Re: [RFC PATCH 1/2] Preserve datatype information when parsing dts

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On Mon, Dec 18, 2017 at 11:20:38AM +0000, Grant Likely wrote:
> On Mon, Dec 18, 2017 at 4:34 AM, David Gibson
> <david@xxxxxxxxxxxxxxxxxxxxx> wrote:
> > On Tue, Dec 12, 2017 at 12:48:10PM +0000, Grant Likely wrote:
> >> On Tue, Dec 12, 2017 at 6:14 AM, David Gibson
> >> <david@xxxxxxxxxxxxxxxxxxxxx> wrote:
> >> > On Tue, Nov 28, 2017 at 11:57:09AM +0000, Grant Likely wrote:
> >> >> The current code throws away all the data type and grouping information
> >> >> when parsing the DTS source file, which makes it difficult to
> >> >> reconstruct the data format when emitting a format that can express data
> >> >> types (ie. dts and yaml). Use the marker list to mark the beginning and
> >> >> end of each integer array block (<> and []), the datatype contained in
> >> >> each (8, 16, 32 & 64 bit widths), and the start of each string.
> >> >>
> >> >> At the same time, factor out the heuristic code used to guess a property
> >> >> type at emit time. It is a pretty well defined standalone block that
> >> >> could be used elsewhere, for instance, when emitting YAML source. Factor
> >> >> it out into a separate function so that it can be reused, and also to
> >> >> simplify the write_propval() function.
> >> >>
> >> >> When emitting, group integer output back into the same groups as the
> >> >> original source and use the REF_PATH and REF_PHANDLE markers to emit the
> >> >> the node reference instead of a raw path or phandle.
> >> >>
> >> >> Signed-off-by: Grant Likely <grant.likely@xxxxxxx>
> >> >
> >> > I'm a bit dubious how well forcing the marker mechanism to do all this
> >> > stuff it was never intended for can work in the long term.  Still,
> >> > it's an interesting experiment.
> >>
> >> As long as the actual data is stored as flat buffer, the markers
> >> mechanism works quite well for this. I tried doing something entirely
> >> separate, and it turned out to be awful. Another alternative is to
> >> break up the flat buffer into a chain of data blocks with attached
> >> type information, but that is a very invasive change.
> >>
> >> This approach has the advantage of being robust on accepting both
> >> typed and anonymous data. If the markers are not there then the
> >> existing behaviour can be maintained, but otherwise it can emit a
> >> higher fidelity of source language.
> >
> > Hm, true.  The approach is growing on me.  I guess what I'm still
> > dubious about is how much this type annotation can get us to approach
> > the YAML model.  For example, YAML can distinguish between [ [1, 2],
> > [3, 4] ] and [1, 2, 3, 4] which isn't really feasible in dtc.
> 
> To start with I'm constraining what is permissible in the YAML
> encoding. So, even though YAML can encode multiple nested lists, I'm
> not permitting that in this iteration. To take an example:
> 
> in dts: reg = <0x1000 0x100> <0x4000 0x300>;
> In YAML I'm encoding as:   reg: [ [0x1000, 0x100], [0x4000, 0x300] ]
> 
> in dts: compatible = "acme,uart9000", "ns16550"
> is in YAML: compatible: [ "acme,uart9000", "ns16550"]
> 
> in dts: #size-cells = <1>;
> in YAML: "#size-cells": [ [ 1 ] ]
> 
> in dts: uint16-prop = /bits/ 16 <31>;
> in YAML: uint16-prop: [ !uint16 [31] ]
> 
> I'm not allowing anything outside that pattern. So, the following are
> all disallowed currently:
> reg: [0x1000, 0x100, 0x4000, 0x300] /* integers need to be in a list -
> maps to <...> in dts */
> compatible: "ns16550" /* not encoded into list */
> reg: [ [ [0x4, 0xffff0000], 0x80000], [ [0x4, 0xfffe0000], 0x40000] ]
> /* Triple nesting not allowed*/

Sorry, I meant to make a comment on this months ago, but never got
around to it.  In terms of the immediate problem here, this seems like
a reasonable approach.  However, it kind of underscores the lingering
worries I have about using YAML as a DT encoding format.  In the
JSON/YAML world, 1 and [ [ 1 ] ] are different things, and using the
later as a way of encoding what's essentially a plain integer would be
pretty perverse.

So, I'm concerned that if we have YAML front-and-centre to the user,
it will be pretty misleading as to what is and isn't possible.

-- 
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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