linux-hardening - Re: [PATCH 2/5] dt-bindings: arm: ti: Add compatible for AM625-based TQMa62xx SOM family and carrier board

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Message-ID: <20241106-happy-anything-46f7293f6aca@spud>
Date: Wed, 6 Nov 2024 16:40:54 +0000
From: Conor Dooley <conor@...nel.org>
To: Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc: Nishanth Menon <nm@...com>, Vignesh Raghavendra <vigneshr@...com>,
	Tero Kristo <kristo@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kees Cook <kees@...nel.org>, Tony Luck <tony.luck@...el.com>,
	"Guilherme G. Piccoli" <gpiccoli@...lia.com>,
	Felipe Balbi <balbi@...nel.org>,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	linux-hardening@...r.kernel.org, Devarsh Thakkar <devarsht@...com>,
	Hari Nagalla <hnagalla@...com>, linux@...tq-group.com
Subject: Re: [PATCH 2/5] dt-bindings: arm: ti: Add compatible for AM625-based
 TQMa62xx SOM family and carrier board

On Wed, Nov 06, 2024 at 01:03:08PM +0100, Matthias Schiffer wrote:
> On Tue, 2024-11-05 at 18:55 +0000, Conor Dooley wrote:
> > On Tue, Nov 05, 2024 at 11:40:20AM +0100, Matthias Schiffer wrote:
> > > On Mon, 2024-11-04 at 18:47 +0000, Conor Dooley wrote:
> > > > On Mon, Nov 04, 2024 at 10:47:25AM +0100, Matthias Schiffer wrote:
> > > > > The TQMa62xx is a SoM family with a pluggable connector. The MBa62xx is
> > > > > the matching reference/starterkit carrier board.
> > > > 
> > > > Why all the wildcards? Why isn't there a compatible per device in the
> > > > family?
> 
> Because all variants use the same Device Tree. There is also only one compatible and one (main) DTSI
> for the AM62 SoC family, which our Device Trees are based on.

So what varies between the members of the family?

> > > For the compatible string we've chosen the TQMa6254 as the representative for the TQMa62xx family.
> > 
> > And all the boards in the family are the exact same?
> 
> There is a single TQMa62xx PCB, which has some AM62 family SoC installed (AM6254 in the case of the
> TQMa6254, but all AM62 are possible). TQMa62xx is also the name used for marketing when not talking
> about a specific SoC variant:
> https://www.tq-group.com/en/products/tq-embedded/arm-architecture/tqma62xx/
> 
> Some SoM variants with different RAM/eMMC/SPI-NOR/... do exist, but they don't have separate device
> trees (firmware may patch some variant information into the DTB however, like the correct RAM size).
> 
> Choosing one representative for the family including the SoC variant number, but not distinguishing
> minor variants matches the level of detail used for our other SOMs already supported by mainline
> Linux (like the TQMa64xxL and various i.MX-based platforms).

I don't like any of this wildcard stuff at all, who is to say that the
next soc you put on your SoM won't be an am62a7, which has a specific
compatible in the kernel? Your fallback then would be ti,am62a7 not
ti,am625. Probably someone will say "that's the am62a family not the
am62 family" - but that exact thing is why I hate all of this
wildcarding. It's barely any more effort to have a tqm6231 and a tqm6254
compatible than what you're doing with wildcard but it is unambiguous.

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