linux-kernel - Re: [PATCH v3 1/2] rtc: omap: update of_device

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Date:	Fri, 16 Aug 2013 21:11:30 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Benoit Cousson <bcousson@...libre.com>
CC:	"Hebbar, Gururaja" <gururaja.hebbar@...com>,
	<mark.rutland@....com>, <a.zummo@...ertech.it>,
	<davinci-linux-open-source@...ux.davincidsp.com>,
	<khilman@...aro.org>, <rtc-linux@...glegroups.com>,
	<devicetree@...r.kernel.org>, <tony@...mide.com>,
	<linux-kernel@...r.kernel.org>, <rob.herring@...xeda.com>,
	<sudhakar.raj@...com>, <rob@...dley.net>,
	<grant.likely@...aro.org>, <akpm@...ux-foundation.org>,
	<linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/2] rtc: omap: update of_device_id to reflect latest
 ip revisions


On 8/16/2013 7:45 PM, Benoit Cousson wrote:
> Hi Gururaja,
> 
> On 16/08/2013 13:36, Hebbar, Gururaja wrote:
>> The syntax of compatible property in DT is to mention the Most specific
>> match to most generic match.
>>
>> Since AM335x is the platform with latest IP revision, add it 1st in
>> the device id table.
> 
> I don't understand why? The order should not matter at all.

Yes, it should not. We are trying to work around a bug in the kernel
where the compatible order is not honored (instead the order in
of_match[] array matters). There were patches being discussed to fix this.

> 
> I've tried to follow the thread you had with Mark on the v2, but AFAIK,
> you've never answered to his latest question.
> 
> Moreover, checking the differences between the Davinci and the am3352
> RTC IP, I would not claim that both are compatible.
> 
> Sure you can use the am3352 with the Davinci driver, but you will lose
> the wakeup functionality without even being notify about that.

When the kernel is fixed for the bug pointed out above, this should not
happen with properly defined compatible property.

> 
> For my point of view, compatible mean that the HW will still be fully
> functional with both versions of the driver, which is not the case here.

I do not think that's the interpretation of compatible. Its goes from
most specific to most generic per the ePAPR spec. That in itself says
that 100% functionality is not expected if you don't find a match for
the more specific property.

> 
> am3352 DTS must use the ti,am3352-rtc to have the expected behavior.

Yes, that's what patch 2/2 does.

> Using the ti,da830-rtc version will not make the board working as
> expected. So we cannot claim the compatibility.

Ideally, the DT file was *always* written as

	compatible = "ti,am3352-rtc", "ti,da830-rtc";

even when there was no kernel support for AM3352 RTC.

That way, there is no need to update the .dts[i] file. As kernel gains
functionality, more features (like rtc wake) is available to users.
Otherwise they get plain RTC functionality - but at least they get
something instead of no RTC.

Thanks,
Sekhar
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