Some slightly random musings on device tree expression syntax

[Stephen Warren]

I was thinking some more about how to expand the device tree syntax to
allow expressions. I wondered if we should use a concept/syntax more
inspired by template processors. Playing with jinja2 and gpp led me
towards (...) being an inline expression syntax that can calculate
integers or strings and get replaced by the string representation of the
expression, and ! at the start of a line introducing a statement
context. So, below are my somewhat wandering thoughts on the matter.
However, the idea still raises a lot of questions that'd need to be
resolved.

I note a few things:

* Using the (...) syntax to indicate which parts of the file should be
evaluated and the substituted solves the issue that David had with Jon's
proposal re: how do you know when a node name is literal text vs.
concatenated to some expression.

* Separating the device tree syntax and pre-processor/... phase allows
them to be decoupled and the pre-processor potentially optional, or even
replaced if things don't work out, or different people could use their
own thing.

* As an aside, I wonder if we couldn't transparently allow <1 2 3> or
<1, 2, 3> for cell list syntax, thus not requiring the brackets in
previously proposed <(1 + 0) (1 + 1) (4 - 1)> syntax, but rather <1 + 0,
1 + 1, 4 - 1>?

Concept
========================================

The .dts syntax that dtc reads is unchanged.

A pre-processing phase occurs on .dts files that handles all aspects of
expressions; all definitions, macro processing, expression process, etc.
are evaluated and fully expanded to strings during the pre-processing
phase. The result of the pre-processing phase should be a source file or
stream that can be handled by the existing dtc.

Whether this pre-processing phase is implemented as:
* A separate executable, manually invoked by the user.
* A separate executable, automatically invoked by dtc itself.
* Something built into dtc itself.
... is not addressed by this proposal.

One potential issue here: if the pre-processing and regular compilation
phases are completely separate, do we need to pay attention that the
int, literal, byte-sequence literal syntax stays the same between the
two phases to reduce confusion, or not?

Pre-processing
========================================

Contexts:

  Pass-through:

    By default, the pass-through context is active.

    Data is passed from input to output without modification, except
    that data is searched for markers that begin other contexts.

  Expression:

    Introduced by: (
    Terminated by: a matching )

    The text within this context is interpreted as an expression. That
    expression is evaluated, the result formatting as a string, and that
    string written to the output stream in place of the ( ) markers and
    the expression between them.

    Expression context can being anywhere within the source stream; no
    note is taken of the tokens that the device tree language

  Statement:

    Introduced by: !
      Notes: Or some other suitable character; # conflicts with property
      names unless we require it to be in the first column, and also
      sounds too much much like regular cpp, so people might get
      confused. @ might work. This is probably bike-shedding at this
      point...
    Terminated by: End of line

Example:

Note: // comments are used below as comments in this document, not
necessarily comments in the actual proposed syntax.

// Simple constant definitions
// Syntax of RHS matches existing .dts syntax

!defint usbbase 0x6000000
!defint usbsize 0x100
!defint usbstride 0x1000
!defstr usb "usb"
!defbytes somebytes [de ad be ef]

// or perhaps implicitly set variable type based on type of the RHS?
!define usbbase 0x6000000
!define usb "usb"

or !assign or !let ...

// RHS may also use expression syntax
// and references to previously defined variables

!defint usb3base usbbase + (2 * usbstride)
!defstr catenated usb + "2"

// Simple use of some variables:

(usbbase) (usbsize) (catenated)

// which yields:
// 0x6000000 0x100 usb2

// A more complex example:

(usb)3@(usb3base) {
    reg = <(usb3base) (usbsize)>;
    name = "(usb)3";
};

// which yields:
// usb3 at 0x60002000 {
//     reg = <0x60002000 0x100>;
//     name = "usb3";
// };

// Question: Do ints always format as 0x%x since that's the most common,
// or do we need explicit control over the base etc.?
//
// Question: How do we know when to format strings with "" around them,
// e.g. for use as property values, and when not to, e.g. for use in
// arbitrary contexts? For example above, it'd be nice if when defining
// the name property, we could write 'name = usb3name;' and have it
// expand to 'name = "usb3";' given a str variable with value "usb3",
// yet we don't want the quotes when using variable usb in the node
// name in the example earlier.
//
// Question: What if we actually wanted the property value "(usb3)". How
// do we stop the expansion; how to escape?
//
// I suppose the solution for the latter 2 questions is that the
// expansion has to actually be sensitive to context in the underlying
// language, and include "" in property value context, but not
// elsewhere. But, what if you write:

!defstr nasty "usb at 0x6000000 { name =";
(nasty) (foo);

// Additional statements could include if, for, while, ...:

!ifdef somevar
foo bar
!else
baz qux
!endif

// I think we don't need e.g.:

foo !ifdef somevar! bar !else! baz !endif! qux

// ... since I think that we can line-break in the middle of any
// property or node definition, so we could just do this instead:

foo
!ifdef somevar
bar
!else
baz
!endif
qux

// If we need to actually concatenate the strings into one, we can do
// that as an expression somehow, assign the result to a variable, and
// expand just that.

!defstr xxx "foo"
!ifdef somevar
!defstr xxx xxx + "bar"
!else
!defstr xxx xxx + "baz"
!endif
!defstr xxx xxx + "qux"
(xxx)

// Perhaps we can delimit large blocks of statements in a way that
// doesn't need a lot of !s:

!!
xxx = "foo"
if somevar:
    xxx += "bar"
else:
    xxx += "baz"
xxx += "qux"
!!
(xxx)

// Then, we can start allowing complex things like macro or function
// definitions within the !! block; a full regular language, and
// perhaps we could even borrow an existing one here.

// About functions: Perhaps cpp-style macros:

!define func(a, b, c) a + b + c

// where the RHS is an expression that can use variables in the
// parameter list
//
// Or, is the RHS/body raw text, so something more like:

!macro func(a, b, c)
   foo {
      prop = (a + b + c);
   }
!end

// Perhaps we need both; one with text RHS accepting escapes into
// expressions, one with an expression on the RHS.

// I wondered if !define's RHS should always be an expression, or
// instead always be raw text with the same (...) escape to expressions
// as in regular text:

// (assuming a, b, c are extant variables)

// all variables are strings?
!define foo (a) + (b) + (c)

or:

// yields an integer variable
!defint foo a + b + c