food for thought for tomorrow's call:

Passing information on what is plugged into the DIMM slots may not be as straightforward as it sounds. You can plug pure DRAM, NVDIMMs or pure flash (from Diablo technologies, acquired by RAMBus in 2019). The pure flash is a module where there is a few MB as bounce buffers to flash located on the DIMM itself. The largest element was 1TB on a single DIMM.

So passing a C structure that details the size of the DIMM fails to fully describe NVDIMM and Flash on DIMM. Those two things required complex Passing a DT fragment that describes the plugged module is generic and can describe both the bounce buffers and the registers needed to access the flash on DIMM. In the future you may find associative memory or whatever AI based modules.

Bottom line, using DT fragment to describe memory is flexible for the three existing cases and future proof.

The document shows that for a 2 DIMM representation and associated SPDs, the additional cost is 88 bytes. 32 are fixed cost, 56 are somewhat name size dependent and some related to FDT format.

There is no need for a full libfdt to actually produce that serialized data on a hob.

So in my view, the benefit of having an alignment, version and evolution proof structure that comes with DT fragment by far exceeds the cost of a few bytes saved, even in the context of SRAM size.

Cheers

On Wed, 2 Jun 2021 at 17:33, Joanna Farley via TF-A <tf-a@lists.trustedfirmware.org> wrote:

Hi All,

The next TF-A Tech Forum is scheduled for Thu 3^rd June 2021 16:00 – 17:00 (BST).



Agenda:

Discussion Session: Static and Dynamic Information Handling in TF-A (Session 2)

Lead by Manish Pandy and Madhukar Pappireddy
On tech forum this week, we would like to continue discussions on HOB list design.
The topics which we would like to cover is

Evaluate different proposals of passing information through boot phases.
If we don't get an agreement on one solution fit for all then we would try to get consensus for Infra segment platform(to solve original problem mentioned by Harb)
Try to get an agreement on size of tags and how "hybrid and tag only" HOB list can co-exist together?

Thanks

Joanna

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