linux-kernel - Re: [PATCH 2/3] coresight: Fail to open with return stacks if they are unavailable

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Message-ID: <70b562dc-adcd-da34-3811-ff872bc7cab8@arm.com>
Date:   Fri, 7 Jan 2022 15:10:45 +0000
From:   James Clark <james.clark@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        coresight@...ts.linaro.org
Cc:     Mike Leach <mike.leach@...aro.org>, Leo Yan <leo.yan@...aro.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH 2/3] coresight: Fail to open with return stacks if they
 are unavailable



On 09/12/2021 11:13, James Clark wrote:
> 
> 
> On 09/12/2021 11:00, Suzuki K Poulose wrote:
>> On 08/12/2021 16:09, James Clark wrote:
>>> Maintain consistency with the other options by failing to open when they
>>> aren't supported. For example ETM_OPT_TS, ETM_OPT_CTXTID2 and the newly
>>> added ETM_OPT_BRANCH_BROADCAST all return with -EINVAL if they are
>>> requested but not supported by hardware.
>>>
>>> The consequence of not doing this is that the user may not be
>>> aware that they are not enabling the feature as it is silently disabled.
>>>
>>> Signed-off-by: James Clark <james.clark@....com>
>>> ---
>>>   drivers/hwtracing/coresight/coresight-etm4x-core.c | 13 +++++++++----
>>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>>> index d2bafb50c66a..0a9bb943a5e5 100644
>>> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
>>> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>>> @@ -674,10 +674,15 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
>>>       }
>>>         /* return stack - enable if selected and supported */
>>> -    if ((attr->config & BIT(ETM_OPT_RETSTK)) && drvdata->retstack)
>>> -        /* bit[12], Return stack enable bit */
>>> -        config->cfg |= BIT(12);
>>> -
>>> +    if (attr->config & BIT(ETM_OPT_RETSTK)) {
>>> +        if (!drvdata->retstack) {
>>> +            ret = -EINVAL;
>>> +            goto out;
>>> +        } else {
>>> +            /* bit[12], Return stack enable bit */
>>> +            config->cfg |= BIT(12);
>>> +        }
>>
>> nit: While at this, please could you change the hard coded value
>> to ETM4_CFG_BIT_RETSTK ?
>>
> I started changing them all because I had trouble searching for bits by name but then
> I thought it would snowball into a bigger change so I undid it.
> 
> I think I'll just go and do it now if it's an issue here.

Hi Suzuki,

I started on this and I think the only worthwhile change is to make them all consistent
with sysreg.h. As in have xxx_SHIFT and xxx_MASK style definitions like:

  #define TRCCONFIGR_INSTP0_SHIFT		1
  #define TRCCONFIGR_INSTPO_MASK		GENMASK(1,0)

This has been done for SPE and some of the new ETM stuff. If that sounds right to you
I will go and do it as a followup patch to this one. It is quite a bit change so I can
see maybe we don't want to do it? (Personally I would vote to do it)

James

> 
>> Otherwise, looks good to me
>>
>> Suzuki