RFC: Add initrd generation to load SSDT tables via ACPI

[Florian Eckert]

Hello,

I have an APU3. This has an I2C bus (SMBUS) to which additional devices 
can be connected. On my APU3 board, an IO expander mcp23s08 [1] is 
connected there.
This IO expander provides additional GPIOs. This additional GPIOs are 
connected to LEDs. The whole thing worked until kernel version 5.4 of 
OpenWrt. But after updating to OpenWrt kernel version 5.10 
(openwrt-22.03). I have made the additional LEDs known to the system via 
a platform driver, which then integrates these LEDs. The support for 
reading out the platform driver data for the driver mcp23s08 has been 
removed in the kernel with the commit [2]. I was unable to compile my 
platform device driver against kernel 5.10, because of the removed 
platform data support of the mcp23s08. I contacted the kernel developer 
Andy Shevchenko [3] who made this change. His suggestion was, that the 
data should be made known to the system via an additional SSDT table via 
ACPI [4]. Then everything should work like a system that supports 
device-tree. I have now played around with SSDT a bit and finally 
managed to get the driver responsible again for this device. I had to 
create a new SSDT file and compile it with a program from the 
acpica-unix package [5]. The file must be moved into an initrd, so this 
could be read by the kernel during boot.

Now to my question:
In order to load this new SSDT file, it must be made available to the 
kernel via an initrd.
However, initrd handling is currently not supported by OpenWrt. 
Manually, it is no problem for me to load the GRUB initrd.
But I would like to have a solution that is generally valid in OpenWrt 
during image generation. I have already prepared a pullrequest for 
openwrt [6] on my fork with the changes that are needed. But I am not 
sure if this is the right solution.
Additional the acpica-unix package from the package feed [7] must also 
be build for the host, to compile the acpi asl source file into acpi aml 
files.


Kind regards

Florian

[1] 
https://github.com/torvalds/linux/blob/master/drivers/pinctrl/Kconfig#L305
[2] 
https://github.com/torvalds/linux/commit/6aba6ed879b3471903c8ada28ba968a268df6143
[3] https://github.com/andy-shev
[4] 
https://lore.kernel.org/all/290741faab199d3e43b6255bf2282075@dev.tdt.de/
[5] https://www.acpica.org/documentation
[6] https://github.com/TDT-AG/openwrt/tree/pr/20230110-config-kernel
[7] 
https://github.com/openwrt/packages/blob/master/utils/acpica-unix/Makefile