linux-kernel - Re: [dmidecode] [Patch v3] firmware: dmi-sysfs: add SMBIOS entry point area raw attribute

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Date:	Sun, 8 Mar 2015 18:38:57 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Jean Delvare <jdelvare@...e.de>
Cc:	"Ivan.khoronzhuk" <ivan.khoronzhuk@...ballogic.com>,
	dmidecode-devel@...gnu.org,
	Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
	Matt Fleming <matt.fleming@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Mark Salter <msalter@...hat.com>,
	Grant Likely <grant.likely@...aro.org>
Subject: Re: [dmidecode] [Patch v3] firmware: dmi-sysfs: add SMBIOS entry
 point area raw attribute

On 8 March 2015 at 18:11, Jean Delvare <jdelvare@...e.de> wrote:
> On Sun, 8 Mar 2015 14:53:04 +0100, Ard Biesheuvel wrote:
>> On 8 March 2015 at 12:31, Jean Delvare <jdelvare@...e.de> wrote:
>> > On Sat, 07 Mar 2015 22:53:32 +0200, Ivan.khoronzhuk wrote:
>> >> The specification doesn't oblige firmware to provide two entry points.
>> >> An implementation may provide either the 32-bit entry point or the 64-bit
>> >> entry point, or both. For compatibility with existing SMBIOS parsers, an
>> >> implementation should provide the 32-bit entry point, but it's not required.
>> >
>> > I expect most implementations will do, as it's trivial to implement.
>>
>> Not quite. First of all, some 64-bit ARM systems do not have any
>> system RAM below 4 GB, so there is not way they can implement the
>> 32-bit entry point.
>
> I didn't know that, thanks for the notice. No big deal anyway, these
> systems did not support SMBIOS before version 3.0 so there cannot be
> any regression on these systems.
>
>> Also, the 64-bit entry point does not limit the
>> structure size or the entire table to 64 KB like the 32-bit one does,
>> so it may be necessary to create a whole separate table with a subset
>> of the contents of the real table to stay within limits for the 32-bit
>> entry point.
>
> I doubt this is an issue in practice. I have been around for quite some
> time now and the largest table I've ever seen was 9043 byte long, which
> is nowhere close to the limit.
>
>> And the 32-bit entry point could well be 3.0 anyway, if
>> it uses any of the new enum values for the data items that were
>> undefined before 3.0.
>
> This is true but irrelevant to the discussion.
>

To clarify, the SMBIOS 3.0 spec explicitly allows the 32-bit entry
point to either point to the same table as the 64-bit entry point, or
point to a separate table, in which case the contents of the latter
should be a subset of the contents of the former. It doesn't specify
anything about the version number to be used in the 32-bit entry point
in case they point to separate tables. This means the presence of the
32-bit entry point does not guarantee that the table contents are
compatible with the pre-3.0 tools. So perhaps it would make sense to
export the 32-bit entry point separately only if it points to a
different table, and has a different version number?

-- 
Ard.
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