From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B18753CF051; Thu, 24 Sep 2026 04:50:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=150.107.74.76 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790225416; cv=none; b=bZEBObMppv97TDJHgd0HVV+9dGD2WgLdbPcq8krMw72tsS2CmNdRd4VJXjk0nkL/2+Uraj1qodjihJGMrHaluVMWIl1WCGGLV8DmLd13qZSPsOpv1NxMBr7JXxdBAqVEtXXL0y9WysC0J/3241N/PVAd0AxA383fPVV3GgKT614= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790225416; c=relaxed/simple; bh=43RMoj05xiKDXP5fYVrwU1nq5jfeUdtdfmfVfIcj7jI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cPOVkRzQuF6CCvXrVXC6keVQT6nyKw7nwwabR14VMlJqFrwWmgFwPVxwr9tgjsu4Tmln2JwAz//FC82g1/0K+to0DQfekAvogmlJERIUhQTdQ+Q2sJE6sKJoVPq0BSIH7Yk4LBmS6UrIJ2FZWBW94YRqgO/0GS4AZze8egOZ2vY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au; spf=pass smtp.mailfrom=gandalf.ozlabs.org; dkim=pass (2048-bit key) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.b=hve2mmx/; arc=none smtp.client-ip=150.107.74.76 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gandalf.ozlabs.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.b="hve2mmx/" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202608; t=1790225380; bh=7f2oetGAe71uHNcIaMbAfa2BIEJq6zVwewOu+CJ7lEw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=hve2mmx/tUtDE6CIHnu4ZtTIyNjLxLtJ2zsakbrWmLuj085BGsmkje2vi/0cfFVLb rtsHmK27Xz7u3X6zUn3bP/J7ry0bUr6dEyXHT4gU3wKNZFllkH/iNeAWlAoIvrrQmS bzBTWcroEnWj4CbAp/P26xe3j+bsjAs5W3PUV02PSFejTsi/T2e0v7zhFunGhcX0Dg 7RuPnW7jBg3MZ5I2C2MX2W3g5hMoX0Kmpmz65oOnq5WyQvU0ra9dcdA3uIwiOmbqWh 3RJ+yWARbtZLeQMGYo311WHj4ER+gKnp5Vi58v69TEDvlMSofHa3jRuo5JY/zwaKiQ 6hPRMkhSA5NpQ== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4hr1b811nqz4wCp; Thu, 24 Sep 2026 14:49:40 +1000 (AEST) Date: Thu, 24 Sep 2026 13:49:55 +1000 From: David Gibson To: Herve Codina Cc: Rob Herring , Krzysztof Kozlowski , Conor Dooley , Laurent Pinchart , David Lechner , Ayush Singh , Geert Uytterhoeven , devicetree-compiler@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, devicetree-spec@vger.kernel.org, Hui Pu , Ian Ray , Luca Ceresoli , Thomas Petazzoni , Frank Li Subject: Re: [PATCH v3 06/15] Introduce structured tag value definition Message-ID: References: <20260911091653.37b22b0f@bootlin.com> <20260914121937.5b7fa2bb@bootlin.com> <20260917090450.770c107a@bootlin.com> <20260918101617.46b3477e@bootlin.com> <20260922084154.576a42d0@bootlin.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="/u7gAoVU7G3ZXZ2N" Content-Disposition: inline In-Reply-To: <20260922084154.576a42d0@bootlin.com> --/u7gAoVU7G3ZXZ2N Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Sep 22, 2026 at 08:41:54AM +0200, Herve Codina wrote: > On Sat, 19 Sep 2026 14:22:09 +1000 > David Gibson wrote: >=20 > > On Fri, Sep 18, 2026 at 10:16:17AM +0200, Herve Codina wrote: > > > Hi David, > > >=20 > > > On Fri, 18 Sep 2026 14:41:11 +1000 > > > David Gibson wrote: > > > =20 > > > > On Thu, Sep 17, 2026 at 09:04:50AM +0200, Herve Codina wrote: =20 > > > > > Hi David, > > > > >=20 > > > > > On Wed, 16 Sep 2026 15:21:15 +1000 > > > > > David Gibson wrote: > > > > > =20 > > > > > > On Mon, Sep 14, 2026 at 12:19:37PM +0200, Herve Codina wrote: = =20 > > > > > > > Hi David, > > > > > > >=20 > > > > > > > On Sat, 12 Sep 2026 12:34:24 +1000 > > > > > > > David Gibson wrote: > > > > > > >=20 > > > > > > > ... > > > > > > > =20 > > > > > > > > > Do you mean that we should avoid the DATA_LEN_ENCODING an= d always have the > > > > > > > > > 32-bit value right after the tag to give the size for all= "skippable" tags? =20 > > > > > > > >=20 > > > > > > > > Yes. > > > > > > > > =20 > > > > > > >=20 > > > > > > > I did a test using a dts file available in kernel sources. I = used (arbitrary > > > > > > > choice) juno.dts [0]. > > > > > > >=20 > > > > > > > Without any new tags, the size of the compiled dtb is 27067 b= ytes. > > > > > > >=20 > > > > > > > With new metadata tags identifying phandles in properties (FD= T_PROPDATA_PHANDLE), > > > > > > > the size of the dtb becomes 29027 bytes and so 29027 - 27067 = =3D 1960 bytes for > > > > > > > those FDT_PROPDATA_PHANDLE tags (+7.2%). > > > > > > >=20 > > > > > > > The tags used are composed of: > > > > > > > 32-bit: FDT_PROPDATA_PHANDLE value encoding 1 x 32-bit for da= ta > > > > > > > 32-bit: offset in the property where a phandle is present. > > > > > > >=20 > > > > > > > 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_P= ROPDATA_PHANDLE > > > > > > > tag (tag + length + offset) instead of the 2 x 32-bit (tag + = offset). > > > > > > >=20 > > > > > > > Back to juno.dts instead of 1960 bytes, the FDT_PROPDATA_PHAN= DLE will need > > > > > > > 1960 * 3 / 2 =3D 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 eas= ily avoided. > > > > > > >=20 > > > > > > > Also, I will not be surprised to see more tags in the future = adding some more > > > > > > > metadata information and so increasing dtb sizes. > > > > > > >=20 > > > > > > > Quite often you have mentioned memory constraints system wher= e libfdt should > > > > > > > be as small as possible. On those system, the dtb itself is e= mbedded in the > > > > > > > binary close to libfdt. The size of dtb should be taken into = account. =20 > > > > > >=20 > > > > > > Yeah, those proportions are high enough that I think it's worth= it. > > > > > > =20 > > > > > > > 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. > > > > > > >=20 > > > > > > > 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 usi= ng > > > > > > > these bits. =20 > > > > > >=20 > > > > > > 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= =2E How about: > > > > > >=20 > > > > > > 0x80000000 structured tag bit > > > > > > 0x7fff0000 tag type > > > > > > 0x0000ffff tag length > > > > > >=20 > > > > > > So we have up to 2^15 (32k) different structured tags each with= a > > > > > > length of [0..65534] bytes (length=3D=3D65535 reserved for thos= e that need > > > > > > a full 32-bit length word). > > > > > >=20 > > > > > > I believe that will avoid the extra length word for everything = you > > > > > > have currently drafted. > > > > > > =20 > > > > >=20 > > > > > 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 che= ck the tag > > > > > value we have to filter out the tag length. > > > > >=20 > > > > > For instance: > > > > > - FDT_PROPDATA_PHANDLE > > > > > fixed data size 4 bytes for offset > > > > > tag value: 0x80010004 > > > > >=20 > > > > > - FDT_PROPDATA_PHANDLE_REF > > > > > data: 4 bytes for offset + N bytes for a string > > > > > tag value 0x8002ssss with ssss for the size > > > > >=20 > > > > > This will lead to code like this: > > > > > tag =3D