Re: [PATCH v4 2/3] cacheinfo: Add arm64 early level initializer implementation

From: Sudeep Holla <sudeep.holla@arm.com>
To: Radu Rendec <rrendec@redhat.com>, Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Pierre Gondois <Pierre.Gondois@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 2/3] cacheinfo: Add arm64 early level initializer implementation
Date: Thu, 13 Apr 2023 11:22:26 +0100	[thread overview]
Message-ID: <20230413102226.yzzd6iccqeamheas@bogus> (raw)
In-Reply-To: <20230412185759.755408-3-rrendec@redhat.com>

Hi Will,

On Wed, Apr 12, 2023 at 02:57:58PM -0400, Radu Rendec wrote:
> This patch adds an architecture specific early cache level detection
> handler for arm64. This is basically the CLIDR_EL1 based detection that
> was previously done (only) in init_cache_level().
> 
> This is part of a patch series that attempts to further the work in
> commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU").
> Previously, in the absence of any DT/ACPI cache info, architecture
> specific cache detection and info allocation for secondary CPUs would
> happen in non-preemptible context during early CPU initialization and
> trigger a "BUG: sleeping function called from invalid context" splat on
> an RT kernel.
> 
> This patch does not solve the problem completely for RT kernels. It
> relies on the assumption that on most systems, the CPUs are symmetrical
> and therefore have the same number of cache leaves. The cacheinfo memory
> is allocated early (on the primary CPU), relying on the new handler. If
> later (when CLIDR_EL1 based detection runs again on the secondary CPU)
> the initial assumption proves to be wrong and the CPU has in fact more
> leaves, the cacheinfo memory is reallocated, and that still triggers a
> splat on an RT kernel.
> 
> In other words, asymmetrical CPU systems *must* still provide cacheinfo
> data in DT/ACPI to avoid the splat on RT kernels (unless secondary CPUs
> happen to have less leaves than the primary CPU). But symmetrical CPU
> systems (the majority) can now get away without the additional DT/ACPI
> data and rely on CLIDR_EL1 based detection.
> 

If you are okay with the change, can I have your Acked-by, so that I can
route this via Greg's tree ?

-- 
Regards,
Sudeep