Propose a new application for reading DIMM SPD directly

[Michael Shen]

On Thu, Feb 10, 2022 at 6:45 AM Ed Tanous <edtanous at google.com> wrote:
>
> On Wed, Feb 9, 2022 at 1:14 PM Patrick Williams <patrick at stwcx.xyz> wrote:
> >
> > On Wed, Feb 09, 2022 at 12:20:00PM -0800, Ed Tanous wrote:
> > > On Wed, Feb 9, 2022 at 11:56 AM Patrick Williams <patrick at stwcx.xyz> wrote:
> > > > On Tue, Feb 08, 2022 at 04:23:12PM +0800, Michael Shen wrote:
> > > > > On Tue, Feb 8, 2022 at 3:11 PM Patrick Williams <patrick at stwcx.xyz> wrote:
> > > > > > On Tue, Feb 08, 2022 at 01:10:37PM +0800, Michael Shen wrote:
> >
> > > > > BIOS owns the MUX select pin and it can decide who owns the SPD(I2C/I3C) bus.
> > > > > From my understanding, BIOS only needs to read SPD during the POST stage.
> > > > > For the rest of time, BIOS will hand over the SPD bus to BMC.
> > > >
> > > > That seems like it might work.  You'll have to deal with the time when the BIOS
> > > > has the mux in the BMC code somehow.  Ideally I'd ask for the mux select to also
> > > > be fed to the BMC as an input GPIO so that you can differentiate between "we
> > > > don't own the mux" and "all the devices are NAKing us".
> > >
> > > This seems like a nitty gritty design detail that's best handled in
> > > code when we review it.  I think the important bit here is that there
> > > are paths where this could work without a significant design issue.
> >
> > Just one subtlety.  I wouldn't expect this, necessarily, to be in _our_ design
> > and/or code, except that we'd want to document the GPIO line like we do all
> > others.  I was trying to hint that "if I were involved in this hardware design,
> > I'd ask for...".  If you leave it out, I'm sure it'll work _most_ of the time
> > just fine and it'll be your problem to debug it when it doesn't.
>
> Understood.

Thanks for all your suggestions. I will keep them in mind during implementation.

> > > > You should take a look at what is already existing in fru-device (part of
> > > > entity-manager repository).  This is already doing this for IPMI-format EEPROM
> > > > data.  We should be able to replicate/enhance this code, in the same repository,
> > > > to handle SPD format.
> > >
> > > I am not sure if it's a good idea to put it into the entity-manager
> > > repo. As you said EM
> > > is designed for IPMI-format EEPROM. Adding another parser into that
> > > repo may violate
> > > EM's design.
> >
> > I'm not sure why it would be an issue.  Hopefully one of the maintainers of that
> > repo can weigh in.  I wouldn't expect "parsing only IPMI-format EEPROMs" is a
> > design but just the current state of implementation.
>
> So long as it can function properly in its current design, i have no
> problem with FruDevice adding more parsing types.  In fact, there's
> already patchsets out to add Linkedins proprietary fru type to
> FruDevice, so in terms of design, Patricks request seems reasonable.

Got it. Then I will push the code to EM.