linux-kernel - Re: [PATCH] riscv: Move call to init_cpu_topology() to later initialization stage

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Message-ID: <20230104125632.ktoyt7mxjjxq5udm@bogus>
Date:   Wed, 4 Jan 2023 12:56:32 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Conor Dooley <conor@...nel.org>
Cc:     Leyfoon Tan <leyfoon.tan@...rfivetech.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ley Foon Tan <lftan.linux@...il.com>
Subject: Re: [PATCH] riscv: Move call to init_cpu_topology() to later
 initialization stage

On Wed, Jan 04, 2023 at 12:18:28PM +0000, Conor Dooley wrote:
> Hey Sudeep,
> 
> On Wed, Jan 04, 2023 at 10:49:00AM +0000, Sudeep Holla wrote:
> > On Wed, Jan 04, 2023 at 09:49:48AM +0000, Conor Dooley wrote:
> > 
> > [...]
> > 
> > > >> Uhh, so where did this "capacity-dmips-mhz" property actually come from?
> > > >> I had a quick check of qemu with grep & I don't see anything there that
> > > >> would add this property.
> > > >> This property should not be valid on anything other than arm AFAICT.
> > > >
> > > >This DT parameter is not in default Qemu. I've added it for testing (see test steps in below). 
> > > >This is preparation to support asymmetric CPU topology for RISC-V.
> > > 
> > > The property is only valid on arm, so how does arm64 deal with such
> > > asymmetric topologies without it?
> > 
> > I don't think we can deal with asymmetric topologies without this.
> > Yes we can detect the difference in the CPU types but we can only assume
> > there are symmetric in terms of performance in absence of this property.
> 
> 
> I looked at the bindings for it and forgot that the arm directory of
> bindings applies to both arm and arm64. I see now that it is also used
> on arm64.
> 
> > 
> > > Why should we "fix" something that may never be a valid dts?
> > >
> > 
> > I would not say invalid. But surely absence of it must be handled and
> > we do that for sure. IIRC, here the presence of it is causing the issue.
> > And if it is present means someone is trying to build it(I do understand
> > this is Qemu but is quite common these days for power and performance
> > balance in many SoC)
> 
> I said "invalid" as the binding is defined for arm{,64} in arm/cpus.yaml
> & documented in the same directory in cpu-capacity.txt, but not yet on
> riscv. All bets are off if your cpu node is using invalid properties
> IMO, at least this one will fail to boot!
> 
> However, I see no reason (at this point) that we should deviate from
> what arm{,64} is doing & that documenation should probably move to a
> shared location at some point.
>

I prefer making this binding generic rather than patching to handle RISC-V
differently in the generic code. Since it is optional, the platform
need not use it if it is not needed.

-- 
Regards,
Sudeep