How to handle named resources with DT?

[David Gibson]

On Tue, Aug 09, 2011 at 11:53:32PM +0200, Cousson, Benoit wrote:
> On 8/9/2011 11:49 PM, Grant Likely wrote:
> >On Tue, Aug 09, 2011 at 11:44:35PM +0200, Cousson, Benoit wrote:
> >>On 8/9/2011 11:17 PM, Grant Likely wrote:
> >>>On Tue, Aug 09, 2011 at 11:08:09PM +0200, Cousson, Benoit wrote:
> >>>>On 8/9/2011 10:57 PM, Grant Likely wrote:
> >>>>>On Tue, Aug 09, 2011 at 01:26:29PM -0500, Scott Wood wrote:
> >>>>>>On 08/09/2011 12:47 PM, Cousson, Benoit wrote:
> >>>>>>>On 8/9/2011 7:23 PM, Grant Likely wrote:
> >>>>>>>>There is no analogous mechanism for _byname in the device tree.  The
> >>>>>>>>DT binding for a device must explicitly state what order the register
> >>>>>>>>ranges are in.  The driver will need to be adapted.
> >>>>>>>
> >>>>>>>That seems to be a small regression for my point of view. Relying on the
> >>>>>>>order is not super safe. This is not very readable either. That's for
> >>>>>>>that exact reason that we changed our drivers to use
> >>>>>>>platform_get_resource_byname. That's probably the reason why that API is
> >>>>>>>there as well.
> >>>>>>>For the same IP, the number of entries can vary depending of the SoC
> >>>>>>>revision.
> >>>>>>>By using the _byname, we can check if the resource is there or not
> >>>>>>>without having to care about the position.
> >>>>>>
> >>>>>>You could have a named u32 property that contains the reg index, e.g.:
> >>>>>>
> >>>>>>dev {
> >>>>>>	reg =<0x20000 0x200 0x24000 0x200>;
> >>>>>>	foo-reg =<0>;
> >>>>>>	bar-reg =<1>;
> >>>>>>};
> >>>>>
> >>>>>That's a little nasty.  A reg-names = "foo", "bar"; would probably be
> >>>>>better.
> >>>>
> >>>>Yep, I agree.
> >>>>
> >>>>And what about something like that?
> >>>>   reg =<0x20000 0x200>, "foo",
> >>>>	<0x20000 0x200>, "bar" ;
> >>>>
> >>>>It is doable?
> >>>
> >>>Definitely not.  It would break all existing 'reg' parsing
> >>>implementations quite badly.
> >>
> >>OK, so what about extending the reg attribute to be a reg node?
> >>
> >>dev {
> >>	reg {
> >>		name = "foo_wrapper";
> >>		start =<0x10000>;
> >>		end =<0x200>;
> >>	}
> >>	reg {
> >>		name = "foo";
> >>		start =<0x20000>;
> >>		end =<0x200>;
> >>	}
> >>}
> >>
> >>A little bit more verbose, but at least we can add any attribute we want.
> >
> >That won't work either because that also breaks the existing 'reg'
> >binding.  Anything you do will need to supplement the existing
> >binding without changing it in an incompatible way.
> 
> OK, but can we add a new attribute then? reg2, reg_ng, reg_plusplus,
> reg_named...?

He already suggested reg-names to be interpreted in parallel with the
existing reg property.  The (serious) problem with replacing the reg
property is that it will break all existing OSes (including old Linux
versions) that don't understand the new property.

Of course, the problem with reg-names is that it will be ignored by
older OSes, and so 'reg' must still be in the correct order.  In which
case you could argue it's more sensible to just have a static place to
name mapping in the Linux driver.

In short, yes, named reg elements in the DT would be nice in theory,
but I'm not convinced it's worth a DT flag day to accomplish it.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson