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

[Thierry Reding]

On Mon, Oct 21, 2013 at 09:05:44PM +0100, Stephen Warren wrote:
> On 10/21/2013 12:44 AM, Guenter Roeck wrote:
> > On 10/20/2013 04:11 PM, Russell King - ARM Linux wrote:
> >> On Sun, Oct 20, 2013 at 10:26:54PM +0100, Stephen Warren wrote:
> >>> 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
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