Date: Wed, 29 Jul 2015 14:31:17 +0200
From: Jiri Olsa <jolsa@redhat.com>
To: kan.liang@intel.com
Cc: a.p.zijlstra@chello.nl, acme@kernel.org, luto@kernel.org, mingo@redhat.com,
        eranian@google.com, ak@linux.intel.com, mark.rutland@arm.com,
        adrian.hunter@intel.com, jolsa@kernel.org, namhyung@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH V3 1/5] perf,tools: introduce generic FEAT for CPU
 attributes
Message-ID: <20150729123117.GF9606@krava.brq.redhat.com>
References: <1438082975-47730-1-git-send-email-kan.liang@intel.com>
 <1438082975-47730-2-git-send-email-kan.liang@intel.com>
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On Tue, Jul 28, 2015 at 07:29:31AM -0400, kan.liang@intel.com wrote:

SNIP

>  
> +static void print_cpu_attributes(struct perf_header *ph, int fd __maybe_unused, FILE *fp)
> +{
> +	fprintf(fp, "# CPU attributes: max frequency = %lu KHz\n", ph->env.cpu_attr[PERF_HEADER_CPU_MAX_FREQ]);
> +}
> +
>  static void print_branch_stack(struct perf_header *ph __maybe_unused,
>  			       int fd __maybe_unused, FILE *fp)
>  {
> @@ -1471,6 +1486,25 @@ static int process_cpuid(struct perf_file_section *section __maybe_unused,
>  	return ph->env.cpuid ? 0 : -ENOMEM;
>  }
>  
> +static int process_cpu_attributes(struct perf_file_section *section __maybe_unused,
> +				  struct perf_header *ph, int fd,
> +				  void *data __maybe_unused)
> +{
> +	ssize_t ret;
> +	u64 nr;
> +
> +	ret = readn(fd, &nr, sizeof(nr));
> +	if (ret != sizeof(nr))
> +		return -1;

you need to ensure backward compatibility, like new perf with
this section extended will be able to read old (current)
perf.data file.. and the other way round probably too

adding a identifier 'tag' for each value might do it

jirka
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