Device tree vs hardware configurations
Hi
I'm currently forward-porting a BSP for imx6-based custom board from pre-devicetree kernel to modern kernel.
In old BSP there was a board setup file, that registered all board's devices. For new BSP, I need to replace that with device tree based solution.
However, old BSP used conditional code to register devices differently based on GPIO inputs and on kernel command line. This approach was used to
- handle board's jumper that switches SPI CS lines: current jumper setting is available over gpio, depending on that old BSP registered chips differently,
- handle different i2c connections on different board revisions: board has 5 i2c busses with quite a few devices connected, these busses are routed to different hardware busses on different board revisions, board revision could be read over gpios.
- handle different possible display connections (lvds vs lcd, 6bit vs 8bit hw interface) based on kernel command line options
- handle different possible camera connections by registered camera differently based on kernel command line option
... and more,
Device tree describes hardware unconditionally. I already have to provide 2 dts files for imx6q and imx6dl based setups (both just include a common dtsi) ... But providing separate dts file for every possible hardware configuration will result into 2^n device trees, which is inconvenient visible regression against old BSP that "just worked" on all hardware configurations.
Is there a sane way to handle hardware configurations like above in device tree based kernel?
Nikita Yushchenko
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Hi Nikita,
Quoted 39 lines · Nikita Yushchenko
Hi
I'm currently forward-porting a BSP for imx6-based custom board from pre-devicetree kernel to modern kernel.
In old BSP there was a board setup file, that registered all board's devices. For new BSP, I need to replace that with device tree based solution.
However, old BSP used conditional code to register devices differently based on GPIO inputs and on kernel command line. This approach was used to
- handle board's jumper that switches SPI CS lines: current jumper setting is available over gpio, depending on that old BSP registered chips differently,
- handle different i2c connections on different board revisions:
board has 5 i2c busses with quite a few devices connected, these busses are routed to different hardware busses on different board revisions, board revision could be read over gpios.- handle different possible display connections (lvds vs lcd, 6bit vs 8bit hw interface) based on kernel command line options
- handle different possible camera connections by registered camera differently based on kernel command line option
... and more,
Device tree describes hardware unconditionally. I already have to provide 2 dts files for imx6q and imx6dl based setups (both just include a common dtsi) ... But providing separate dts file for every possible hardware configuration will result into 2^n device trees, which is inconvenient visible regression against old BSP that "just worked" on all hardware configurations.
Quoted 2 lines
Is there a sane way to handle hardware configurations like above in device tree based kernel?
This depends on your definition of sane, but here the options I know of:
- Let the bootloader modify the device tree based on the GPIOs. This seems doable in new projects, but not when you want to keep the old bootloader.
- Use device tree overlays in the Kernel which is something Pantelis Antoniou works on. I think the intention here is more to plug a daughter board to a CPU board. I don't know how good this works when you have to manipulate many different places in the device tree.
- I heard people working on a shim to place between bootloader and kernel. The shim translates the bootloader information into the device tree. I can't remember who it was though. For going this way I wouldn't rewrite the shim but use barebox instead of course ;)
BTW where do the command line options in your setup come from? Are they autogenerated from other informations in the bootloader or are they entered there by a user?
Sascha
Signature · 8 lines
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