Re: [PATCH v3 06/15] Introduce structured tag value definition

[Herve Codina <herve.codina@bootlin.com>]

Hi David,

On Thu, 24 Sep 2026 13:49:55 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Sep 22, 2026 at 08:41:54AM +0200, Herve Codina wrote:
> > On Sat, 19 Sep 2026 14:22:09 +1000
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Fri, Sep 18, 2026 at 10:16:17AM +0200, Herve Codina wrote:  
> > > > Hi David,
> > > > 
> > > > On Fri, 18 Sep 2026 14:41:11 +1000
> > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > >     
> > > > > On Thu, Sep 17, 2026 at 09:04:50AM +0200, Herve Codina wrote:    
> > > > > > Hi David,
> > > > > > 
> > > > > > On Wed, 16 Sep 2026 15:21:15 +1000
> > > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > > >       
> > > > > > > On Mon, Sep 14, 2026 at 12:19:37PM +0200, Herve Codina wrote:      
> > > > > > > > Hi David,
> > > > > > > > 
> > > > > > > > On Sat, 12 Sep 2026 12:34:24 +1000
> > > > > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > > > > > 
> > > > > > > > ...
> > > > > > > >         
> > > > > > > > > > Do you mean that we should avoid the DATA_LEN_ENCODING and always have the
> > > > > > > > > > 32-bit value right after the tag to give the size for all "skippable" tags?          
> > > > > > > > > 
> > > > > > > > > Yes.
> > > > > > > > >         
> > > > > > > > 
> > > > > > > > I did a test using a dts file available in kernel sources. I used (arbitrary
> > > > > > > > choice) juno.dts [0].
> > > > > > > > 
> > > > > > > > Without any new tags, the size of the compiled dtb is 27067 bytes.
> > > > > > > > 
> > > > > > > > With new metadata tags identifying phandles in properties (FDT_PROPDATA_PHANDLE),
> > > > > > > > the size of the dtb becomes 29027 bytes and so 29027 - 27067 = 1960 bytes for
> > > > > > > > those FDT_PROPDATA_PHANDLE tags (+7.2%).
> > > > > > > > 
> > > > > > > > The tags used are composed of:
> > > > > > > > 32-bit: FDT_PROPDATA_PHANDLE value encoding 1 x 32-bit for data
> > > > > > > > 32-bit: offset in the property where a phandle is present.
> > > > > > > > 
> > > > > > > > Removing the '1 x 32-bit' information from the tag value and adding a 32-bit
> > > > > > > > 'length' in all cases will lead 3 x 32-bit values for a FDT_PROPDATA_PHANDLE
> > > > > > > > tag (tag + length + offset) instead of the 2 x 32-bit (tag + offset).
> > > > > > > > 
> > > > > > > > Back to juno.dts instead of 1960 bytes, the FDT_PROPDATA_PHANDLE will need
> > > > > > > > 1960 * 3 / 2 = 2640 bytes (+9.7%). This leads to around +2.5% of the whole
> > > > > > > > dtb just to have the 32-bit for length. This +2.5% can be easily avoided.
> > > > > > > > 
> > > > > > > > Also, I will not be surprised to see more tags in the future adding some more
> > > > > > > > metadata information and so increasing dtb sizes.
> > > > > > > > 
> > > > > > > > Quite often you have mentioned memory constraints system where libfdt should
> > > > > > > > be as small as possible. On those system, the dtb itself is embedded in the
> > > > > > > > binary close to libfdt. The size of dtb should be taken into account.        
> > > > > > > 
> > > > > > > Yeah, those proportions are high enough that I think it's worth it.
> > > > > > >       
> > > > > > > > If the SAFE_SKIP bit is removed, I even plan to use this now free bit in the
> > > > > > > > length encoding part:
> > > > > > > >   0b000: No data
> > > > > > > >   0b001: 1 fdt32
> > > > > > > >   0b010: 2 fdt32
> > > > > > > >   ...
> > > > > > > >   0b110: 6 fdt32
> > > > > > > >   0b111: On additional fdt32 to encode the length of data.
> > > > > > > > 
> > > > > > > > IHMO, length encoding bits in tag value definition should be kept and used
> > > > > > > > for all tags where the length is fixed and can be encoded using
> > > > > > > > these bits.        
> > > > > > > 
> > > > > > > Well, I'm convinced we want some sort of compact encoding of the
> > > > > > > length, but I think we can do better than the current proposal.  It
> > > > > > > seems implausible to me that we'll need 2^29 different metadata tags,
> > > > > > > so I think we can spend some more of the tag bits on the length.  How about:
> > > > > > > 
> > > > > > >    0x80000000		structured tag bit
> > > > > > >    0x7fff0000		tag type
> > > > > > >    0x0000ffff		tag length
> > > > > > > 
> > > > > > > So we have up to 2^15 (32k) different structured tags each with a
> > > > > > > length of [0..65534] bytes (length==65535 reserved for those that need
> > > > > > > a full 32-bit length word).
> > > > > > > 
> > > > > > > I believe that will avoid the extra length word for everything you
> > > > > > > have currently drafted.
> > > > > > >       
> > > > > > 
> > > > > > Yes, this will avoid the extra length field. The drawback is the that the
> > > > > > tag value is no more a well fixed value. Each time we have to check the tag
> > > > > > value we have to filter out the tag length.
> > > > > > 
> > > > > > For instance:
> > > > > > - FDT_PROPDATA_PHANDLE
> > > > > >   fixed data size 4 bytes for offset
> > > > > >   tag value: 0x80010004
> > > > > > 
> > > > > > - FDT_PROPDATA_PHANDLE_REF
