From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtpout-02.galae.net (smtpout-02.galae.net [185.246.84.56]) (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 E24FE3B9617; Thu, 10 Sep 2026 07:41:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.246.84.56 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789026104; cv=none; b=UTAl/euaaFaWCne3bYJwFxoxyXJV3zIKLVWX6+j74ewg8D9848FHqk/4JP3O1+Igl6fNJjISxftEmftxP5vCJmuk2oMaoyIopZCsvDiPKcF/tV8iRuiTddHcSHlwgbZpwvGY7l1MXyvERXeioPoiWU3tQTA7RmU8NFh44+1rWGU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789026104; c=relaxed/simple; bh=s4xzi3/ChXeiGi5vZRgpZuxVbGsK8ogXz7GRWM8ND0s=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=qbA+y271ypMTXp16tt/lisKICwyQBxJ+FmlLRw1kOlgH/ZWcMUCAHAEOLGZ/SjAv7Hx98DrMJ7n4zFmRUAXvHnmQycMpfmpxpscQVGJMB/VZz9CjfwCps9Lq6K33q5vq1Fsot3flFIriiqcsjO4EJGye7KJUgPl8HrZ6N/0KwGY= 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=WLgD+O3/; arc=none smtp.client-ip=185.246.84.56 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="WLgD+O3/" Received: from smtpout-01.galae.net (smtpout-01.galae.net [212.83.139.233]) by smtpout-02.galae.net (Postfix) with ESMTPS id 9D9641A1A89; Thu, 10 Sep 2026 07:41:33 +0000 (UTC) Received: from mail.galae.net (mail.galae.net [212.83.136.155]) by smtpout-01.galae.net (Postfix) with ESMTPS id 6FE21602B8; Thu, 10 Sep 2026 07:41:33 +0000 (UTC) Received: from [127.0.0.1] (localhost [127.0.0.1]) by localhost (Mailerdaemon) with ESMTPSA id 06D4C11C78279; Thu, 10 Sep 2026 09:41:27 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bootlin.com; s=dkim; t=1789026092; h=from:subject:date:message-id:to:cc:mime-version:content-type: content-transfer-encoding:in-reply-to:references; bh=kMPeRTm+aw2EyETunRUKyMK/qtl1oCjDcIMJwGBX7QA=; b=WLgD+O3/ZyiuGyaqtS3Or/k/RfHEj3r+T6xSVxf3VeOm6WGIu74WIOVf9lnf85EBbLi3+J njr/CbKF2Lg2ZQK+jI3XhbP5MpNMcwF0TGAtkub7brsKL8Br67LJtT6vfGG2P2bawMmtnT bJ4hBf7gjPkJWTsEBUea+Dr6x5298mZO91poC/u0Il6Lb1eYUm7vzu7EqRriYDvoZ4k9rV xIIilJLaW5D99yOP7ES4TN15KFi7WiynKL5JGXZFn/7tjEm35EF+LHxiXV55/dCkuKzBn0 XOhrXjGKPiTmhTKLg9NRnoVAL7tynDsx6dP6sLR2PjgTkcz5kI+ChkFjkpsPZQ== Date: Thu, 10 Sep 2026 09:41:26 +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: <20260910094126.4bf4cae6@bootlin.com> In-Reply-To: References: <20260826083146.304291-1-herve.codina@bootlin.com> <20260826083146.304291-7-herve.codina@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 Hi David, On Thu, 10 Sep 2026 14:51:08 +1000 David Gibson wrote: > On Wed, Aug 26, 2026 at 10:31:37AM +0200, Herve Codina wrote: > > The goal of structured tag values is to ease the introduction of new > > tags in future releases with the capability for an already existing > > release to ignore those structured tags. In order to do that data length > > related to the unknown tag needs to be identified. > > > > Also add a flag to tell an old release if this tag can be simply skipped > > or must lead to an error. > > I suggested/insisted on this in the past, but I've since realised this > isn't actually useful. The "structured tag" format is only useful > because it allows you to skip over unknown tags, which means there's > no point using it for ags that can't be skipped. If we need new tags > that can't be safely skipped, they can be added as old-style tags > instead. > > In which case "old style" versus "new style" is no longer a good > characterization: they would exist side by side, so the distinction is > more "skippable" versus "non skippable" tags. Which suggests that > "skippable tags" or "metadata tags" might be a better term than > "structured tags" - that would focus more on the why than the how. Do you mean that the SKIP_SAFE bit should be removed ? I like the defined structure with the DATA_LEN_ENCODING part. Even for tags which are not "skippable". This ensures a kind of standardized format for all future tags instead of a specific definition (related to the length of the data) for each new tag. For this kind