linux-kernel - Re: [PATCH] perf: CXL: fix mismatched number of counters mask

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Date:   Tue, 5 Sep 2023 15:28:54 +0100
From:   Will Deacon <will@...nel.org>
To:     Jeongtae Park <jtp.park@...sung.com>
Cc:     linux-cxl@...r.kernel.org,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Kyungsan Kim <ks0204.kim@...sung.com>,
        Wonjae Lee <wj28.lee@...sung.com>,
        Hojin Nam <hj96.nam@...sung.com>,
        Junhyeok Im <junhyeok.im@...sung.com>,
        Jehoon Park <jehoon.park@...sung.com>
Subject: Re: [PATCH] perf: CXL: fix mismatched number of counters mask

On Tue, Sep 05, 2023 at 09:33:09PM +0900, Jeongtae Park wrote:
> The number of Count Units field is described as 6 bits long
> in the CXL 3.0 specification. However, its mask value was
> only declared as 5 bits long.
> 
> Signed-off-by: Jeongtae Park <jtp.park@...sung.com>
> ---
>  drivers/perf/cxl_pmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
> index 0a8f597e695b..365d964b0f6a 100644
> --- a/drivers/perf/cxl_pmu.c
> +++ b/drivers/perf/cxl_pmu.c
> @@ -25,7 +25,7 @@
>  #include "../cxl/pmu.h"
>  
>  #define CXL_PMU_CAP_REG			0x0
> -#define   CXL_PMU_CAP_NUM_COUNTERS_MSK			GENMASK_ULL(4, 0)
> +#define   CXL_PMU_CAP_NUM_COUNTERS_MSK			GENMASK_ULL(5, 0)
>  #define   CXL_PMU_CAP_COUNTER_WIDTH_MSK			GENMASK_ULL(15, 8)
>  #define   CXL_PMU_CAP_NUM_EVN_CAP_REG_SUP_MSK		GENMASK_ULL(24, 20)
>  #define   CXL_PMU_CAP_FILTERS_SUP_MSK			GENMASK_ULL(39, 32)

I don't have access to the CXL spec, but widening this mask looks like
it puts us out-of-whack with CXL_PMU_MAX_COUNTERS.

Did v3.0 of the spec bump the number of counters? If so, can you please
check that this is a backwards-compatible change?

Will