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Comment by Khol | Hacker News Reader

I'm curious as to what things DT cannot access. Is this a general issue (and if so have you spoken to the DT maintainers?), or is this because the systems you are working on are doing something unexpected?

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I've had situations where we had to add a small functionality to a driver. For instance a secondary functionality that wasn't supported in the upstream version. It's very rare for drivers of complex components to support 100% of the functionality.

An example that comes to mind was a video chip that also had a GPIO pin that we needed to control. The chip had a driver in the kernel but it didn't have support for the GPIO (it is after all a very secondary function). That required a small patch.

It would of course have been possible to upstream it but then that meant implementing a "clean" patch (using the kernel GPIO API etc... instead of a one liner to toggle the PIN high on dereset) then submitting it upstream, maybe doing a couple of back and forth... Some will bother to do it, many won't.

Drivers can be built as modules most of the time. Google could still ship a single kernel binary and any vendor that had a weird quirk could load their own module. In fact I wouldn't be surprised if Google requires every driver to be built as a module and they ship a minimal core kernel.

Unfortunately there is no binary compatibility between modules and different kernels. Moreover Google recommends CONFIG_MODVERSIONS=y, which prevents to load even practically compatible modules.

The vendor would need to recompile their modules for every kernel Google releases, but that's not a huge issue - how many times does Google update the kernel?

All the ARM boards i know share our same kernel issues. The problem is that DT is basically a configuration, which is accessed by drivers. If a driver do not read it, you must edit the driver. For instance a 2 lines patch [1] is about the fec (ethernet) driver. DT can specify a sleep time before the PHY reset GPIO is toggled. However we need to sleep also after the toggle, hence the patch.

[1] https://github.com/UDOOboard/linux_kernel/commit/b2fc4a3cba4...

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