linux-kernel - Re: [PATCH] perf: ARM_SMMU_V3_PMU should depend on ARM_SMMU

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Message-ID: <CAMuHMdVTNP8YyUCvA_LxH1uHWPrF6oLK6gmo4f998nSCXha59g@mail.gmail.com>
Date:   Wed, 16 Aug 2023 13:36:15 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: ARM_SMMU_V3_PMU should depend on ARM_SMMU_V3

Hi Robin,

On Wed, Aug 16, 2023 at 1:23 PM Robin Murphy <robin.murphy@....com> wrote:
> On 2023-08-15 16:36, Geert Uytterhoeven wrote:
> > There is no point in monitoring transactions passing through the SMMU
> > when ARM Ltd. System MMU Version 3 (SMMUv3) Support is disabled.
> > Hence replace the dependency on ARM64 by a dependency on ARM_SMMU_V3
> > (which implies the former).
> >
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > ---
> > This caught my eye after commit 7c3f204e544dfa37 ("perf/smmuv3: Remove
> > build dependency on ACPI") in arm64/for-next/core.
> >
> > Perhaps my understanding is wrong? Is there anything to monitor when
> > ARM_SMMU_V3=n?
>
> Yes, at least TBU event 1 still counts bypass transactions even when the
> SMMU is disabled, so PMCGs can be useful as basic traffic monitors in
> their own right. Plus the original design intent was that PMCGs may also
> be implemented by other things that interact with the SMMU, like
> ATS-capable PCIe root complexes, or devices with their own internal
> TLBs, thus they could potentially count any manner of
> implementation-defined events that aren't necessarily related to SMMU
> translation.

Thanks for the explanation!
Hereby I withdraw my patch.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds