Some slightly random musings on device tree expression syntax

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.

* 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>?

Quoted 2 lines

I was thinking some more about how to expand the device tree syntax to allow expressions.

Excellent!

Quoted 9 lines

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.

Quoted 6 lines

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.

So the M4 solution then.

Quoted 4 lines

* 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>?

That's the sort of direction I advocated earlier.

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@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@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

Quoted 3 lines · 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.

Quoted 6 lines

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.

Quoted 4 lines

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?

I'm not sure quite what you're getting at here.

Hrm. I'm pretty dubious about doing the expression evaluation (as opposed to macro/constant expansion) within the preprocessor, then resubstituting as a string.

It would work ok for integer expressions, but for bytestring expressions, it seems likely we'd have to duplicate the lexical/grammar constructs for [...], <...> and basic literals between preproc and dtc, which seems a bit horrible.

In addition this approach means that an expression can never express a value which a literal couldn't. No problem in most cases, but one thing I had in mind is that an expression syntax could be used to specify a node or property name with illegal characters in it (mostly relevant for ensuring that doing -I dtb -O dts then -I dts -O will always end up exactly where you started, even when the original dtb is corrupted or otherwise contains things it shouldn't.

Quoted 4 lines

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

Quoted 3 lines

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

Hrm. If using defines is based on textual substitution, then type should be irrelevant. If they're not based on textual substitution, then the "preprocessor" is doing something rather more involved than something with that name normally would.

Quoted 6 lines

// A more complex example:

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

Oh. You *intended* for expression substitution within strings. Nack, nack nackity nack. That violates least surprise seven ways to sunday. If the user wants something like this they can do:

	name = (usb + "3");

Ugh. Well, I think you've pretty much proved the case that attempting to put all the expression evaluation into the preprocessor is a really bad idea. It requires the preproc to be at least somewhat type aware which (a) is likely to lead to grammar duplication and (b) is absolutely not what someone familiar with cpp will expect.

Quoted 4 lines · David Gibson

Quoted 3 lines · 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.

...

Quoted 11 lines

Quoted 10 lines

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?

Quoted 1 line

I'm not sure quite what you're getting at here.

Well, it's the point you make right below. Namely that if expression evaluation happens during pre-processing (either only there, or both there and during the separate final "compilation" phase), that the pre-processor must be able to parse and manipulate literals of all types, so the expressions it calculates can use values of those types.

...

Quoted 8 lines

Hrm. I'm pretty dubious about doing the expression evaluation (as opposed to macro/constant expansion) within the preprocessor, then resubstituting as a string.

It would work ok for integer expressions, but for bytestring expressions, it seems likely we'd have to duplicate the lexical/grammar constructs for [...], <...> and basic literals between preproc and dtc, which seems a bit horrible.

Don't we have to allow the pre-processor to parse and manipulate constants of all types (both scalars and perhaps even complete nodes)? If we don't, then how would you do something like:

var = [00 11 aa 55]
for byte in var:
do_something_with(byte)

or:

var = "Some long string"
for word in var.split():
do_something_with(word)

Quoted 7 lines

In addition this approach means that an expression can never express a value which a literal couldn't. No problem in most cases, but one thing I had in mind is that an expression syntax could be used to specify a node or property name with illegal characters in it (mostly relevant for ensuring that doing -I dtb -O dts then -I dts -O will always end up exactly where you started, even when the original dtb is corrupted or otherwise contains things it shouldn't.

Well, one might imagine:

s = "Some text" + chr(128)

That's an expression that expresses something that I think can't currently be a literal string.

...

Quoted 4 lines

Quoted 4 lines

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

Quoted 8 lines

Quoted 3 lines

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

Hrm. If using defines is based on textual substitution, then type should be irrelevant. If they're not based on textual substitution, then the "preprocessor" is doing something rather more involved than something with that name normally would.

True. I was more leaning to describing this as a template processor than a pre-processor. Related, my thoughts started out simpler, but became more complex and raised a lot of open questions when thinking through some of the details, so became a lot less clear!

Quoted 11 lines

Quoted 6 lines

// A more complex example:

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

Oh. You *intended* for expression substitution within strings. Nack, nack nackity nack. That violates least surprise seven ways to sunday. If the user wants something like this they can do:

	name = (usb + "3");

That works for the name property, but what about the node's name:

    (usb)3@(usb3base) {

Even if we required that the whole thing be calculated elsewhere and placed into a variable, how do we know whether:

    foo {

is meant to expand variable foo or be literal "foo"? That seemed to be one of your main objections to Jon's implementation. I proposed solving that by explicitly marking the source to indicate where expansion was desired:

  (foo) {

or not:

  foo {

So, () act as "start and end of expression".

Given that, why not allow complete expressions with () rather than just a single variable or macro call?

This is pretty much the core point of why I was referring to a templating engine rather than a pre-processor. Of course, templating engines often use e.g. <%= %> instead of ( ) or a wide variety of other syntaxes.

...

Quoted 5 lines

Ugh. Well, I think you've pretty much proved the case that attempting to put all the expression evaluation into the preprocessor is a really bad idea. It requires the preproc to be at least somewhat type aware which (a) is likely to lead to grammar duplication and (b) is absolutely not what someone familiar with cpp will expect.

Well, I don't necessarily agree that people would be by default expecting the syntax/... must match cpp specifically; there are many many other pre-processors, macro-processors, template languages etc. out there.