended in the future for
dynamic board discovery using 1-wire eeprom present in some MikroBUS
addon boards.

While it is a stub driver, disabling it will make the GPIO connector
nexus node unreachable (any driver attempting to use it will enter
differed probing). So it is required.

MikroBUS connector Devicetree
------------------------------

The connector devicetree defines the MikroBUS GPIO nexus node. This
allows using pin numbering relative to MikroBUS connector pins in the
addon boards overlay. Currently, the patch uses a clockwise numbering:

  0: PWM
  1: INT
  2: RX
  3: TX
  4: SCL
  5: SDA
  6: MOSI
  7: MISO
  8: SCK
  9: CS
  10: RST
  11: AN

Additionally, in case PWM pin is not using channel 0, a nexus node for pwm
should also be used and go in the connector devicetree.

MikroBUS connector symbols overlay
----------------------------------

To make an overlay generic we need a standard name scheme which we
use across base boards. For the connector pins the pinmux phandle
shall be:

<connector-name>_<pin-name>_mux_<pin-function>

For the parent provider phandle, we use a similar naming scheme:

<connector-name>_<pin-name>_<pin-function>

For GPIO controller, I am using `MIKROBUS_GPIO` name since with nexus
nodes, we do not need to define individual pin gpio controllers.

The string symbols can be replaced with phandles once [5] is accepted.
That will make connector stacking much simpler.

MikroBUS addon board overlay
----------------------------

The patch puts the addon board overlays in their own directory. I am
using the following naming scheme for MikroBUS addon boards:

<vendor>-<product_id>.dtso

Mikroe [6] lists this for all boards in their website, but I am not sure
if other vendors have a product_id.

This naming also makes future dynamic discovery easier, since click id
spec [7] contains vendor_id and product_id in the header.

Usage
-----

So what does this all look like? Let's take an example of a BeaglePlay
with one MikroBUS connectors for which we have physically attached a
Wather click module to the first connector. Doing ahead of time
command-line DT overlay application:

./fdtoverlay \
	-o output.dtb \
	-i k3-am625-beagleplay.dtb \
		k3-am625-beagleplay-mikrobus-connector0.dtbo mikroe-5761.dtbo

Open Items
----------

- SPI Support: 
  Currently SPI dt requires `reg` property to specify the
  chipselect to use. This, makes the SPI device overlay dependent on the
  SPI controller. Thus for SPI support, we need a way to allow defining
  SPI chipselect relative to MikroBUS pins, and not the actual device
  controller.

  One possible solution is to introduce something like `named-reg` and
  allow selecting the chipselect by string array. But this probably
  requires more discussion so I have left out SPI support for now.

  NOTE: pins other than the CS MikroBUS pin can be used as chipselect.
  See [8].

- Controller symbol duplication
  The current symbol scheme has multiple symbols for the same underlying
  controller (Eg: MIKROBUS_SCL_MUX_I2C_SCL and MIKROBUS_SDA_MUX_I2C_SDA)
  point to the same i2c controller.

  I think both of them will always use the same i2c controller, but
  maybe there are some exceptions? So I have left it as it is for this
  patch series. The same thing also applies to UART and SPI.

  NOTE: with the use of nexus node for GPIO, the GPIO controller symbol
  will be the same for all pins.

- Nexus node dt-bindings
  I am not quite sure how to deal with the nexus node properties
  (#gpio-cells, gpio-map, gpio-map-mask, gpio-map-pass-thru) since they
  seem to conflict with upstream gpio schema (gpio-controller is a
  dependency of #gpio-cells).

Please submit a fix to dtschema. Either GH PR or patch to 
devicetree-spec list.

Rob