linux-kernel - Re: [RFC][PATCH v3 1/3] runtime interpreted power sequences

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Date:	Tue, 31 Jul 2012 20:55:37 +0800
From:	Mitch Bradley <wmb@...mworks.com>
To:	Thierry Reding <thierry.reding@...onic-design.de>
CC:	Alex Courbot <acourbot@...dia.com>,
	"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 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.

> 
> Thierry
> 
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