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

[Ard Biesheuvel]

On 8 March 2015 at 12:31, Jean Delvare <jdelvare@xxxxxxx> wrote:
> Hi Ivan,
>
> On Sat, 07 Mar 2015 22:53:32 +0200, Ivan.khoronzhuk wrote:
>> On 03/05/2015 09:46 AM, Jean Delvare wrote:
>> > It's not just two tables (I don't expect a lot of BIOSes to provide two
>> > tables in practice, and they would have essentially the same format
>> > anyway) but more importantly two entry points. The _SM3_ entry point is
>> > brand new and most applications (including dmidecode) don't support it
>> > yet. It doesn't matter if the kernel itself can parse it, as it passes
>> > the raw entry point to applications anyway.
>> >
>> > It happens that we are introducing this new sysfs raw interface at the
>> > same time _SM3_ is being introduced, so we do not have to care about
>> > backwards compatibility. Both the kernel and dmidecode will need to be
>> > updated to support the new interface, so we can keep things simple and
>> > let the kernel expose only the best entry point.
>> >
>> > If the sysfs raw interface was already present at the time _SM3_
>> > support was being added, then we would have had to present both entry
>> > points for backwards compatibility. And if some _SM4_ entry point is
>> > ever added in the future with a new format, we will have to export it
>> > as a new sysfs attribute so as to not break compatibility.
>> >
>> > As a summary, I agree that a single entry point file is OK for now, but
>> > only because we are lucky that the timing is right.
>>
>> Despite of timing is right.
>>
>> 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. 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. 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.

More info here:
http://sourceforge.net/p/edk2/mailman/message/33550425/

Regards,
Ard.


>> you can
>> be sure in backward compatibility. But at least for now you can't.
>>
>> It's obvious, if kernel found two entry points then it can create two
>> sysfs attributes.
>> But, what kernel should do in case if only one new entry point is present.
>> Generate entry point of old version..., sorry but it's bad idea. At
>> least because
>> where guarantee that we have enough information for this. Only field we
>> can bring
>> thought entry point versions is magic string _SM*_, and based on it, if util
>> don't support new version it can warn. It's used for differ versions and
>> there is nothing we can do more.
>
> I agree that the kernel should not fake an entry point which does not
> exist (I'm not sure if you misunderstood me but I never suggested that.)
>
> --
> Jean Delvare
> SUSE L3 Support
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