Re: [PATCH v3 1/2] rtc: omap: update of_device_id to reflect latest ip revisions

[Benoit Cousson <bcousson@baylibre.com>]

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