of information related to the length of data, I prefer a global rule instead of tag-specific rules. > > > > Structured tag value is defined on 32bit and is defined as follow: > > > > Bits | 31 | 30 | 29 28 | 27 0| > > ------+----+-----------+-------------------+--------+ > > Fields| 1 | SKIP_SAFE | DATA_LEN_ENCODING | TAG_ID | > > ------+----+-----------+-------------------+--------+ > > > > Bit 31 is always set to 1 to identify a structured tag value. > > > > Bit 30 (SKIP_SAFE) is set to 1 if the tag can be safely ignored when its > > TAG_ID value is not a known value (unknown tag). If the SKIP_SAFE bit is > > set to 0 this tag must not be ignored and an error should be reported > > when its TAG_ID value is not a known value (unknown tag). > > > > Bits 29..28 (DATA_LEN_ENCODING) indicates the length of the data related > > to the tag. Following values are possible: > > - 0b00: No data. > > The tag is followed by the next tag value. > > I think "next tag." would be clearer than "next tag value." - "next > tag value" could be confused as meaning the data that comes after the > tag. Ok, will be updated. > > > - 0b01: 1 cell data > > The tag is followed by a 1 cell (u32) data. The next tag is > > available after this cell. > > Remove "available", it doesn't add any information. Ok, will be updated. > > > - 0b10: 2 cells data > > The tag is followed by a 2 cells (2 * u32) data. The next tag > > is available after those two cells. > > Ditto. Will be update as well. > > > - 0b11: Data length encoding > > The tag is followed by a cell (u32) indicating the size of the > > data. This size is given in bytes. Data are available right > > after this cell. > > > > The next tag is available after the data. Padding is present > > after the data in order to have the next tag aligned on 32bits. > > This padding is not included in the size of the data. > > I'm guessing the 1 & 2 cell cases are pretty common so it saves a > moderate amount of dtb size to have this encoding. However, it does > come at the cost of greater code complexity. Is it worth it? I'm > willing to believe it is, but I think the case needs to be made. In term of code complexity, FDT_PROPDATA_PHANDLE introduced in the addons series, patch 7/74 [0], is a 1-cell (or 1-fdt32) "structured" tag. It can be compared with FDT_PROPDATA_PHANDLE_REF, introduced in patch 11/74 [1]. FDT_PROPDATA_PHANDLE_REF is a "Data length encoding" tag. Modifications in libfdt/fdt.c are pretty similar for both tags. [0] https://lore.kernel.org/all/20260826094950.1088288-8-herve.codina@bootlin.com/ [1] https://lore.kernel.org/all/20260826094950.1088288-12-herve.codina@bootlin.com/ > > I'd also avoid the term "cell" here. To me a "cell" is specifically a > term applying to a 32-bit value _within a device tree property_. > These are 32-bits, but not part of a property. Plus if Segher > reappears, he'll complain that in the old days of OF, a cell wasn't > necessarily 32-bits :). ok, I will avoid the term "cell" in the next iteration. > > > Bits 27..0 (TAG_ID) is the tag identifier defining a specific tag. > > > > Introduce the structured tag values definition and some specific tags > > reserved for tests based on this structure definition. > > > > Signed-off-by: Herve Codina > > Reviewed-by: Frank Li > > --- > > libfdt/fdt.h | 23 +++++++++++++++++++++++ > > 1 file changed, 23 insertions(+) > > > > diff --git a/libfdt/fdt.h b/libfdt/fdt.h > > index a07abfcc..f41a355f 100644 > > --- a/libfdt/fdt.h > > +++ b/libfdt/fdt.h > > @@ -49,6 +49,7 @@ struct fdt_property { > > > > #define FDT_MAGIC 0xd00dfeed /* 4: version, 4: total size */ > > #define FDT_TAGSIZE sizeof(fdt32_t) > > +#define FDT_CELLSIZE sizeof(fdt32_t) > > I'd avoid creating this constant for similar reasons to avoid the term > "cell" above. Ok, I will avoid the constant and use sizeof(fdt32_t) directly when needed in the code. Also, I will avoid the CELL in all other part of this file. Best regards, Hervé