Re: Please backport commit 13e3a512a290 (i2c: smbus: Support up to 8 SPD EEPROMs)

[Jean DELVARE <jdelvare@suse.com>]

Hi Greg, Paul,

On Thu, 2024-03-28 at 07:10 +0100, Greg KH wrote:
> On Wed, Mar 27, 2024 at 09:35:38PM +0100, Paul Menzel wrote:
> > Am 27.03.24 um 17:52 schrieb Greg KH:
> > > On Wed, Mar 27, 2024 at 04:13:26PM +0100, Paul Menzel wrote:
> > 
> > > > Please apply commit 13e3a512a290 (i2c: smbus: Support up to 8
> > > > SPD EEPROMs) [1] to the stable series to get rid of a warning
> > > > and to support more SPDs.
> > > > That commit is present since v6.8-rc1.
> > > 
> > > How far back?
> > 
> > I’d say 6.1.
> > 
> > > But isn't this a new feature, why is it needed in older kernels?
> > > It's not a fix for a regression.
> > decode-dimm does not work on systems with more than four SPD
> > EEPROMs, so I’d say it’s a fix.
>
> But it's never worked on such systems so it's not a regression fix,
> right?

It's hard to qualify whether this is a regression or not.

On the one hand, automatic detection of SPD EEPROMs only ever supported
4 modules maximum (since kernel v5.8):

01590f361e94 ("i2c: i801: Instantiate SPD EEPROMs automatically")
5ace60859e84 ("i2c: smbus: Add a way to instantiate SPD EEPROMs automatically")


OTOH, this was implemented using the at24 driver, which replaced the
legacy eeprom driver. Said legacy driver was removed in kernel v6.7:

0113a99b8a75 ("eeprom: Remove deprecated legacy eeprom driver")

As it would be possible to see up to 8 SPD EEPROMs using the legacy
eeprom driver, and only 4 when using the at24 driver, you could say
that kernel v6.7 suffers from a regression. So backporting commit
13e3a512a290 to 6.7-stable would make sense.

> Anyway, I'll defer to the i2c maintainers as to what they want to
> have happen here, as they did not originally tag this commit for
> stable inclusion.

I'm definitely in favor of backporting 13e3a512a290 to 6.7-stable.

For older kernels, I'm not so sure, as there's a fairly easy
workaround: loading the legacy eeprom driver should let decode-dimms
see all memory modules (modules 1-4 using the at24 driver and modules
5-8 using the eeprom driver). Paul, can you try and confirm that this
does work?

-- 
Jean Delvare
SUSE L3 Support