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

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Date: Tue, 19 Mar 2024 06:58:16 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Ayush Singh <ayushdevel1325@...il.com>, Michael Walle
 <mwalle@...nel.org>, open list <linux-kernel@...r.kernel.org>
Cc: jkridner@...gleboard.org, robertcnelson@...gleboard.org,
 lorforlinux@...gleboard.org, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Nishanth Menon <nm@...com>,
 Vignesh Raghavendra <vigneshr@...com>, Tero Kristo <kristo@...nel.org>,
 Derek Kiernan <derek.kiernan@....com>, Dragan Cvetic
 <dragan.cvetic@....com>, Arnd Bergmann <arnd@...db.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Vaishnav M A <vaishnav.a@...com>, Mark Brown <broonie@...nel.org>,
 Johan Hovold <johan@...nel.org>, Alex Elder <elder@...nel.org>,
 "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
 <devicetree@...r.kernel.org>,
 "moderated list:ARM/TEXAS INSTRUMENTS K3 ARCHITECTURE"
 <linux-arm-kernel@...ts.infradead.org>,
 "open list:SPI SUBSYSTEM" <linux-spi@...r.kernel.org>,
 "moderated list:GREYBUS SUBSYSTEM" <greybus-dev@...ts.linaro.org>,
 Vaishnav M A <vaishnav@...gleboard.org>
Subject: Re: [PATCH v4 1/5] dt-bindings: misc: Add mikrobus-connector

On 18/03/2024 18:20, Ayush Singh wrote:
> On 3/18/24 17:52, Michael Walle wrote:
> 
>> On Sun Mar 17, 2024 at 8:37 PM CET, Ayush Singh wrote:
>>> Add DT bindings for mikroBUS interface. MikroBUS is an open standard
>>> developed by MikroElektronika for connecting add-on boards to
>>> microcontrollers or microprocessors.
>>>
>>> mikroBUS is a connector and does not have a controller. Instead the
>>> software is responsible for identification of board and setting up /
>>> registering uart, spi, i2c, pwm and other buses. Thus it needs a way to
>>> get uart, spi, i2c, pwm and gpio controllers / adapters.
>>>
>>> A mikroBUS addon board is free to leave some of the pins unused which
>>> are marked as NC or Not Connected.
>>>
>>> Some of the pins might need to be configured as GPIOs deviating from their
>>> reserved purposes Eg: SHT15 Click where the SCL and SDA Pins need to be
>>> configured as GPIOs for the driver (drivers/hwmon/sht15.c) to work.
>>>
>>> For some add-on boards the driver may not take care of some additional
>>> signals like reset/wake-up/other. Eg: ENC28J60 click where the reset line
>>> (RST pin on the mikrobus port) needs to be pulled high.
>>>
>>> Here's the list of pins in mikroBUS connector:
>>> Analog - AN
>>> Reset - RST
>>> SPI Chip Select - CS
>>> SPI Clock - SCK
>>> SPI Master Input Slave Output - MISO
>>> SPI Master Output Slave Input - MOSI
>>> VCC-3.3V power - +3.3V
>>> Reference Ground - GND
>>> PWM - PWM output
>>> INT - Hardware Interrupt
>>> RX - UART Receive
>>> TX - UART Transmit
>>> SCL - I2C Clock
>>> SDA - I2C Data
>>> +5V - VCC-5V power
>>> GND - Reference Ground
>>>
>>> Additionally, some new mikroBUS boards contain 1-wire EEPROM that contains
>>> a manifest to describe the addon board to provide plug and play
>>> capabilities.
>>>
>>> Link: https://www.mikroe.com/mikrobus
>>> Link:
>>> https://download.mikroe.com/documents/standards/mikrobus/mikrobus-standard-specification-v200.pdf
>>> mikroBUS specification
>>> Link: https://www.mikroe.com/sht1x-click SHT15 Click
>>> Link: https://www.mikroe.com/eth-click ENC28J60 Click
>>> Link: https://www.mikroe.com/clickid ClickID
>>>
>>> Co-developed-by: Vaishnav M A <vaishnav@...gleboard.org>
>>> Signed-off-by: Vaishnav M A <vaishnav@...gleboard.org>
>>> Signed-off-by: Ayush Singh <ayushdevel1325@...il.com>
>>> ---
>>>   .../connector/mikrobus-connector.yaml         | 113 ++++++++++++++++++
>> See also
>> https://lore.kernel.org/r/YmFo+EntwxIsco%2Ft@robh.at.kernel.org/
>>
>> Looks like this proposal doesn't have the subnodes. How do you
>> attach a kernel driver to it's spi port for example? Only through
>> the manifest files?
>>
>> -michael
> 
> 
> So I looked at the Patch, and it seems the approach of fundamentally 
> different than this PR. So, let me try to explain what this patch set 
> does for an add-on board using SPI.
> 
> 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 

As I understood Mikrobus does not have EEPROM.

> in manifest, it registers SPI device along with setting properties such 
> as `chip_select`, `max_speed_hz`, `mode`, etc., which are defined in the 
> manifest. On board removal, it unregisters the SPI device and waits for 
> a new mikroBUS board to be detected again.

You explained drivers, not hardware for DT.

> 
> It is also possible for SPI not to be used by a device, in which case, 
> no SPI device is registered to the controller. It is also possible that 
> the SPI pins will be used as normal GPIOs. Everything is identified from 
> the manifest.


Best regards,
Krzysztof