Re: [Patch V2 4/4] ARM: dts: AM33XX: update rtc node compatibility

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Jul 30, 2013 at 05:21:14PM +0100, Sekhar Nori wrote:
> On 7/30/2013 8:25 PM, Mark Rutland wrote:
> > On Tue, Jul 30, 2013 at 06:05:52AM +0100, Gururaja Hebbar wrote:
> >> Hi,
> >>
> >> On 7/3/2013 2:17 PM, Hebbar Gururaja wrote:
> >>> Since AM33xx  RTC IP has RTC_IRQWAKEEN to support Alarm Wake-up.
> >>>
> >>> Update the rtc compatible property to "ti,am3352-rtc" to enable handling
> >>> of this feature inside rtc-omap driver.
> >>
> >> The other 2 rtc driver related patches have been pulled up. If you have 
> >> no comments, can you please pull this up.
> >>
> >> Regards
> >> Gururaja
> >>
> >>>
> >>> Signed-off-by: Hebbar Gururaja <gururaja.hebbar@ti.com>
> >>> ---
> >>> :100644 100644 77aa1b0... dde180a... M	arch/arm/boot/dts/am33xx.dtsi
> >>>   arch/arm/boot/dts/am33xx.dtsi |    2 +-
> >>>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/arch/arm/boot/dts/am33xx.dtsi b/arch/arm/boot/dts/am33xx.dtsi
> >>> index 77aa1b0..dde180a 100644
> >>> --- a/arch/arm/boot/dts/am33xx.dtsi
> >>> +++ b/arch/arm/boot/dts/am33xx.dtsi
> >>> @@ -297,7 +297,7 @@
> >>>   		};
> >>>
> >>>   		rtc@44e3e000 {
> >>> -			compatible = "ti,da830-rtc";
> >>> +			compatible = "ti,am3352-rtc";
> > 
> > Given that this is backwards compatible with the old binding, it would
> > be nicer to have this as:
> > 
> > 	compatible = "ti,am3352-rtc", "ti,da830-rtc";
> > 
> > We must get into the habit of changing dts files in a
> > backwards-compatible fashion.
> 
> Right, I suggested this when v1 was posted. It turns out the current
> kernel does not handle the compatilble list correctly and the string
> selected actually depends on the order in which it appears in match
> table in driver instead.
> 
> I saw there were patches being discussed to fix this issue, but until
> that is fixed, we cannot really use what you (and I before) suggested.

A temporary solution would be to list the "ti,am3352-rtc" first in the
of_match_table kernel-side. That way you can keep the old compatible
string in the dts compatible list, and maintain backwards compatibilty
with older kernels.

Later if the way Linux matches compatible strings is changed, this
shouldn't break the probe order, and the of_match_table order could be
changed.

Have I missed something?

Thanks,
Mark.