linux-kernel - Re: [PATCH] drivers: perf: arm_pmuv3: Update 'pmc

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Message-ID: <ZSe27i20fjvh6zSk@FVFF77S0Q05N>
Date:   Thu, 12 Oct 2023 10:05:50 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers: perf: arm_pmuv3: Update 'pmc_width' based on
 actual HW event width

On Mon, Oct 09, 2023 at 10:07:24AM +0530, Anshuman Khandual wrote:
> This updates 'perf_event_mmap_page->pmc_width' based on actual HW event's
> width that are currently missing i.e ARMPMU_EVT_63BIT and ARMPMU_EVT_47BIT.
> 
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
> This applies on v6.6-rc5.
> 
>  drivers/perf/arm_pmuv3.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
> index fe4db1831662..94723d00548e 100644
> --- a/drivers/perf/arm_pmuv3.c
> +++ b/drivers/perf/arm_pmuv3.c
> @@ -1375,6 +1375,10 @@ void arch_perf_update_userpage(struct perf_event *event,
>  	if (userpg->cap_user_rdpmc) {
>  		if (event->hw.flags & ARMPMU_EVT_64BIT)
>  			userpg->pmc_width = 64;
> +		else if (event->hw.flags & ARMPMU_EVT_63BIT)
> +			userpg->pmc_width = 63;
> +		else if (event->hw.flags & ARMPMU_EVT_47BIT)
> +			userpg->pmc_width = 47;

The PMUv3 driver *never* uses 63-bit or 47-bit counters, so the PMUv3 driver
doesn't need this. The ARMPMU_EVT_63BIT and ARMPMU_EVT_47BIT flags are only
used by the Apple PMU driver, which doesn't support user access, and similarly
doesn't need to handle this.

This code is not necessary as the two new branches can never run. It will also
confuse people into thinking the PMUv3 driver can use these widths when it
cannot.

NAK to this patch for the reasons above.

Mark

>  		else
>  			userpg->pmc_width = 32;
>  	}
> -- 
> 2.25.1
>