linux-kernel - Re: [PATCH v4 4/5] arm64/perf: Enable PMCR long cycle counter bit

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Date:	Mon, 29 Feb 2016 15:39:35 +0000
From:	Will Deacon <will.deacon@....com>
To:	Jan Glauber <jglauber@...ium.com>
Cc:	Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, marc.zyngier@....com
Subject: Re: [PATCH v4 4/5] arm64/perf: Enable PMCR long cycle counter bit

Hi Jan,

I've queued this lot on my perf/updates branch, but I just noticed an
oddity whilst dealing with some potential conflicts with the kvm tree.

On Thu, Feb 18, 2016 at 05:50:13PM +0100, Jan Glauber wrote:
> With the long cycle counter bit (LC) disabled the cycle counter is not
> working on ThunderX SOC (ThunderX only implements Aarch64).
> Also, according to documentation LC == 0 is deprecated.
> 
> To keep the code simple the patch does not introduce 64 bit wide counter
> functions. Instead writing the cycle counter always sets the upper
> 32 bits so overflow interrupts are generated as before.
> 
> Original patch from Andrew Pinksi <Andrew.Pinksi@...iumnetworks.com>
> 
> Signed-off-by: Jan Glauber <jglauber@...ium.com>
> ---
>  arch/arm64/kernel/perf_event.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index 0ed05f6..c68fa98 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -405,6 +405,7 @@ static const struct attribute_group *armv8_pmuv3_attr_groups[] = {
>  #define ARMV8_PMCR_D		(1 << 3) /* CCNT counts every 64th cpu cycle */
>  #define ARMV8_PMCR_X		(1 << 4) /* Export to ETM */
>  #define ARMV8_PMCR_DP		(1 << 5) /* Disable CCNT if non-invasive debug*/
> +#define ARMV8_PMCR_LC		(1 << 6) /* Overflow on 64 bit cycle counter */
>  #define	ARMV8_PMCR_N_SHIFT	11	 /* Number of counters supported */
>  #define	ARMV8_PMCR_N_MASK	0x1f
>  #define	ARMV8_PMCR_MASK		0x3f	 /* Mask for writable bits */

You haven't extended this mask to cover the LC bit, so it will be ignored
by armv8pmu_pmcr_write afaict.

How did you test this? I can easily update the mask, but it would be
good to know that it doesn't end up cause a breakage.

Will