Date:	Wed, 1 Aug 2012 10:47:39 +0900
From:	Alex Courbot <acourbot@...dia.com>
To:	Mitch Bradley <wmb@...mworks.com>
CC:	Thierry Reding <thierry.reding@...onic-design.de>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	Stephen Warren <swarren@...dia.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <rob.herring@...xeda.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>
Subject: Re: [RFC][PATCH v3 1/3] runtime interpreted power sequences

On 07/31/2012 09:55 PM, Mitch Bradley wrote:
> On 7/31/2012 8:38 PM, Thierry Reding wrote:
>> On Tue, Jul 31, 2012 at 08:22:17PM +0800, Mitch Bradley wrote:
>>> On 7/31/2012 6:56 PM, Thierry Reding wrote:
>>>> On Tue, Jul 31, 2012 at 07:32:20PM +0900, Alex Courbot wrote:
>>>>> On 07/31/2012 07:45 AM, Stephen Warren wrote:
>>>>>> I wonder if using the same structure/array as input and output would
>>>>>> simplify the API; the platform data would fill in the fields mentioned
>>>>>> above, and power_seq_build() would parse those, then set other fields in
>>>>>> the same structs to the looked-up handle values?
>>>>>
>>>>> The thing is that I am not sure what happens to the platform data
>>>>> once probe() is done. Isn't it customary to mark it with __devinit
>>>>> and have it freed after probing is successful?
>>>>
>>>> No, platform data should stay around forever. Otherwise, consider what
>>>> would happen if your driver is built as a module and you unload and load
>>>> it again.
>>>>
>>>>> More generally, I think it is a good practice to have data
>>>>> structures tailored right for what they need to do - code with
>>>>> members that are meaningful only at given points of an instance's
>>>>> life tends to be more confusing.
>>>>
>>>> I agree. Furthermore the driver unload/reload would be another reason
>>>> not to reuse platform data as the output of the build() function.
>>>>
>>>> But maybe what Stephen meant was more like filling a structure with data
>>>> taken from the platform data and pass that to a resolve() function which
>>>> would fill in the missing pieces like pointers to actual resources. I
>>>> imagine a managed interface would become a little trickier to do using
>>>> such an approach.
>>>>
>>>>>> If the nodes have a unit address (i.e. end in "@n"), which they will
>>>>>> have to if all named "step" and there's more than one of them, then they
>>>>>> will need a matching reg property. Equally, the parent node will need
>>>>>> #address-cells and #size-cells too. So, the last couple lines would be:
>>>>>>
>>>>>> 		power-on-sequence {
>>>>>> 			#address-cells = <1>;
>>>>>> 			#size-cells = <0>;
>>>>>> 			step@0 {
>>>>>> 				reg = <0>;
>>>>>
>>>>> That's precisely what I would like to avoid - I don't need the steps
>>>>> to be numbered and I certainly have no use for a reg property. Isn't
>>>>> there a way to make it simpler?
>>>>
>>>> It's not technically valid to not have the reg property. Or
>>>> #address-cells and #size-cells properties for that matter.
>>>
>>> I'm not keen on this representation where individual steps are nodes.
>>> That seems like it could end up being too "heavyweight" for a long sequence.
>>
>> The other alternative would involve using a single property to encode
>> one sequence. I think that was the initial proposal, though using proper
>> phandle encoding it could probably be enhanced a bit. However anything
>> that involves a single property has the problem that we need to encode
>> the type of resource as an integer, and that makes things very hard to
>> read.
>>
>> So it would look something like this:
>>
>> 	power-on = <1  &gpio 6 0            1
>> 		    0                   10000
>> 		    2  &reg                 1
>> 		    3  &pwm  0 5000000      1>;
>>
>> 	power-off = <3  &pwm  0 5000000      0
>> 		     2  &reg                 0
>> 		     0                   10000
>> 		     1  &gpio 6 0            0>;
>>
>> So the first cell would encode the type:
>>    0: delay
>>    1: gpio
>>    2: regulator
>>    3: PWM
>>
>> The next n cells would be the phandle and the specifier, while the last
>> cell would encode a resource-specific parameter:
>>    delay: time in microseconds
>>    gpio: set level (0: low, 1: high)
>>    regulator: 0: disable, 1: enable
>>    pwm: 0: disable, 1: enable
>>
>> I guess this would be more compact, but it is also very hard to read. Is
>> that something you would be happier with? Perhaps you were thinking of
>> something completely different?
>
>
> Perhaps a compact/flexible encoding could be designed, with a textual
> encoding that is easy to read.  A separate tool could convert the text
> encoding to the integer format, annotated with comments containing
> the "source text".  A file containing that output could be #included
> into the dts file.

Do you mean having a external compiler that would run before dtc just 
for producing the power sequences? That sounds a little bit overkill for 
something that ough to remain simple.

Also, although I admit I don't have the whole picture of where they 
could be used, I don't expect the power sequences to grow to sizes that 
would make us bother about their footprint.

Alex.

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