Re: [Ksummit-2013-discuss] ARM topic: Is DT on ARM the solution, or is there something better?

On Mon, Oct 21, 2013 at 09:05:44PM +0100, Stephen Warren wrote:

On 10/21/2013 12:44 AM, Guenter Roeck wrote:

quoted

On 10/20/2013 04:11 PM, Russell King - ARM Linux wrote:

quoted

On Sun, Oct 20, 2013 at 10:26:54PM +0100, Stephen Warren wrote:

quoted

The only thing we've really moved out of the kernel is the exact IDs of
which GPIOS, interrupts, I2C/SPI ports the devices are connected to; the
simple stuff not the hard stuff. The code hasn't really been simplified
by DT - if anything, it's more complicated since we now have to parse
those values from DT rather than putting them into simple
data-structures.

Here's my random thoughts this evening on DT, orientated mostly on a
problem area I've been paying attention to recently.

In some ways, DT has made things much harder.  I don't know whether

On the other side, DT has made some things much simpler.

Problem case: Chip A's interrupt pin is connected to gpio pin of chip B.
Chip B's interrupt pin is connected to a gpio pin on chip C. Chip C's
interrupt pin is connected to the interrupt controller. Chips B and C
are multi-function FPGAs. Exact gpio pins used vary from board to board.

With DT, this relationship is easy to describe, and none of the chip
drivers
really needs to know what is connected to what. Without DT, it would be
much more complex.

I thought that was very easy with board files. For every chip that gets
instantiated, you tell it which base IRQ ID to use. For every chip that
outputs a IRQ, you tell it which IRQ ID that signal is, simply by adding
the recipient chip's IRQ base and an offset. That seems very simple. If
you don't want to add the base/offset together, we could always have
invented some lookup table similar to what regulators had, and pinctrl
has now, which says for each (device name, IRQ name/number) output,
here's the (device name, IRQ name/number) of the associated input.

Well, DT has certainly increased the level of dynamicity within the
kernel. I for one consider not having to rely on a fixed IRQ or GPIO
base per-chip a good thing. Deferred probing is a very powerful yet
extremely simple tool that was created as part of the DT conversion
effort. There is obviously much more that can be done, but I think
that we have come a long way.

But as you said, that could all have been done even with board files.

Thierry