Re: linux-next: manual merge of the arm-soc tree with the i2c-embeddedtree

From: Lee Jones
Date: Wed Jul 18 2012 - 06:29:26 EST

On 18/07/12 10:59, Mark Brown wrote:
On Wed, Jul 18, 2012 at 01:33:42PM +0800, Shawn Guo wrote:

I have an IP block getting different FIFO size on different IMX SoCs.
We could introduce a new compatible string for driver to figure it out,
but I think the simpler way is just have the data encoded in device
tree. In any case DT is not completely limited in encoding board
specific data. Today, we have IO region and interrupt number of
hardware blocks encoded in DT, and to me, FIFO size could just be
another aspect of hardware block we could choose to encode in DT.

So, this is part of the problem with encoding the description of the SoC
into the DT - we're not doing a good job of splitting out the silicon
description from the board specific description which is not a triumph
for maintainability since it means that we end up needing to modify the
DT for every board using the silicon if we want to use the new feature
(assuming people maintain binary compatibility with old DTs, which we
don't do a good job at either even for established DT architectures).

It's not the using device tree bit that creates concern for me here,
it's the fact that the board and silicon aren't being separated.

What's the difference?

The Device Tree describes the hardware (hardware == silicon + peripherals). So different platforms are different pieces of hardware. Everything the driver can't figure itself out from the silicon itself needs to be described in the DT.

We can describe the relationship with a hierarchical layout of the dts and dtsi files. So for my recent venture we have:

+- db8500.dtsi // silicon
+-- snowball.dts // board

We also have plans to split it up further:

db8xx0.dtsi // shared silicon
/ \
db8500.dtsi db8540.dtsi // differing silicon
\ \
+-- snowball.dts +-- new_board.dts // board

Lee Jones
Linaro ST-Ericsson Landing Team Lead
M: +44 77 88 633 515 â Open source software for ARM SoCs
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