> > > > > >   data: 4 bytes for offset + N bytes for a string
> > > > > >   tag value 0x8002ssss with ssss for the size
> > > > > > 
> > > > > > This will lead to code like this:
> > > > > >   tag = fdt_next_tag();
> > > > > >   if (tag == FDT_PROPDATA_PHANDLE)
> > > > > >     /* Do something */
> > > > > >      
> > > > > >   if (TAG_GET_ID(tag) == FDT_PROPDATA_PHANDLE_REF)
> > > > > >     /* Do something */      
> > > > > 
> > > > > True.  But.. a similar problem kind of exists with the original
> > > > > proposed encoding too: we *expect* a tag with fixed 4-byte contents to
> > > > > use the "1 cell" flags, but we need to consider the case of encoding
> > > > > it as VARLEN with a length field of 4.  We could choose to make that
> > > > > forbidden, but we'd still need to consider who's responsible for
> > > > > enforcing that.
> > > > > 
> > > > > Similarly, if a variable length metadata tag happens to have length 4
> > > > > or 8 in a particular place, is it valid to encode it with the 1-cell
> > > > > or 2-cell flag?    
> > > > 
> > > > My position was: if we expect a tag with "1-cell" flag, using varlen
> > > > field encoding is considered as an other tag and so either an error
> > > > or a skippable unknown tag.    
> > > 
> > > So essentially tags of different (fixed) length live in different
> > > namespaces.  Ok, that makes good sense to me, but wasn't initially
> > > obvious to me.  So, I think we need to spell this out a bit better.
> > >   
> > > > The same apply for varlen defined tag. Even if the data is, let's
> > > > say 8 bytes, the tag cannot be moved to a "2-cells" tag.
> > > > 
> > > > The kind of data length encoding (1-cell, 2-cells, varlength) is done
> > > > when the tag is defined and cannot be changed.
> > > > 
> > > > Who is responsible for enforcing that ?
> > > > I would say the documentation of the tag should clearly set the data
> > > > encoding used for the tag and the documentation of the "skippable"
> > > > format should say that data encoding is fixed for a given tag. It is
> > > > set when the tag is defined and any changes at runtime should be
> > > > considered as a different tag.
> > > > 
> > > > Of course we can introduce dynamic length encoding to set the data length
> > > > encoding in the tag according to the exact data length found at runtime.
> > > >     
> > > > > As a variant on my proposal, I'd also be fine with dividing the
> > > > > structed tags into several classes with bits indicating which is
> > > > > which.  Either:
> > > > > 
> > > > >  * "short" vs "long": "short" always has the length within the tag
> > > > >    word (and so cannot exceed 64k, or however many bits we set aside)
> > > > >    whereas long always has a length word
> > > > >  * "fixed" vs "variable", fixed length tag types always have the same
> > > > >    length, so the length can be considered part of the tag.  Variable
> > > > >    would have a length word.    
> > > > 
> > > > Well, only strings, or more generally arrays, need a varlen. For those
> > > > item, I would use the varlen word and so "long" in your definition.
> > > > 
> > > > For all others where the sizeof(data) is well known when the tag is
> > > > defined, I would use "fixed" and "long" only if sizeof(data) cannot
> > > > be encoded by "fixed" (lengh > limit of dedicated bits).    
> > > 
> > > Right, now understanding your thoughts on the originally proposed
> > > encoding, the "fixed" versus "variable" distinction makes more sense I
> > > think.  I do see the advantage of never having a variable length
> > > encoded within the tag word.
> > >   
> > > > Without any additional bits for any category, all of these fit with the
> > > > following length encoding:
> > > >   000...00: No data
> > > >   000...01: 1 x 32-bit
> > > >   111...10: N x 32-bit
> > > >   111...11: varlen word    
> > > 
> > > Right, so revising my suggestions in light of a better understanding
> > > of what you had in mind originally, it comes down to:
> > > 
> > >  * I think adding more bits to the size field would be worthwhile to
> > >    allow a wider variety of future fixed length metadata tags.  
> > 
> > Right, I have planned to add one more bit compared to the original proposal.
> > This leads to 3 bits and so 0 to 6 32-bit data words (0b111 means varlen word).
> > 
> > Do you think we should add one more bits?  
> 
> I don't think we particularly need them for the tag type, so yes, I
> think that would be worthwhile.
> 
> > >  * Originally I was thinking that having a length in bytes rather than
> > >    just a length in 32-bit words would be worth it.  But thinking
> > >    further, there's probably no benefit.  The length rounded up to
> > >    32-bit words is all we need for skipping over it when unknown.
> > >    Even if we want a fixed length tag with, say, 3 bytes of data, we
> > >    can pad that out to a 1-word tag with a reserved byte.  
> > 
> > The varlen word could be kept in bytes.  
> 
> The varlen word _must_ be kept in bytes.
> 
> > > Ok, so more length bits and better documentation of the fixed
> > > vs. variable distinction are the only remaming suggestions.  
> 

