linux-kernel - Re: [PATCH] perf: Change PMCR write to read-modify-write

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Message-ID: <20190402165054.GB1442@fuggles.cambridge.arm.com>
Date:   Tue, 2 Apr 2019 17:50:54 +0100
From:   Will Deacon <will.deacon@....com>
To:     Prasad Sodagudi <psodagud@...eaurora.org>
Cc:     mingo@...hat.com, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org, mark.rutland@....com
Subject: Re: [PATCH] perf: Change PMCR write to read-modify-write

[+MarkR]

On Wed, Mar 20, 2019 at 07:07:46PM -0700, Prasad Sodagudi wrote:
> Preserves the bitfields of PMCR_EL0(AArch64) during PMU reset.
> Reset routine should write a 1 to PMCR.C and PMCR.P fields only
> to reset the counters. Other fields should not be changed
> as they could be set before PMU initialization and their
> value must be preserved even after reset.
> 
> Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
> ---
>  arch/arm64/kernel/perf_event.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index 4addb38..0c1afdd 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -868,8 +868,8 @@ static void armv8pmu_reset(void *info)
>  	 * Initialize & Reset PMNC. Request overflow interrupt for
>  	 * 64 bit cycle counter but cheat in armv8pmu_write_counter().
>  	 */
> -	armv8pmu_pmcr_write(ARMV8_PMU_PMCR_P | ARMV8_PMU_PMCR_C |
> -			    ARMV8_PMU_PMCR_LC);
> +	armv8pmu_pmcr_write(armv8pmu_pmcr_read() | ARMV8_PMU_PMCR_P |
> +			ARMV8_PMU_PMCR_C | ARMV8_PMU_PMCR_LC);

No, I don't think this is right at all. Many of these bits are UNKNOWN
out of reset, so you're throwing away our means of getting the PMU into
a deterministic state.

Who is setting which bits that you want to preserve and why?

Will