[PATCH v2 11/14] Documentation: dt: binding: omap: am43x timer

[Mohammed, Afzal]

Hi Jon,

On Wed, May 29, 2013 at 03:35:10, Stephen Warren wrote:
> On 05/28/2013 03:25 PM, Jon Hunter wrote:

> >>  			ti,am335x-timer	(applicable to AM335x devices)
> >>  			ti,am335x-timer-1ms (applicable to AM335x devices)
> >> +			"ti,am4372-timer-1ms", "ti,am335x-timer-1ms" for AM43x 1ms timer
> >> +			"ti,am4372-timer", "ti,am335x-timer" for AM43x timers other than 1ms one

> > If you are adding more compatibility strings, then this implies that the
> > AM43x timers are not 100% compatible with any other device listed (such
> > as am335x or any omap device). That's fine but you should state that in
> > the changelog. If the AM43x timer registers are 100% compatible with
> > existing devices you should not add these.
> 
> I'm not sure that's true; .dts files should always include a compatible
> value that describes the most specific model of the HW, plus any
> baseline compatible value that the HW is compatible with. This allows
> any required quirks/fixes/... to be applied for the specific HW model
> later even if nobody knows right now they'll be needed. Hence, defining
> new compatible values doesn't necessarily mean incompatible HW.

Stephen took words out of my finger ;)
 
Some explanations,

1. first compatible should be exact device [A], followed by compatible
model (if one)
2. Minor effort in getting DT right the first time may help prevent
difficult effort later modifying it (if a necessity comes), considering
the fact that DT sources has  to move out of Kernel at some point of
time. And DT is not supposed to be modified, which may cause difficulty
for the users (I had been a minor victim of this during rebase).

As we both were in GPMC land earlier, an example,

If my memory is right, GPMC IP in am335x is rev 6, and IP has 8 chip
select, but one is not pinned out. Now assume that same IP is integrated
in another SoC (probably OMAP4 has rev 6). Here if we use same compatible
for both, driver cannot handle it properly (w/o knowledge about platform).
But if exact compatible is mentioned, without modifying DT (which should
be considered as a firmware) just by modifying Kernel, deciding based on
compatible would help achieve what is required.

Regards
Afzal

[A] http://devicetree.org/Device_Tree_Usage#Understanding_the_compatible_Property