[Ksummit-2013-discuss] DT bindings as ABI [was: Do we have people interested in device tree janitoring / cleanup?]

[David Gibson]

On Mon, Jul 29, 2013 at 10:15:12PM -0400, jonsmirl at gmail.com wrote:
> On Mon, Jul 29, 2013 at 9:44 PM, David Gibson
> <david at gibson.dropbear.id.au> wrote:
> > On Sat, Jul 27, 2013 at 10:11:16PM -0700, James Bottomley wrote:
> >> On Sat, 2013-07-27 at 21:28 -0600, Grant Likely wrote:
> >> > On Sat, Jul 27, 2013 at 2:25 PM, Grant Likely <grant.likely at secretlab.ca> wrote:
> >> > > On Sat, Jul 27, 2013 at 2:01 PM, jonsmirl at gmail.com <jonsmirl at gmail.com> wrote:
> >> > >> On Sat, Jul 27, 2013 at 3:45 PM, Grant Likely <grant.likely at secretlab.ca> wrote:
> >> > >>> On Sat, Jul 27, 2013 at 4:59 AM, Arend van Spriel <arend at broadcom.com> wrote:
> >> > >>>> Let's see how many people go and scream if I say this: Too bad .dts files
> >> > >>>> are not done using XML format as DT bindings could be described using XML
> >> > >>>> Schema.
> >> > >>>
> >> > >>> Draft an example and show us how it would look!  :-)  There is
> >> > >>> absolutely nothing preventing us from expressing a DT in XML format,
> >> > >>> or even using XSLT to define DT schema while still using our current
> >> > >>> .dts syntax. It would be trivial to do lossless translation between
> >> > >>> .dts syntax and xml.
> >> > >>>
> >> > >>> The problem that I have with XML and XSLT is that it is very verbose
> >> > >>> and not entirely friendly to mere-mortals. However, I'm more than
> >> > >>> willing to be proved wrong on this point.
> >> > >>
> >> > >> I considered this approach a while ago and discarded it. It would work
> >> > >> but it is just too much of a Frankenstein monster.
> >> > >>
> >> > >> Much cleaner to modify dtc to take a schema as part of the compilation
> >> > >> process. The schema language itself has no requirement to look like
> >> > >> DTS syntax. Whoever wrote dtc probably has a favorite language that
> >> > >> would be good for writing schemas in.
> >> > >
> >> > > Making it part of dtc is a required feature as far as I'm concerned.
> >> > > Using XML/XSLT and dtc-integration are not mutually exclusive, but I
> >> > > digress.
> >> >
> >> > Oops, ignore the XSLT bit. XSLT isn't schema and has no bearing on the
> >> > discussion of schema. Sorry for the noise.
> >>
> >> XSLT is a transform language ... you'd use it say to transform xml to
> >> dtc, so it would be an integral component of an xml/xslt based schema.
> >>
> >> If you want actually to describe and have validated the xml schema
> >> itself, then you'd use xsd (XML schema description language) and its
> >> associated tools.
> >>
> >> I'm not saying you *should* do this, just that it's possible (plus I've
> >> just blown my kernel cred by knowing about xml, sigh).
> >
> > Heh.  So, it was said in jest, but that actually raises an important
> > point.
> >
> > There are basically two criteria to keep in mind for our
> > representation of schemas:
> >    1) Adequate expressiveness to validate a sufficiently large part,
> > of a sufficiently large number of bindings to be useful.
> >    2) Ease of use and ease of learning **for the target audience**.
> >
> > To the best of my knowledge xsd would do well on (1), but I'm not
> > convinced it does very well on (2).  In an environment where XML was
> > already widely used, XSD would make perfect sense.  Here, I think it
> > would be pretty ugly to wire onto the existing DT tools and
> > infrastructure, and unpleasantly unfamiliar for many kernel and board
> > developers trying to work with DT schemas.
> >
> >
> > So, by way of investigation, let me propose an alternative expression
> > of schemas, that I'm also not convinced we should do, but is possible
> > and expressive.  It's illustrative, because it's kind of the polar
> > opposite approach to XSD: just use C.
> >
> > dtc already has a (so far limited) "checks" mechanism which verifies
> > various aspects of DT content.  These are implemented by C functions
> > in checks.c.  There's obviously ample expressiveness - you can express
> > any constraint you want that way.  It can be pretty verbose, and
> > fiddly.  A good library of helper functions can mitigate that, but
> > it's not clear how much.  On the other hand, a very good fraction of
> > people working with this will already be familiar with C, which is a
> > big plus.  This is, after all, the reason that the dts syntax is
> > chiefly C inspired.
> >
> > Now, in practice, I think we will want a more convenient schema
> > language (just as we wanted dts, rather than manually constructing
> > FDTs as C structures).  But I absolutely do think, that the schema
> > handling should be handled as plugins to the checks mechanism -
> > basically we'd have a validate_schemas() check function.
> >
> > I also think we should consider the option of having a simple and
> > straightforward schema language which handles, say, 80% of cases with
> > a fall back to C for the 20% of curly cases.  That might actually be
> > simpler to work with in practice than a schema language which can
> > express absolutely anything, at the cost of being awkward for simple
> > cases or difficult to get your head around.
> 
> Would C++ work? You can use operating overloading and templates to
> change the syntax into something that doesn't even resemble C any
> more.

Well, in theory.  But given that dtc and the kernel are both in plain
C, I don't think it's a good idea.

-- 
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
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