Boolean properties

[Mitch Bradley]

On 12/7/2011 7:12 AM, David Gibson wrote:
> On Mon, Dec 05, 2011 at 04:29:22PM -0800, Simon Glass wrote:
>> Hi,
>>
>> As I understand it, boolean properties in the device tree are defined
>> as follows:
>>
>> - if the property is present, then it is true
>> - if the property is absent, then it is false
>
> That's the common convention.  Although an individual binding could
> define something else, and I think some already do define a
> boolean~=int approach instead (e.g. the ibm,dfp cpu property).  In
> some cases that's anticipating future expansion to values other than 0
> and 1.
>
>> The fdtget and fdtput utilities deal with reading and changing these.
>> At present fdtget returns the value of the property and is not
>> suitable for boolean properties. fdtput allows setting of a property
>> value but not removing it.
>>
>> To deal with boolean properties these utilities need to change. One
>> option is to use something like:
>>
>> $ fdtget -t f /path/to/node property
>> (prints 'true' or 'false')
>> $ fdtput -t f /path/to/node property false/true
>> $ fdtput -t f /path/to/node property 0/1
>> (deletes the property for false and 0; adds it with no value for true and 1)
>>
>> (Unfortunately the 'b' type is taken for byte at present, so it is
>> unclear to be so far what letter we should use for 'f' in the -t flag)
>
> Hrm.  Rather than treating this as a boolean "type", I think it would
> be less ambiguous to instead add options to explicitly delete a
> property (either e.g. fdtput -d or a new fdtdel), and to create an
> empty valued property.
>
>> But first I would like to suggest that we support boolean properties
>> with a value, i.e. change to:
>>
>> - if the property is absent, then it is false
>> - if the property is present, then it is true
>>      - unless the property has a 0 value, iwc it is false
>
> So the first question here is who/what exactly constitutes "we" here -
> kernel, u-boot, dtc?  Using the above logic sounds like a good idea
> for fdt readers on the "be liberal in what you accept" principle.  But
> it's something I'd like to see Mitch Bradley and/or Segher comment on
> - they'd have a better idea of whether there's existing practice that
> might break with that.

Since the producer and consumer of a named property must agree on the 
interpretation of its value, you can do whatever you want with a 
newly-named property without breaking anything else - although you might 
not be able to use existing encoding/decoding methods if you make up 
something new.

The use case for absence/presence boolean properties was as follows:

You develop a device and describe it via a set of properties.  Some time 
later, someone develops a newer version of that device that has a new 
feature that was not contemplated in the original design.  You need to 
declare that the feature exists, but the device node for the older 
version has no property relating to that hitherto-unknown feature.  So 
you invent a new property, with the understanding that the absence of 
that property indicates that the feature is not present.

That scheme does not preclude a "name"=0/1 formulation, with the 
understanding that absence implies 0, but it's unclear to me that the 
0/1 value adds much.  Either way, for your system to work, you have to 
get the right information in the device node.

The =0/1 formulation could in principle be used for 3 states, with 
absence meaning either "don't know" or "error".  To my way of thinking, 
that just makes things more complicated, putting the burden on the 
programmer to somehow handle a vague condition.  I prefer "get it right 
and be done with it".



>
>> There are three reasons:
>> 1. it is very confusing for the poor user to set a property value to 0
>> and have it come back as 'true' from a function that checks these
>> sorts of things. This is particularly true since fdt properties do not
>> have types, or even type hints.  The function I have in mind is
>> something like this (modified to fit the discussion):
>>
>> /**
>>   * Look up a boolean property in a node and return it.
>>   *
>>   * A boolean properly is true if present in the device tree and false if not
>>   * present (regardless of any property data/value).
>>   *
>>   * @param blob	FDT blob
>>   * @param node	node to examine
>>   * @param prop_name	name of property to find
>>   * @return 1 if the properly is present; 0 if it isn't present
>>   */
>> int fdtdec_get_bool(const void *blob, int node, const char *prop_name);
>>
>> An alternative might be to print an error (and refuse to return a
>> value) when a boolean property has a value:
>>
>>   * @return 1 if the properly is present; 0 if it isn't present, -1 if
>> it has any data (and is therefore illegal as a boolean property)
>
> Kind of a pain for C.
>
>> 2. it allows us to adjust the boolean value without requiring a change
>> in the size of the device tree blob (faster, easier)
>>
>> 3. Related to 2 it allows us to put template values in the fdt which
>> can then be *optionally* updated in the build system or in U-Boot to
>> the required value before passing to the kernel.
>>
>> I presume this discussion has happened before - can anyone please
>> point me to the thread?
>>
>> Regards,
>> Simon
>>
>