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On Wed, 2013-07-31 at 16:56 +0100, Mark Rutland wrote:

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Ah, I guess the question more: Do we want generic bindings that describe
the low-level details of the HW in a completely generic fashion so that
new HW can be supported with zero kernel changes, or do we want a simple
driver with a lookup table that maps from compatible value to the HW
configuration? One of the potential benefits of DT is to be able to
support new HW without code changes, although perhaps that's more
typically considered in the context of new boards rather than new IP
blocks or SoCs.

... or FPGAs that can be synthesized with random collection of standard
IP blocks. With Xilinx's Zynq and Altera's SOCFPGA this is getting
simpler and simpler...

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I think that going forward it would be better to have have a compatible
string per different device. As Olof pointed out, we're leaking the way
we currently handle things in Linux into the binding, rather than
precisely describing the hardware (with a unique compatible string). I'm
not sure if this is much better than embedding a bytecode describing how
to poke devices.

This really isn't leaking information about how Linux handles the
device. It's simply saying that there is a GPIO controller whose HW is
able to be described by a simple/generic binding, and that binding
provides full details of the register layout. Other OSs can handle this
differently; see below ...

I worry that it doesn't provide a full description, but rather a
description of the subset of the hardware used by the driver.

I don't see that as a problem.

A particular DT file provides a description of an interface to HW. To my
mind, if that particular DT doesn't describe everything about a
particular HW module (e.g. some advanced feature can't be enabled),
that's basically equivalent to not describing aspect of the board/system
(so e.g. some I2C device isn't represented at all, and hence some
temperature probe can't be monitored).

My concern is simply that if people can get basic functionality with the
generic driver they won't bother describing the details of the more
specific binding, and you'll never be able to use the board with a
better driver without having to modify the dt later.

I guess the question boils down to how much we care about that
situation.

I personally think it's fine. If people only want to expose the basic
functionality and can do so with simple-gpio, I don't see any problem at
all with that. (Why should we care that a binding doesn't expose all
features of the device? I'm sure there are many undocumented features,
e.g. debug/backdoor, in HW that we don't even know exist and so don't
know aren't exposed by various bindings)

If someone later finds it useful to expose more functionality than is
sensible to expose through simple-gpio, then a HW-specific binding can
be added for that purpose, and the .dts file amended to use that.

The more I think about this, the more I think that simply having
disjoint simple and HW-specific bindings makes sense.

In that case, the new DT won't be compatible with an old kernel, but I
think that's reasonable, since it was explicitly changed to implement
new features.

I think we should resolve this aspect first before considering the
details. I'm curious what other DT maintainers and users think about
this topic?

I want to add from myself that the ability to use a simple-gpio interface
makes things very simple :) As an example, I cite one of the architectures
on which I work. ARM CLPS711X. I can freely use a simple-gpio driver instead
of the special driver, since the architecture does not have any other capacity
for the pins (interrupts, etc.).
But mostly, I intended to use this driver for such things like PLD, 74HC24x,
74HC57x, etc. on the bus.
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