Re: Propose a new application for reading DIMM SPD directly

From: Patrick Williams <patrick@stwcx.xyz>
To: Michael Shen <gpgpgp@google.com>
Cc: openbmc@lists.ozlabs.org
Subject: Re: Propose a new application for reading DIMM SPD directly
Date: Tue, 8 Feb 2022 01:11:29 -0600	[thread overview]
Message-ID: <YgIXodbEP9hmae6Q@heinlein> (raw)
In-Reply-To: <CAD1rtg8sa2OG4jUL8x7Gr9ExzH-tEswhHwSDaZtWhcAN4sAvuw@mail.gmail.com>

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On Tue, Feb 08, 2022 at 01:10:37PM +0800, Michael Shen wrote:
> Hi Openbmc,
> 
> We would like to propose an application that directly reads the DIMM
> SPD over HW interface(mostly I2C/I3C).

Who is "we"?

> If I understand correctly, the main method for obtaining the SPD
> information in openbmc is from SMBIOS which is prepared by BIOS. And
> We are exploring another way that excludes the involvement of BIOS.

Unless you're proposing that the BIOS itself comes to the BMC to get the SPD
data, I'm somewhat surprised you could come up with a hardware design to make
this work.  Due to the number of DIMM channels and the limited number of CS pins
on JEDEC DIMMs, you're going to have muxes on the bus somewhere.  Mixing muxes
and multi-master access is pretty problematic.  Either the BIOS and BMC are
fighting over the mux, which is going to mess with the mux driver's view of the
world, or you've got one mux for each in which case you're switching masters
onto a bus, which violates a few i2c design rules.

> The architecture of this application will be similar to the
> openbmc/smbios-mdr (the dimm part). The main difference will be the
> data source (changed from SMBIOS to SPD).
> 
> Currently the code is still being implemented, and we plan to support
> DDR5 SPD first, then expand to other DDR generation(if needed).

Hopefully you're leveraging the existing kernel drivers for reading SPD EEPROMs.
This creates you a sysfs file containing the whole of the EEPROM content.  You'd
just need to write a parser for JEDEC SPD format data.

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.

> If this proposal looks good to you, please help to create a repo for
> this application.
> 
> Best,
> Michael

-- 
Patrick Williams

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