linux-kernel - Re: [PATCH v5 2/4] perf tools arm64: Add support for get_cpuid

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Date:   Wed, 23 Aug 2017 11:17:24 +0100
From:   Will Deacon <will.deacon@....com>
To:     Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        catalin.marinas@....com, mark.rutland@....com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, peterz@...radead.org,
        mingo@...hat.com, jnair@...iumnetworks.com,
        zhangshaokun@...ilicon.com, Robert.Richter@...ium.com,
        gpkulkarni@...il.com
Subject: Re: [PATCH v5 2/4] perf tools arm64: Add support for get_cpuid_str
 function.

On Wed, Aug 16, 2017 at 12:40:46PM +0530, Ganapatrao Kulkarni wrote:
> function get_cpuid_str returns MIDR string of the first online
> cpu from the range of cpus associated with the pmu core device.
> 
> Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>
> ---
>  tools/perf/arch/arm64/util/Build    |  1 +
>  tools/perf/arch/arm64/util/header.c | 61 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 62 insertions(+)
>  create mode 100644 tools/perf/arch/arm64/util/header.c
> 
> diff --git a/tools/perf/arch/arm64/util/Build b/tools/perf/arch/arm64/util/Build
> index cef6fb3..b1ab72d 100644
> --- a/tools/perf/arch/arm64/util/Build
> +++ b/tools/perf/arch/arm64/util/Build
> @@ -1,3 +1,4 @@
> +libperf-y += header.o
>  libperf-$(CONFIG_DWARF)     += dwarf-regs.o
>  libperf-$(CONFIG_LOCAL_LIBUNWIND) += unwind-libunwind.o
>  
> diff --git a/tools/perf/arch/arm64/util/header.c b/tools/perf/arch/arm64/util/header.c
> new file mode 100644
> index 0000000..713ef17
> --- /dev/null
> +++ b/tools/perf/arch/arm64/util/header.c
> @@ -0,0 +1,61 @@
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <api/fs/fs.h>
> +#include "header.h"
> +
> +#define MIDR "/regs/identification/midr_el1"
> +#define MIDR_SIZE 128

Why is this so big?

> +char *get_cpuid_str(struct perf_pmu *pmu)
> +{
> +	char *buf = NULL;
> +	char *temp = NULL;
> +	char path[PATH_MAX];
> +	const char *sysfs = sysfs__mountpoint();
> +	int cpu;
> +	unsigned long long midr = 0;
> +	struct cpu_map *cpus;
> +	FILE *file;
> +
> +	if (!sysfs || !pmu->cpus)
> +		return NULL;
> +
> +	buf = malloc(MIDR_SIZE);
> +	if (!buf)
> +		return NULL;
> +
> +	/* read midr from list of cpus mapped to this pmu */
> +	cpus = cpu_map__get(pmu->cpus);
> +	for (cpu = 0; cpu < cpus->nr; cpu++) {
> +		scnprintf(path, PATH_MAX, "%s/devices/system/cpu/cpu%d"MIDR,
> +				sysfs, cpus->map[cpu]);
> +
> +		file = fopen(path, "r");
> +		if (!file) {
> +			pr_err("fopen failed for file %s\n", path);

pr_debug instead? It's not fatal if a CPU is hotplugged off and you can't
read its midr.

> +			continue;
> +		}
> +
> +		temp = fgets(buf, MIDR_SIZE, file);
> +		if (!temp)
> +			continue;

Do you actually need a temporary variable for this?

> +		fclose(file);
> +
> +		/* Ignore/clear Variant[23:20] and
> +		 * Revision[3:0] of MIDR
> +		 */
> +		midr = strtoll(buf, NULL, 16);
> +		midr &= (~(0xf << 20 | 0xf));

If midr really needs to be unsigned long long (u64 might be better), then
you need to fix these immediates to have a ULL suffix.

Can you use the GENMASK_ULL macro to generate the MIDR mask, or is that
not available here?

> +		scnprintf(buf, MIDR_SIZE, "0x%016llx", midr);
> +		/* got midr break loop */
> +		break;
> +	}
> +
> +	if (!midr) {
> +		free(buf);
> +		buf = NULL;

If you want the pr_err, here would seem more appropriate because at this
point we've actually failed.

Will