linux-kernel - RE: [PATCH v3] dcdbas: Add support for WSMT ACPI table

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Message-ID: <73c53c23c0824e4595292bfafc6054ee@ausx13mpc120.AMER.DELL.COM>
Date:   Mon, 2 Jul 2018 16:15:12 +0000
From:   <Mario.Limonciello@...l.com>
To:     <andy.shevchenko@...il.com>
CC:     <stuart.w.hayes@...il.com>, <dvhart@...radead.org>,
        <linux-kernel@...r.kernel.org>,
        <platform-driver-x86@...r.kernel.org>
Subject: RE: [PATCH v3] dcdbas: Add support for WSMT ACPI table

> 
> > I don't believe SMM communication ACPI table has ever been implemented by
> Dell
> > on server or client BIOS.  I do agree this table describes the behavior that DCDBAS
> driver
> > has used since before even UEFI BIOS pretty accurately.
> 
> So, EPS table has been for ages in Dell machines?
> Can we consider it as a predecessor of that SMM communication ACPI table?

No, EPS is new this year, specifically for server BIOS to be able to support SMM communication
when WSMT is enabled.  The code tests in Stuart's patch will detect if WSMT is enabled
and if it's enabled test if EPS was defined.  On server BIOS when EPS is defined dcdbas
will be able to communicate using addresses defined in EPS.

Server BIOS will support EPS for applications using dcdbas interface and may at a later time
introduce same WMI interface as client too (but applications will need time to update so
they need to support both).

Actually Stuart's patch will cause client BIOS that has WSMT enabled make dcdbas fail
initialization (as it should because dcdbas doesn't have a region that it can successfully
communicate).  

In client machines we moved this communication to ACPI buffer allocated by WMI, which
is why we have dell-smbios-wmi now in kernel.

I think once some variation of Stuart's patch is merged, I'll send a follow
up patch to drop this test because it's no longer necessary:
https://github.com/torvalds/linux/blob/master/drivers/platform/x86/dell-smbios-smm.c#L106

> 
> > Stuart and I did discuss with server BIOS (who uses this EPS mechanism) to see if
> its possible
> > to move EPS to SMM communication ACPI table however since it's been
> deprecated by
> > UEFI 2.7 they weren't willing to adopt it.
> 
> It's pity, but the motivation to deprecate is "lack of use" which is
> not true. That's why I would suggest to escalate this to UEFI
> committee.
> 
> > Stuart, anything else you want to add here?
> 
> Darren, what's your opinion about this?
> 
> P.S. I'm not against this approach (just some technical comments I
> already shared), but on the other hand it would be nice to have undo
> that deprecation and follow the standard in new firmwares.
> Would you agree?

Sure.  Due to the timing of how long this will take, even if SMM communication
ACPI table is undone from deprecation we may have to still support both EPS
and SMM communication ACPI table though (maybe it would be order of preference).