From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtpout-04.galae.net (smtpout-04.galae.net [185.171.202.116]) (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 C9B3A4DD3A7; Tue, 22 Sep 2026 06:42:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.171.202.116 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790059333; cv=none; b=ZG0E1Wa5qUr7mUbxDcAUccpRe+eCOyoxkSOKfFwVaXn97221KiLBC1Nz/93d93FFrpzTt/oMdA6/kTE0wvjeSvbcakrV8t1BUpWwaaqWNN/2oh3P+loF63iRLo1Aggs3B05WUA0o3+HrTCm125m9raUQdsa1TwMbhCteHxiXdcI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790059333; c=relaxed/simple; bh=szeWvxi07zaZ8VBuWTXfsFEpNXFbl3+Oq+wxER2IuTY=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=IYnAfim3y6OhWbbSxMdo7eZ7sjU4OgEcYh6aBgde8KVlGclDCL0B6v2QHKv6lW0s2So4PIK5R8i0oFj6mCLF28FXFkA7soTAxpuZzZpxp81c/yUJnn0OL94Dt3q/Kn78LNkdY6KvtiiiRqJ06J0fONIWyzGoxhp4AEIlTfZm2CY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=bootlin.com; spf=pass smtp.mailfrom=bootlin.com; dkim=pass (2048-bit key) header.d=bootlin.com header.i=@bootlin.com header.b=Xaic2v4j; arc=none smtp.client-ip=185.171.202.116 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=bootlin.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=bootlin.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=bootlin.com header.i=@bootlin.com header.b="Xaic2v4j" Received: from smtpout-01.galae.net (smtpout-01.galae.net [212.83.139.233]) by smtpout-04.galae.net (Postfix) with ESMTPS id 5E681C23DB0; Tue, 22 Sep 2026 06:42:55 +0000 (UTC) Received: from mail.galae.net (mail.galae.net [212.83.136.155]) by smtpout-01.galae.net (Postfix) with ESMTPS id C006860580; Tue, 22 Sep 2026 06:42:08 +0000 (UTC) Received: from [127.0.0.1] (localhost [127.0.0.1]) by localhost (Mailerdaemon) with ESMTPSA id 86EC510328173; Tue, 22 Sep 2026 08:41:56 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bootlin.com; s=dkim; t=1790059326; h=from:subject:date:message-id:to:cc:mime-version:content-type: content-transfer-encoding:in-reply-to:references; bh=Q3Vf+8PWFtWZUIskQ6CRyBCwIFSfIMrS0KXNG+qbKwU=; b=Xaic2v4jLtt8XX/zhvYaeYDc8ugTaD/X14ypw5QxtPpPTys/SyKAEPG6rXST+syZA8Yo5s FOZA2kFPQK9BJjBx/qbnds0FM6vWIEoRueFk81jskBW67A0kxVrmLeoGPJCllKJR6vO4aJ 2vC+YVsVPxgkKqGDV5a8nFRGOA9PP637r8RCEvsdZqtmjfcx9fQMqWvvhLo97asUA/DpJE tI1ItkptkspVLoUTavwqxLkHUOxOzbIfqiKuHcL9QqdF4sLoH3ZoQM0vbRlHCbDtAtj6Ju GzPtMUsd6duO8uMhnn2Lc1nEepsF40MIDdzd7Sekqg7HD1nSnulZ0UQnO3/pgg== Date: Tue, 22 Sep 2026 08:41:54 +0200 From: Herve Codina To: David Gibson 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: <20260922084154.576a42d0@bootlin.com> In-Reply-To: References: <20260910094126.4bf4cae6@bootlin.com> <20260911091653.37b22b0f@bootlin.com> <20260914121937.5b7fa2bb@bootlin.com> <20260917090450.770c107a@bootlin.com> <20260918101617.46b3477e@bootlin.com> Organization: Bootlin X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Last-TLS-Session-Version: TLSv1.3 On Sat, 19 Sep 2026 14:22:09 +1000 David Gibson 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 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. 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? > > * 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. > > Ok, so more length bits and better documentation of the fixed > vs. variable distinction are the only remaming suggestions. > Best regards, Hervé -- Hervé Codina, Bootlin Embedded Linux and Kernel engineering https://bootlin.com