[PATCHv4] arm64: dmi: Add SMBIOS/DMI support

[Leif Lindholm]

Hi Jon,

On Sat, Jan 17, 2015 at 12:57:38AM -0500, Jon Masters wrote:
> Hi Yi Li,

Yi has gone back to work inside Huawei, so probably did not receive
your messags.

> I would like some advice on the below patch. First, a little background.
> 
> I /may/ need to use SMBIOS provided data on ARMv8 systems to enumerate
> the number of physical underlying CPU sockets present.

This sounds like a very risky precedent to set. Hardware description for
the kernel comes in the form of DT or ACPI. SMBIOS is hardware
description for userland.

x86 added internal access to SMBIOS tables to be able to work around
known-bad firmware. I would say we're in a much better place today
with regards to people actually testing hardware with Linux than when
that happened, so would really like to avoid opening those floodgates
on ARM*.

The rest of this email is therefore assuming we are talking about a
temporary out-of-tree patch kept for development purposes until a
better solution can be devised to upstream.

> This is provided
> in one Type4 DMI structure per physical socket. Using that fact (such
> data is already provided by all of the reference ARMv8 server systems),
> and a variable interpretation of the CPU affinity data, we can refactor
> the topology.c code to understand threads vs cores vs sockets without
> getting confused by the extra levels of hierarchy for clusters.
> 
> I am going back over some older patches, including this one to
> understand the DMI port, and I notice that you call dmi_scan_machine
> from an early_initcall. This will run from the init thread, after we've
> done basic SMP setup in smp_prepare_cpus, which is where we stash
> cpu_topology data, which is after I already need DMI data available.

Yep.

> So. If I wanted this available prior to early_initcall time, would it be
> reasonable to follow e.g. x86 and move this early in the boot? I'm not
> necessarily going to ask to upstream such a change (since I expect to be
> able to solve the underlying problem I am looking to address in a
> different fashion soon), but first want to know if it would be
> reasonable to ask to do that anyway.

My suggestion would be to not try to over-engineer a patch not going
upstream. Move the call before smp_prepare_cpus() in init/main.c.

Or, if you can be fairly certain your SMBIOS table is covered by the
linear mapping, move the call to setup_arch().

> P.S. I'm not interested at all in replies pointing me to the existing DT
> cpu topology binding, which I am well aware of, but it does also not
> actually solve the problem I have :)

Not going to point, but perhaps the correct answer is to ensure the
information you need is provided via DT/ACPI. As your email hints at.

/
    Leif