Subject: Re: dtc: Add python source code output

From: Milton Miller <[email-masked]>
Date: Mon, 10 Nov 2008 10:11:53 -0600

On 2008-11-07 at 02:31:40, David Gibson wrote:
> On Thu, Nov 06, 2008 at 06:55:44PM +1100, Michael Ellerman wrote:
>> This commit adds an output format, which produces python
>> code. When run, the python produces a data structure that
>> can then be inspected in order to do various things.
...
>> I'm not sure if this is generally useful (or sane) but it was for me 
>> so
>> I thought I'd post it.
>
> Hrm, well the idea of langauge source output seems reasonable.  But
> the actual data structure emitted, and the method of construction in
> Python both seem a bit odd to me.
>
>> I have a dts that I want to use to configure a simulator, and this
>> seemed like the nicest way to get there. dtc spits out the pythonised
>> device tree, and then I have a 10 line python script that does the
>> configuring.

[snip]
> These branches also result in the value having different Python types
> depending on the context.  That's not necessarily a bad thing, but
> since which Python type is chosen depends on a heuristic only, it
> certainly needs some care.  You certainly need to be certain that you
> can always deduce the exact, byte-for-byte correct version of the
> property value from whatever you put into the Python data structure.
>>  +
>>  +out:
>>  +     fprintf(f, "    n.properties.append(p)\n");
>
> So, emitting Python procedural code to build up the data structure,
> rather than a great big Python literal that the Python parser will
> just turn into the right thing seems a bit of a roundabout way of
> doing this.

I would think so too.   I haven't looked at the output, only at Davids 
comments.  If the data structure is ambiguous, then I do think more 
thought is needed.

Have you considered just parsing the flat tree binary?   Either 
creating a python binding to libfdt or even just parsing the dtb 
directly?

I have written perl code to parse a dtb and query it for nodes and 
properties, it wasn't too bad.  I need to look at a bug report by 
another user and comment it, then I should seek the okays post it.  It 
is currently read-only and iterative callback based (like the kernels 
early-scan-flat-tree stuff), but I have planned creating a tree for 
querying, editing, and re-flattening.  Perl strings are counted length 
binary blobs, so property contents are interpreted with pack and 
unpack.  The library has been used to search a dtb to build a list of 
cpu instances and memory blocks, and it has been used to query the 
properties of a known node in the tree.

milton