Re: RFC: oftree based setup of composite board devices

On Tue, Feb 9, 2021 at 12:25 AM Enrico Weigelt, metux IT consult
[off-list ref] wrote:

Hello folks,

here's an RFC for using compiled-in dtb's for initializing board devices
that can't be probed via bus'es or firmware.

Use cases are boards with non-oftree firmware (ACPI, etc) where certain
platform devices can't be directly enumerated via firmware. Traditionally
we had to write board specific drivers that check for board identification
(DMI strings, etc), then initialize the actual devices and their links
(eg. gpio<->leds/buttons, ...). Often this can be expressed just by DT.

In ACPI we support DT compatible strings, and we support overlays for
a long time. Would it work for you?

This patch queue does a bunch of preparations in oftree code, so we can
support multiple fully independent DT's (not using DT overlays). And then
adds a generic driver parses compiled-in fdt blobs, checks for mathing
DMI strings and initializes the devices. As an example, the last patch
adds an alternative implementation for the PC engines APU2/3/4 board
family based on device tree.

Sounds weird, but let's see...

The approach can be easily be extended to other kinds of composite devices,
eg. PCI cards or USB dongles.

What do you mean? PCI and USB are self-enumerated. What's wrong with them?

Yet some drawbacks of the current implementation:

 * individual FDT's can't be modularized yet (IMHO, we don't have DMI-based
   modprobing anyways)

What?! https://lwn.net/Articles/233385/
`git grep -n 'MODULE_DEVICE_TABLE(dmi'`

 * can't reconfigure or attach to devices outside the individual DT's
   (eg. probed by PCI, etc)

-- 
With Best Regards,
Andy Shevchenko