Re: [RFC 4/5] RTC: rtc-at91sam9: add device-tree support

[Nicolas Ferre <nicolas.ferre-AIFe0yeh4nAAvxtiuMwx3w@public.gmane.org>]

On 04/08/2013 11:00 AM, Johan Hovold :
> On Mon, Apr 08, 2013 at 09:38:07AM +0200, Jean-Christophe PLAGNIOL-VILLARD wrote:
>> On 17:12 Sun 07 Apr     , Johan Hovold wrote:
>>> Add device-tree support.
>>>
>>> The AT91 RTT can be used as an RTC if the atmel,at91-rtt-as-rtc-gpbr
>>> property is present and set to the general-purpose backup register to
>>> use to store the RTC time base.
>>>
>>> Signed-off-by: Johan Hovold <jhovold-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>>> ---
>>>  .../devicetree/bindings/rtc/rtc-at91sam9.txt       | 19 ++++++++++++
>>>  drivers/rtc/rtc-at91sam9.c                         | 36 +++++++++++++++++++++-
>>>  2 files changed, 54 insertions(+), 1 deletion(-)
>>>  create mode 100644 Documentation/devicetree/bindings/rtc/rtc-at91sam9.txt
>>>
>>> diff --git a/Documentation/devicetree/bindings/rtc/rtc-at91sam9.txt b/Documentation/devicetree/bindings/rtc/rtc-at91sam9.txt
>>> new file mode 100644
>>> index 0000000..0f54988
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/rtc/rtc-at91sam9.txt
>>> @@ -0,0 +1,19 @@
>>> +Atmel AT91 RTT as RTC
>>> +=====================
>>> +
>>> +Required properties:
>>> +- compatible: Should be "atmel,at91sam9260-rtt"
>>> +- reg: Should contain register location and length
>>> +- interrupts: Should contain interrupt for the RTT which is the IRQ line
>>> +  shared across all System Controller members.
>>> +- atmel,rtt-as-rtc-gpbr: Should contain the backup-register to use to store
>>> +  the RTC time base
>>> +
>>> +Example:
>>> +
>>> +rtt@fffffd20 {
>>> +		compatible = "atmel,at91sam9g45-rtt", "atmel,at91sam9260-rtt";

No, there is no visible difference between the sam9g45 RTT and the
sam9260 one. So the most precise compatibility string is still sam9260.
If one day we feel the need for a advanced feature that exists on a more
recent SoC, we have the possibility to add it at that time...

>>> +		reg = <0xfffffd20 0x10>;
>>> +		interrupts = <1 4 7>;
>>> +		atmel,at91-rtt-as-rtc-gpbr = <0>;
>> no you miss the point of the DT
>>
>> you need to describe the hardware no a particular use of it
> 
> That was what I was trying to achieve by adding the two use-neutral
> rtt and gpbr-nodes. But then the question is how would you influence
> which out of two rtt-drivers to use?
> 
> Adding a property as above in the final board descriptions seemed
> preferable to adding rtt-as-rtc to the compatible string of the rtt as
> that would mean describing use rather than just hardware.

Well, re-reading the Device_Tree_Usage page, I found this sentence:
"
Understanding the compatible Property

Every node in the tree that represents a device is required to have the
compatible property. compatible is the key an operating system uses to
decide which device driver to bind to a device.
"
or ePARP:

"
The compatible property value consists of one or more strings that
define the specific programming model for the device.
"

We have the notion of link between hardware and software in this
*compatible* sting, even if the *node* itself is about hardware description.

>> the RTT is a general purpose timer backuped that we use in linux as a
>> RTC with a gpbr to store the time
>>
>> you need 2 binding on for the RTT one the RTT-rtc
> 
> As in adding some virtual hardware-node which uses the rtt and gpbr as
> resources?

So, why not simply having a compatibility string that collects the uses
of this RTT node:

compatible = "atmel,at91sam9260-rtt-as-rtc", "atmel,at91sam9260-rtt";

And then "decide which device driver to bind to [the RTT] device"...
If the rtt-as-rtc driver is not selected, the device can still be used
as a simple "rtt". The board .dts can overload a compatibility string
according to the use, etc.

Then the way do describe which GPBR to use has still to be discussed.
But for the RTT itself, I would keep it simple like that.

Best regards,
-- 
Nicolas Ferre