Hi Conor, On 19/03/24 23:49, Conor Dooley wrote:
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_SupportWhat 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?
Yes, those would need a device tree overlay, but most mikroBUS add-on boards does not need this, almost all the boards need the standard bus properties (SPI/I2C properties), IRQ, named-gpios, named properties, regulators, clocks and the current implementation supports this.
Looking at the example Andrew provided above (SPI-I2C converter with sensors .etc on the I2C bus and GPIO controller) - usually you will not find such a mikroBUS add-on board, because if there are I2C devices they would directly be on the mikroBUS I2C bus rather than on the converter, now someone can do this in their custom solution but then it is no different than an I2C adapter on USB/PCIe, does the standard discovery mechanism on those buses help instantiate devices on the I2C? my understanding is NO and you will need to write a custom device tree overlay for the same and same will be the case here.
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, theWhy 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?
Agreed, but the comparison need to limited to the expansion interface (mikroBUS) under discussion as the idea is not to create an alternate interface for describing generic devices, the class of add-on boards that can fit in the mikroBUS add-on board form factor and on the buses exposed by mikroBUS requires only simple descriptions - namely standard bus properties (SPI/I2C properties), IRQ, named-gpios, named properties, regulators, clocks and the extensions to manifest were made for those properties only. Also the extensions were done to support the properties under unified device property interface under include/linux/property.h
Thanks and Regards, Vaishnav
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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