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* [RFC] Remove PI spec TemporaryRamSupport PPI from PEI Core
@ 2019-04-24  7:51 Jordan Justen
  2019-04-24 16:36 ` Laszlo Ersek
  2019-04-25  0:09 ` [edk2-devel] " Nate DeSimone
  0 siblings, 2 replies; 4+ messages in thread
From: Jordan Justen @ 2019-04-24  7:51 UTC (permalink / raw)
  To: devel
  Cc: Ni, Ray, Andrew Fish, Laszlo Ersek, Leif Lindholm, Mike Kinney,
	Vincent Zimmer

Short summary: Despite being part of the Platorm Initialization
Specificication, this proposal would remove support for the
TemporaryRamSupport PPI from PEI Core. Arguably, this would mean the
PEI Core does not support the PI spec, but we do not currently know if
any platforms require this PPI.

Main question: Does anyone know of a platform that requires this PPI,
or does anyone have major concerns with removing it before the PI
specification can be updated to remove the PPI?

Alternatives: Continue to support the PPI, but we would need to merge
a bug-fix patchset which adds some assembly code into the PEI Core.

More details:

I discussed this topic with the Tianocore Stewards, Vincent and Ray.
Although the path forward was not unanimous, it appears that the
majority thought we should propose to remove support for the
TemporaryRamSupport PPI from the PEI Core.

This PPI is defined in MdePkg/Include/Ppi/TemporaryRamSupport.h. I
believe it has been present in all versions of the Platorm
Initialization Specificication, including the latest, version 1.7.
(https://uefi.org/specsandtesttools)

The TemporaryRamSupport PPI is defined as "optional" in the PI spec,
but I believe this only applied to producers of the PPI (PI
platforms). The PEI Core (which is the consumer of this PPI) should
arguably support the PPI in order to fully support PI spec based
platforms.

But, there are some subtle issues with the PPI that make it difficult
to implement for both the consumer (PEI Core) and the producer (the
platform). In fact, there is a bug currently with the PEI Core's
implementation, which I sent a patch series to address on April 10th,
with the subject of "[PATCH v2 0/6] Fix PEI Core issue during
TemporaryRamMigration".

Unfortunately the fix is not very simple, and requires adding assembly
code to the PEI Core module. Based on this, we discussed if the
TemporaryRamSupport PPI is no longer required by current PI platforms,
despite being present in the PI spec. The only known example of it
being required was a platform based on the IPF architecture, which is
no longer supported by EDK II.

Several EDK II sample platforms produce this PPI in EDK II, but only
as sample code. Clearly the removal would have to take this into
account.

There are two replacements for the TemporaryRamSupport PPI, but they
do not cover all types of potential hardware. If a platform cannot
access main memory at the same time as the Temporary RAM, then it
would not be supported by the alternatives. Nevertheless, the two
replacements are:

1. Stop producing the PPI, and the PEI Core will automatically copy
   the required memory from Temporary RAM to the main memory. This
   will work for platforms that do not require special code to shut
   down Temporary RAM.

2. Produce the MdePkg/Include/Ppi/TemporaryRamDone.h PPI. This will
   cause the PEI Core to notify the platform after it has copied
   memory, and the platform can take special actions to disable
   Temporary RAM mode if required.

Since both of these options will cause the PEI Core to do a memory
copy from Temporary RAM to the main memory, there might be platforms
that they cannot work with as described above.

Thanks for your time,

-Jordan

^ permalink raw reply	[flat|nested] 4+ messages in thread

end of thread, other threads:[~2023-01-12 10:16 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2019-04-24  7:51 [RFC] Remove PI spec TemporaryRamSupport PPI from PEI Core Jordan Justen
2019-04-24 16:36 ` Laszlo Ersek
2019-04-25  0:09 ` [edk2-devel] " Nate DeSimone
2023-01-12 10:16   ` Ni, Ray

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