linux-kernel - Re: [PATCH 4/7] perf cs-etm: Don't use hard coded config bits when setting up ETMCR

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Message-ID: <96c54895-cebe-4247-86e6-d41b8be0dd40@linaro.org>
Date: Thu, 4 Dec 2025 10:55:37 +0000
From: James Clark <james.clark@...aro.org>
To: Leo Yan <leo.yan@....com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Mike Leach
 <mike.leach@...aro.org>, John Garry <john.g.garry@...cle.com>,
 Will Deacon <will@...nel.org>, Leo Yan <leo.yan@...ux.dev>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 4/7] perf cs-etm: Don't use hard coded config bits when
 setting up ETMCR



On 02/12/2025 11:53 am, James Clark wrote:
> 
> 
> On 02/12/2025 11:43 am, Leo Yan wrote:
>> On Mon, Dec 01, 2025 at 04:41:07PM +0000, Coresight ML wrote:
>>
>> [...]
>>
>>> @@ -746,7 +779,7 @@ static void cs_etm_get_metadata(struct perf_cpu 
>>> cpu, u32 *offset,
>>>       case CS_ETMV3:
>>>           magic = __perf_cs_etmv3_magic;
>>>           /* Get configuration register */
>>> -        info->priv[*offset + CS_ETM_ETMCR] = cs_etm_get_config(itr);
>>> +        info->priv[*offset + CS_ETM_ETMCR] = cs_etm_guess_etmcr(itr);
>>
>> I still think cs_etm_get_config() is better than cs_etm_guess_etmcr().
>>
>> For ETMv3, we directly pass CONFIG to the kernel, and after validation
>> in the dirver, then the value will be set to ETMCR.  If we already know
>> the config value is consistent between user space and kernel, why

One other note is that since moving the timestamp field, this is no 
longer true either. The value in attr.config isn't directly put into ETMCR.

>> introduce a redundant "guess" operation here?
>>
>> Thanks,
>> Leo
> 
> Because userspace doesn't always come up with the same value as the 
> driver. For example right now in ETM3, ETMCR_RETURN_STACK isn't set 
> depending on certain conditions that userspace doesn't know about.  ETM4 
> has the same for TRCCONFIGR_RS and maybe some others. In the future, 
> other versions of the driver could do different things as long as we 
> don't break decoding.
> 
> I didn't want the function name to imply it was doing something it 
> wasn't as that confused me a little bit. It's definitely not "getting" 
> the value. Maybe "guess" isn't the best it could be, but it's not far off.
>