Date:	Wed, 01 Aug 2012 10:15:07 +0800
From:	Mitch Bradley <wmb@...mworks.com>
To:	Alex Courbot <acourbot@...dia.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 8/1/2012 9:47 AM, Alex Courbot wrote:
> 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.

It is axiomatic that every "language", if it succeeds at all, eventually
grows into a complete programming language.  The more special-purpose
that it starts as, the uglier that it ends up.

> 
> Alex.
> 
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