linux-kernel - Re: [PATCH v8] ARM: dts: aspeed: Adding Facebook Bletchley BMC

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Date: Tue, 2 Jan 2024 21:21:54 -0600
From: Patrick Williams <patrick@...cx.xyz>
To: Lukas Wunner <lukas@...ner.de>
Cc: Howard Chiu <howard10703049@...il.com>, robh+dt@...nel.org,
	joel@....id.au, andrew@...id.au, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-aspeed@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, potin.lai@...ntatw.com,
	Howard Chiu <howard.chiu@...ntatw.com>,
	linux-integrity@...r.kernel.org, rentao.bupt@...il.com
Subject: Re: [PATCH v8] ARM: dts: aspeed: Adding Facebook Bletchley BMC

On Sat, Dec 23, 2023 at 09:36:23AM +0100, Lukas Wunner wrote:
> On Fri, Dec 22, 2023 at 04:38:12PM -0600, Patrick Williams wrote:
> > On Wed, Dec 20, 2023 at 06:00:12PM +0100, Lukas Wunner wrote:
> > > If chips are dual-sourced or triple-sourced, as you say, and they
> > > behave identically, then I think it is fine to specify all of their
> > > compatible strings plus the generic compatible.  
> > 
> > This has explicitly been rejected before; having multiple incompatible
> > chips listed in the same compatible.  I've tried to search lore but I
> > can't find a reference unfortunately.
> 
> I'll let devicetree maintainers comment on that.
> 
> 
> > Furthermore, what you're suggesting does not jive with what is in the
> > devicetree binding documentation for tpm_tis-spi [2]:
> > 
> > - compatible: should be **one** of the following (emphasis mine)
> 
> That's superseded by:
> 
> https://lore.kernel.org/all/cover.1702806810.git.lukas@wunner.de/
> 
> I don't really have a dog in this fight, I merely stepped up to
> convert TPM DT bindings to YAML.  There have been multiple attempts
> to convert them in the past but none of them have been pursued into
> mainline.

Thank you for the effort and context.  I wasn't aware of this pending
change.

> I looked at compatible string usage in arch/arm{,64}/boot/dts
> and was under the impression that the majority of devicetrees
> use a combo matching this pattern:
> "vendor,chip", "tcg,tpm[_-]tis-{spi,i2c,mmio}"
> 
> So that's what I went for in the conversion.  It would be inconsistent
> to enforce a generic compatible for i2c and mmio, but not for spi.
> 
> I ran the validator against all arm/arm64 devicetrees and there are
> four devicetrees which only use a generic compatible and not a
> "vendor,chip" compatible:
> arch/arm/boot/dts/aspeed/aspeed-bmc-facebook-bletchley.dts
> arch/arm/boot/dts/ast2600-facebook-netbmc-common.dtsi
> arch/arm/boot/dts/aspeed-bmc-facebook-wedge400.dts
> arch/arm/boot/dts/am335x-moxa-uc-2100-common.dtsi

After some investigation, it should be safe to use "infineon,slb9670"
for all of the facebook systems.  If you want to add that to your patch
set you can cc me and Tao Ren (rentao.bupt@...il.com) and I will at
least give my Reviewed-by.

> So, three Aspeed Facebook and one Moxa.  There's a fifth case (phyTEC)
> but the devicetree author clarified it's an Infineon SLB9670.
> The authors of the other four devicetrees listed above did not respond.
> 
> Patches to fix up schema violations are here:
> https://github.com/l1k/linux/commit/7813a455ed15393df7d9d353173635b98ae23387
> https://github.com/l1k/linux/commit/a958be44952b1de170100be1007780a72ce7d861
> 
> 
> > As I said,
> > these are pluggable modules and not simply second-source builds.  There
> > are a collection of modules that can all be plugged into the same header.
> > They might not even be shipped with the device...
> 
> If those TPM modules might not even be plugged in or are interchangeable,
> I think they ought to be represented as DT overlays.

I still don't think DT overlays are appropriate for TPMs as it
effectively extends the attack surface for the kernel PCRs all the way
until you can run enough code to load the appropriate DT overlay,
which is likely somewhere in userspace.  This seriously diminishes the
value of measured boot.

> Honestly I'm wondering how common the scenario you're describing is.
> If it's an edge case, it might not be worth holding up the YAML
> conversion because of it.  The missing YAML conversion is a constant
> cause of pain for a lot of people.

Understood.

Since any of the chips we might be using are currently equivalent from a
driver perspective with the generic TCG spec (and the infineon,slb9670
compatible) we should be fine.  If there becomes an incompatibility in
the future with the tpm_tis_spi code we'll cross that bridge at that
time.

-- 
Patrick Williams

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