Re: [PATCH v3 1/2] arm64: topology: move store_cpu_topology() to shared code

From: <Conor.Dooley@microchip.com>
To: <sudeep.holla@arm.com>, <gregkh@linuxfoundation.org>
Cc: <mail@conchuod.ie>, <paul.walmsley@sifive.com>,
	<palmer@dabbelt.com>, <palmer@rivosinc.com>,
	<aou@eecs.berkeley.edu>, <catalin.marinas@arm.com>,
	<will@kernel.org>, <rafael@kernel.org>,
	<Daire.McNamara@microchip.com>, <niklas.cassel@wdc.com>,
	<damien.lemoal@opensource.wdc.com>, <geert@linux-m68k.org>,
	<zong.li@sifive.com>, <kernel@esmil.dk>, <hahnjo@hahnjo.de>,
	<guoren@kernel.org>, <anup@brainfault.org>,
	<atishp@atishpatra.org>, <heiko@sntech.de>,
	<philipp.tomsich@vrull.eu>, <robh@kernel.org>, <maz@kernel.org>,
	<viresh.kumar@linaro.org>, <linux-riscv@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>,
	<linux-arm-kernel@lists.infradead.org>, <Brice.Goglin@inria.fr>
Subject: Re: [PATCH v3 1/2] arm64: topology: move store_cpu_topology() to shared code
Date: Mon, 11 Jul 2022 16:39:01 +0000	[thread overview]
Message-ID: <4090fbfa-648c-5299-d3db-01efd2ebc62b@microchip.com> (raw)
In-Reply-To: <20220711152423.djfqk6wbhmrshbkr@bogus>

On 11/07/2022 16:24, Sudeep Holla wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Mon, Jul 11, 2022 at 04:50:38PM +0200, Greg Kroah-Hartman wrote:
>> On Mon, Jul 11, 2022 at 03:35:42PM +0100, Sudeep Holla wrote:
>>> On Sat, Jul 09, 2022 at 04:23:54PM +0100, Conor Dooley wrote:
>>>> From: Conor Dooley <conor.dooley@microchip.com>
>>>>
>>>> arm64's method of defining a default cpu topology requires only minimal
>>>> changes to apply to RISC-V also. The current arm64 implementation exits
>>>> early in a uniprocessor configuration by reading MPIDR & claiming that
>>>> uniprocessor can rely on the default values.
>>>>
>>>> This is appears to be a hangover from prior to '3102bc0e6ac7 ("arm64:
>>>> topology: Stop using MPIDR for topology information")', because the
>>>> current code just assigns default values for multiprocessor systems.
>>>>
>>>> With the MPIDR references removed, store_cpu_topolgy() can be moved to
>>>> the common arch_topology code.
>>>>
>>>
>>> Looks good. FWIW,
>>>
>>> Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
>>>
>>>> CC: stable@vger.kernel.org
>>>
>>> However, while I understand the reason why this is needed in stable trees
>>> for RISC-V, I am not sure if we want this for stable tree at-least on arm64.
>>> I leave that part to Greg and Will.
>>
>> Why would it be good for one arch but bad for another?
> 
> Not really bad as such. Just needs testing and must not change much ideally,
> but it really depends on which stable trees we will target and what is the
> original state there. As mentioned in the commit, this changed a bit around
> v5.8/9 on arm64 and not sure what kernels RISC-V needs this. There could
> be some surprises on some Andriod platforms but that is something we can
> look at when if and when there are complaints.
> 
> I am in general not sure what is the -stable tree rules is such situation and
> hence made the noise so that we are aware that we may need more work than just
> backporting this patch. Also this is just my opinion. If we decide to backport
> esp. to kernels older than the one containing 3102bc0e6ac7, then arm64 may need
> more changes or probably we can pull that commit if that makes it easier. Based
> on what is decided and what are the targeted -stable trees, we can dig deeper.

There's always the option of, for the older kernels, not migrating arm64 at all
and just wrap store_cpu_topo with "if RISCV" rather than "if RISCV || ARM64".

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