ARM, SoC: About the use DT-defined properties by 3rd-party drivers

On Mon, Sep 12, 2016 at 02:29:37PM +0200, Sebastian Frias wrote:

My question is about submitting DT properties/nodes (describing some HW) for
which there is no Linux driver. Like register addresses for HW blocks,
including HW capabilities of said HW blocks, which may or may not be setup
by Linux directly.

The idea being that since DT describes the HW and is usually shared with the
bootloader (yet stored in the Linux kernel tree), all layers of the stack
could use the same DT and each layer would use relevant properties. So the
DT would describe the whole SoC even if not all HW blocks have a Linux
driver.

3rd party users of said SoC could then write kernel modules for such HW
blocks using the DT description. The DT would thus become the authoritative
source of information regarding register programming for the SoC.

I don't follow this part entirely. Why are you expecting thrid parties
to write a driver for those blocks rather than upstreaming a driver for
them?

Currently, HW blocks for which there is no public driver (that it is
accessed through user-mode libraries or firmware) require a separate
HW description (be it Documentation, headers, etc.)

Since the DT describes the HW, it would make sense to expose the HW through
DT, that would centralise the HW description.

I would generally agree that the hardware should be described in DT.
The difficulty is that without a 'real' user it's not always possible to
tell if we're describing the thing correctly.

Putting smoething together that's only sufficient to support some
out-of-tree driver with implicit assumptions that we are not aware of is
far from fantastic.

However, after discussing over IRC, it looks like there was no guidance on
this. Some people think submitting DT properties/nodes without a corresponding
Linux driver is frowned upon, while others thought it was an odd limitation
and suggested asking here.

Unfortunately, I think that the area is sufficiently vague that there
simply is no clear and general answer.

For the sake of discussion, an example of a particular block, along with
what you expect/need to describe would be helpful.

Thanks,
Mark.