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

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Date:	Fri, 16 Aug 2013 20:12:46 +0200
From:	Benoit Cousson <bcousson@...libre.com>
To:	Mark Rutland <mark.rutland@....com>
CC:	"Hebbar, Gururaja" <gururaja.hebbar@...com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"khilman@...aro.org" <khilman@...aro.org>,
	"tony@...mide.com" <tony@...mide.com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"a.zummo@...ertech.it" <a.zummo@...ertech.it>,
	"rob@...dley.net" <rob@...dley.net>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"davinci-linux-open-source@...ux.davincidsp.com" 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	"sudhakar.raj@...com" <sudhakar.raj@...com>
Subject: Re: [PATCH v3 1/2] rtc: omap: update of_device_id to reflect latest
 ip revisions

Hi Mark,

On 16/08/2013 19:20, Mark Rutland wrote:
> Hi Benoit,
>
> On Fri, Aug 16, 2013 at 03:15:57PM +0100, 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.
>>
>> 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.
>
> Could you describe the wakeup functionality, and how it differs between
> the am3352-rtc and the da830-rtc?

AFAIK, da830-rtc does not have that functionality at all. This is 
something that was added to the am3352-rtc.

> As I understand it, the am3352 functionality is a superset of the da830
> functionality. You can use the old driver, and get some functionality,
> or use the new driver and get it all.

Mmm, what your are saying now seems to make sense to me as well. So I'm 
even more confused :-)

> That means that am3352-rtc is compatible with da830. As long as the
> kernel first checks for am3352-rtc, there will be *no* loss of
> functionality. All this does is enable older kernels to use the hardware
> in some fashion, and given the older kernel didn't have support for the
> am3352-rtc features, this is a *gain* in functionality, not a loss.
>
>>
>> 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.
>
> What? A driver for any entry in the compatible list should be able to
> drive the hardware to *some* level of functionality. We list from
> most-specific to most-general to allow a graceful degradation from fully
> supported to bare minimum functionality.

OK, but where is it written in the DT spec that this is what the 
compatible is supposed to mean?

I'm quoting it again:
"
The compatible property value consists of one or more strings that 
define the specific programming model for the device. This list of 
strings should be used by a client program for device driver selection. 
The property value consists of a concatenated list of null terminated
strings, from most specific to most general. They allow a device to 
express its compatibility with a family of similar devices, potentially 
allowing a single device driver to match against several devices.
"

The graceful degradation or the loss of functionality is not something 
that I really understand in that text.

Anyway, I'm probably too tired... I'll go back home, and think about 
that after the week-end.

Regards,
Benoit

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