linux-kernel - Re: [PATCH v2] perf tool: fix endianness handling of u32 data in samples

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Message-ID: <4E612A9E.5000807@gmail.com>
Date:	Fri, 02 Sep 2011 13:12:30 -0600
From:	David Ahern <dsahern@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
CC:	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu, peterz@...radead.org, fweisbec@...il.com,
	paulus@...ba.org, tglx@...utronix.de, anton@...ba.org
Subject: Re: [PATCH v2] perf tool: fix endianness handling of u32 data in
 samples

On 09/02/2011 12:18 PM, Arnaldo Carvalho de Melo wrote:
> Em Fri, Sep 02, 2011 at 10:03:46AM -0600, David Ahern escreveu:
>>  	if (type & PERF_SAMPLE_RAW) {
>> -		u32 *p = (u32 *)array;
>> +		u.val64 = *array;
>> +		if (swapped) {
>> +			static bool show_warn = true;
>> +
>> +			/* undo swap of u64, then swap on individual u32s */
>> +			u.val64 = bswap_64(u.val64);
>> +			u.val32[0] = bswap_32(u.val32[0]);
>> +			u.val32[1] = bswap_32(u.val32[1]);
>> +
>> +			if (show_warn) {
>> +				pr_warning("Endianness of raw data not corrected!\n");
>> +				show_warn = false;
>> +			}
>> +		}
> 
> Can you use WARN_ONCE? Would become:
> 	
> 		if (WARN_ONCE(swapped, "Endianness of raw data not corrected!\n")) {
> 			/* undo swap of u64, then swap on individual u32s */
> 			u.val64 = bswap_64(u.val64);
> 			u.val32[0] = bswap_32(u.val32[0]);
> 			u.val32[1] = bswap_32(u.val32[1]);
> 		}

That's not quite what we need. The bswap's happen all the time; the warn
once is to tell the user one time that samples in the file contain raw
data and those cannot be programmatically adjusted for endianness.

ie., more like:
    WARN_ONCE(swapped, "Endianness of raw data not corrected!\n");
and no action taken (the 'if (WARN_ONCE())'part).

David


> 
> We have that and its the usual idiom in the kernel proper :-)
> 
> See tools/perf/util/include/asm/bug.h and net/can/af_can.c can_rcv for
> an example.
> 
> Thanks,
> 
> - Arnaldo
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