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 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 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 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. * 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. Ok, so more length bits and better documentation of the fixed vs. variable distinction are the only remaming suggestions. > > Not sure if that makes things any easier, but they might, and I'd be > > fine with either option (or some combination). In any of these cases > > we do need to spell out what the requirements are: for dtb writers, > > for dtb readers and for whoever defines a new tag. > > > > > Further more, in the code you will both a mix of both construction: > > > while (tag == FDT_NOP || tag == FDT_BEGIN_NODE || > > > TAG_GET_ID(tag) == FDT_BEGIN_NODE_REF); > > > > > > with #define TAG_GET_ID(tag) ((tag) & 0xffff0000) > > > > > > Also when we write dtbs either in libfdt or dtc, tags value > > > have to be built with the length when needed. > > > #define TAG_VALUE(tag_id, length) (((tag_id) & 0xffff0000) || \ > > > ((length) & 0x0000ffff)) > > > > > > I am totally fine with that but we need to have it in mind. > > > > Right, that's not a deal breaker for me - especially since I think > > we'll need some similar stuff even with the original proposal. > > > > > Of course, I can encode FDT_PROPDATA_PHANDLE_REF with 0x8002ffff + length > > > field but we lose all the benefits > > > > > > Ready to see TAG_GET_ID(tag) and TAG_VALUE(tag_id, length) when needed in > > > the code? > > > > I think so, though that could change depending on what it ends up > > looking like in practice. > > > > Best regards, > Hervé > -- David Gibson (he or they) | 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