fdt_next_tag(); > > > > > if (tag =3D=3D FDT_PROPDATA_PHANDLE) > > > > > /* Do something */ > > > > > =20 > > > > > if (TAG_GET_ID(tag) =3D=3D FDT_PROPDATA_PHANDLE_REF) > > > > > /* Do something */ =20 > > > >=20 > > > > 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. > > > >=20 > > > > 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? =20 > > >=20 > > > 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. =20 > >=20 > > 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. > >=20 > > > 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. > > >=20 > > > The kind of data length encoding (1-cell, 2-cells, varlength) is done > > > when the tag is defined and cannot be changed. > > >=20 > > > 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. > > >=20 > > > Of course we can introduce dynamic length encoding to set the data le= ngth > > > encoding in the tag according to the exact data length found at runti= me. > > > =20 > > > > 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: > > > >=20 > > > > * "short" vs "long": "short" always has the length within the tag > > > > word (and so cannot exceed 64k, or however many bits we set asid= e) > > > > whereas long always has a length word > > > > * "fixed" vs "variable", fixed length tag types always have the sa= me > > > > length, so the length can be considered part of the tag. Variab= le > > > > would have a length word. =20 > > >=20 > > > 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. > > >=20 > > > 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). =20 > >=20 > > 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. > >=20 > > > Without any additional bits for any category, all of these fit with t= he > > > following length encoding: > > > 000...00: No data > > > 000...01: 1 x 32-bit > > > 111...10: N x 32-bit > > > 111...11: varlen word =20 > >=20 > > Right, so revising my suggestions in light of a better understanding > > of what you had in mind originally, it comes down to: > >=20 > > * I think adding more bits to the size field would be worthwhile to > > allow a wider variety of future fixed length metadata tags. >=20 > Right, I have planned to add one more bit compared to the original propos= al. > This leads to 3 bits and so 0 to 6 32-bit data words (0b111 means varlen = word). >=20 > 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. >=20 > 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. --=20 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 --/u7gAoVU7G3ZXZ2N Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmq0ndwACgkQzQJF27ox 2Gefig//SJSuaWh8Aq8xRZo1lKaEMMP5kF/Lnc7orn71+MdBW6GlzJyaAX9JewSh BEEbuYc/9nfWlxeE+4nY7ie1FPnVNKfbyE7AW2sCkUMcEKTNN2b8i8YVU/M1Foiz agWo1VXZ1vMJ4DAuPGowM4CHWcRIqZWRwmExdDjXsUEqR27oBCbTNkVnVVTCCWll Cvda+5qkS+PICOEUSFKJi5nYUmdgvR2ZtCURQy81rEW3fnNMOeij1byJFYGdojqp Q5Fb+4k5SCpPgcVR1/PuJjYDDOcsPzD0aweozOb+15v8+DkohycIlF1On4Fz83d3 k/c+jk84SAiF1avSiB5p8oX/YhlqIyVMw1JOjejUMySu+h/bgec78I3Qz7qipafz 6s6jJihdW0gnXZLmBHw7V885tIDTBUzgJXQ7zjVn9dB98K0mxdhh1bT89dpBRz+E fyWONuvsz7pKQA1j0s6xs9zTmQYT1k0sb4QSqIneaiMIHORuJZNqVLzX8s9P9BJR YX2/22l5Xnflwk+x0VdTNU25bqn/UY4OeGO0fe23cFi8rFt76FIccrtU5jedurja ZZ0Ae5yC4hTHRQrMMJwOMViqe5QtSlAU2d0QVg6CKw9qL9A0zscgPBrNwoSxZFUi W3RBqlpZdw2N1bv+h2iKgn44UsZ1tpROUkir79mSvEFszMHHsJE= =VzK1 -----END PGP SIGNATURE----- --/u7gAoVU7G3ZXZ2N--