So the name "structured" tag will be replace my "skippable" tag. I don't
want to use "metadata" because we don't know if future tags will be only
metadata tags or something else that shouldn't be qualified by "metadata".

Skippable tag format:
  - bit 31: set to 1
      Indentify a skippable tag.
  - bits 30..24: Data length encoding
      Encode the length of data attached to the tag.
      0b0000000: No data
      0b0000001: 1 32-bit word
      0b0000010: 2 x 32-bits words
      0b0000011: 3 x 32-bits words
      ...
      0b1111101: 125 x 32-bits words
      0b1111110: 126 x 32-bits words
      0b1111111: varlen encoding. The length of data is encode by an 32-bit
                 word available right after the tag. This word gives the
                 length of data in bytes.

  - bits 23..0: Tag identifier

The full tag value are used to identify a tag. Two skippable tags with the
same tag identifier field but with different data lengh encoding should be
considered as two different tags.

The exact tag value is completely known when the new tag is defined and its
data length encoding must not be determine at runtime when the exact length
of data is known.

Any tags where the length of the data is not known when the tag is defined
(i.e. sizeof(data) cannot be determined when the tag is defined) must use the
varlen encoding value (0b1111111). Tags with strings or arrays where the number
of items in the array can vary should use the varlen encoding.

Also for tags where the length of data is greater than 126 x 32-bits words
(maximum value without using varlen encoding), the varlen encoding must be used
even if the length of data is known when the tag is defined.

Tags reserved for tests purpose:
  Any tags with the tag identifier set to 0xffffff are reserved for test
  purpose. When a parser find this kind of tag, it must ignore the tag (skip
  it). A generator must never use this kind of tags, except, of course, if
  the generator is used to generate a dtb for test purpose.


What is you opinion about this description?
Do you agree on the description? If so I will implement it on next iteration.

Best regards,
Hervé