linux-kernel - Re: [PATCH v2 7/8] du-bindings: mips: cpu: Add img,mips compatible

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Message-ID: <20240615-reaction-movie-b4c21f2c7d91@spud>
Date: Sat, 15 Jun 2024 13:28:57 +0100
From: Conor Dooley <conor@...nel.org>
To: Jiaxun Yang <jiaxun.yang@...goat.com>
Cc: Rob Herring <robh@...nel.org>, Lee Jones <lee@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	"paulburton@...nel.org" <paulburton@...nel.org>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	"linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>
Subject: Re: [PATCH v2 7/8] du-bindings: mips: cpu: Add img,mips compatible

On Thu, Jun 13, 2024 at 08:40:18PM +0100, Jiaxun Yang wrote:
> 
> 
> 在2024年6月13日六月 下午7:59，Rob Herring写道：
> > On Wed, Jun 12, 2024 at 05:59:24PM +0100, Jiaxun Yang wrote:
> >> 
> >> 
> >> 在2024年6月12日六月 下午5:39，Conor Dooley写道：
> >> > On Wed, Jun 12, 2024 at 12:56:26PM +0100, Jiaxun Yang wrote:
> >> >> This compatible is used by boston.dts.
> >> >> 
> >> >> Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
> >> >> ---
> >> >> note: This is a wildcard compatible for all MIPS CPUs,
> >> >>       I think we should use something like "riscv" for riscv.
> >> >
> >> > riscv systems, other than simulators etc are not meant to use the
> >> > "riscv" compatible. All of the real CPUs use "vendor,cpu", "riscv".
> >> > I'd suggest you add specific compatibles for your CPUs.
> >> 
> >> Boston can be combined with many different CPUs, thus we need to have
> >> such compatibles.
> >
> > Then you'll need different DTs. Different h/w, different DT.
> 
> The board have 9 CPU types in total, with hundreds of different possible
> CPU topologies. Maintaining separate DT for them seems impossible in kernel.

But you could definitely add 9 different compatibles for each of these
different CPUs.

> We can potentially patch this in bootloader, but for existing firmware it's
> being doing like this for years. I can see for RISC-V QEMU generated DTB is
> using a single "riscv" compatible and I do think it's a similar problem.

That "riscv" compatible is only supposed to be used for
simulators/software models. Real CPUs are not meant to use it. AFAICT,
your boston is a real platform, even if the CPUs are implemented on an
FPGA they should still have one. If you take the OpenC906 RISC-V CPU and
put it on an FPGA, you're still meant to put "thead,c906" in your DT.

> I think it's better to document it and warn people only to use it in limited
> circumstances, instead of keeping such usage in grey area.
> 
> >
> > No way we're taking a generic compatible like this.

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