linux-kernel - Re: [PATCH 4/5] dt-bindings: mips: Document mti,mips-cm

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Message-ID: <20240508-puzzle-directive-b6f771f92fe9@spud>
Date: Wed, 8 May 2024 18:01:03 +0100
From: Conor Dooley <conor@...nel.org>
To: Jiaxun Yang <jiaxun.yang@...goat.com>
Cc: "paulburton@...nel.org" <paulburton@...nel.org>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	"linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 4/5] dt-bindings: mips: Document mti,mips-cm

On Tue, May 07, 2024 at 07:16:25PM +0100, Jiaxun Yang wrote:
> 
> 
> 在2024年5月7日五月 下午5:50，Conor Dooley写道：
> > On Tue, May 07, 2024 at 10:01:52AM +0100, Jiaxun Yang wrote:
> >> Add devicetree binding documentation for MIPS Coherence Manager.
> >> 
> >> Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
> >> ---
> >>  .../devicetree/bindings/mips/mips-cm.yaml          | 37 ++++++++++++++++++++++
> >>  1 file changed, 37 insertions(+)
> >> 
> >> diff --git a/Documentation/devicetree/bindings/mips/mips-cm.yaml b/Documentation/devicetree/bindings/mips/mips-cm.yaml
> >> new file mode 100644
> >> index 000000000000..b92b008d7758
> >> --- /dev/null
> >> +++ b/Documentation/devicetree/bindings/mips/mips-cm.yaml
> Hi Cornor,
> 
> Thanks for your comments.
> 
> >
> > Filename matching the compatible please.
> Ok.
> 
> >
> >> @@ -0,0 +1,37 @@
> >> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> >> +%YAML 1.2
> >> +---
> >> +$id: http://devicetree.org/schemas/mips/mips-cm.yaml#
> >> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> >> +
> >> +title: MIPS Coherence Manager
> >> +
> >> +description: |
> >> +  Defines a location of the MIPS Coherence Manager registers.
> >> +
> >> +maintainers:
> >> +  - Jiaxun Yang <jiaxun.yang@...goat.com>
> >> +
> >> +properties:
> >> +  compatible:
> >> +    const: mti,mips-cm
> >
> > Is it actually only available on mips? Google seems to report there
> > being Coherence Managers on their RISC-V offerings too.
> 
> I think for MIPS's RISC-V system, it is only used by SBI and transparent
> to kernel, so it won't present in DT. 

Devicetree isn't just for Linux, things that only the SBI implementation
cares about should also be documented in bindings - or at least I try to
get them to be, where I have enough sway to have it happen..

> Register fields for RISC-V system is totally different with MIPS one, and
> there is no driver to be reused. In MIPS system CM code is highly coupled
> with arch code, so for RISC-V if we want to expose it to kernel we'll need
> a new set of driver and a new binding.

Right, that's a reasonable reason (lol) for having it be declared as
mips-specific.

> >> +  reg:
> >> +    description: |
> >
> > The | isn't needed, there's no formatting to preserve.
> Ok.
> 
> >
> >> +      Base address and size of an unoccupied memory region, which will be
> >> +      used to map the MIPS CM registers block.
> >
> > This sounds like it should actually be a memory-region that references
> > some reserved memory, not a reg, given the description. I think the
> > commit message here is lacking any information about what the intentions
> > are for this binding.
> So it's actually a register block that can be remapped to anywhere in
> MMIO address space. DeviceTree usually passes firmware's mapping location
> to kernel.
> 
> There are some other similar bindings like mti,mips-cdmm and mti,mips-cpc,
> I just copied phraseology from them, should I try to explain it more here?

The description that you've given here is of something that sounded
awfully like mapping into a location in DDR etc, is it actually being
mapped into a non-memory address?

Thanks,
Conor.

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