Re: Boolean properties

Hi David,

On Thu, Dec 8, 2011 at 5:15 PM, David Gibson
[off-list ref] wrote:

On Tue, Dec 06, 2011 at 04:01:20PM -0800, Simon Glass wrote:

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Hi David,

On Tue, Dec 6, 2011 at 3:12 PM, David Gibson
[off-list ref] wrote:

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On Mon, Dec 05, 2011 at 04:29:22PM -0800, Simon Glass wrote:

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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.

OK I didn't know that.

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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.

That was my first thought, but I thought it would be painful since
callers would need to do something like:

if [ $value -eq 0 ]
   fdtput -d /path/to/node property
else
   fdtput /path/to/node property <some tag that means nothing>
    (or maybe    fdtput -z /path/to/node property)
fi

so better IMO to provide boolean support in the tool. But it does
illustrate that presence/absence is not always ideal.

Hrm.  I think that's a lesser evil than having a data "type" that can
actually completely delete the property.

Well isn't that the nature of the boolean property. In a way that is
exactly my point, that to change a boolean property to zero you have
to delete it. This is not something that fdtput would mandate, merely
make easier for the user. I agree it is odd, and really that is my
point.

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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.

I mean device tree users (kernel, u-boot) rather than dtc (which I
suspect would be unaffected).

Well, like I say, I have no in principle objection to using this as
the usual boolean test, but actually changing it is obviously
something you'd need to convince the maintainers of the relevant
projects.

OK will see about that...

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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.

I don't like it either. But we currently have a situation where a zero
value is considered true, and this can be confusing. I would prefer to
expand the meaning.

Actually, on further thought, I don't mind the above so much, because
even if you use it without checking for the error value, you'll at
least get the right behaviour for the unambiguous cases.

Yes that's right. Of course you will get the 'right' behaviour when
the value is 0 (i.e. the boolean will be considered true).

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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?

--
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                               | _way_ _around_!
http://www.ozlabs.org/~dgibson

Regards,
Simon