_DSD standardization note (WAS: Re: [PATCH 2/2] net, thunder, bgx: Add support for ACPI binding.)

[Jon Masters <jcm@redhat.com>]

Following up on this thread after finally seeing it...figured I would
send something just for the archive mainly (we discussed this in person
recently at a few different events and I think are aligned already).

On 08/07/2015 08:28 PM, Rafael J. Wysocki wrote:
> Hi David,
> 
> On Sat, Aug 8, 2015 at 2:11 AM, David Daney <ddaney@caviumnetworks.com> wrote:
>> On 08/07/2015 05:05 PM, Rafael J. Wysocki wrote:
> 
> [cut]
> 
>>>
>>> It is actually useful to people as far as I can say.
>>>
>>> Also, if somebody is going to use properties with ACPI, why whould
>>> they use a different set of properties with DT?

Generally speaking, if there's a net new thing to handle, there is of
course no particular problem with using DT as an inspiration, but we
need to be conscious of the fact that Linux isn't the only Operating
System that may need to support these bindings, so the correct thing (as
stated by many of you, and below, and in person also recently - so we
are aligned) is to get this (the MAC address binding for _DSD in ACPI)
standardized properly through UEFI where everyone who has a vest OS
interest beyond Linux can also have their own involvement as well. As
discussed, that doesn't make it part of ACPI6.0, just a binding.

FWIW I made a decision on the Red Hat end that in our guidelines to
partners for ARM RHEL(SA - Server for ARM) builds we would not generally
endorse any use of _DSD, with the exception of the MAC address binding
being discussed here. In that case, I realized I had not been fully
prescriptive enough with the vendors building early hw in that I should
have realized this would happen and have required that they do this the
right way. MAC IP should be more sophisticated such that it can handle
being reset even after the firmware has loaded its MAC address(es).
Platform flash storage separate from UEFI variable storage (which is
being abused to contain too much now that DXE drivers then load into the
ACPI tables prior to exiting Boot Services, etc.) should contain the
actual MAC address(es), as it is done on other arches, and it should not
be necessary to communicate this via ACPI tables to begin with (that's a
cost saving embedded concept that should not happen on server systems in
the general case). I have already had several unannounced future designs
adjusted in light of this _DSD usage.

In the case of providing MAC address information (only) by _DSD, I
connected the initial ARMv8 SoC silicon vendors who needed to use such a
hack to ensure they were using the same properties, and will followup
off list to ensure Cavium are looped into that. But, we do need to get
the _DSD property for MAC address provision standardized through UEFI
properly as an official binding rather than just a link on the website,
and then we need to be extremely careful not to grow any further
dependence upon _DSD elsewhere. Generally, if you're using that approach
on a server system (other than for this MAC case), your firmware or
design (or both) need to be modified to not use _DSD.

Jon.