linux-kernel - Re: [PATCH v2 12/17] coresight: trbe: Add a helper to fetch cpudata from perf handle

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Date:   Thu, 7 Oct 2021 10:18:42 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        maz@...nel.org, catalin.marinas@....com, mark.rutland@....com,
        james.morse@....com, anshuman.khandual@....com, leo.yan@...aro.org,
        mike.leach@...aro.org, will@...nel.org, lcherian@...vell.com,
        coresight@...ts.linaro.org
Subject: Re: [PATCH v2 12/17] coresight: trbe: Add a helper to fetch cpudata
 from perf handle

On 06/10/2021 18:15, Mathieu Poirier wrote:
> On Tue, Oct 05, 2021 at 11:35:13PM +0100, Suzuki K Poulose wrote:
>> Hi Mathieu
>>
>> On 04/10/2021 18:42, Mathieu Poirier wrote:
>>> On Tue, Sep 21, 2021 at 02:41:16PM +0100, Suzuki K Poulose wrote:
>>>> Add a helper to get the CPU specific data for TRBE instance, from
>>>> a given perf handle. This also adds extra checks to make sure that
>>>> the event associated with the handle is "bound" to the CPU and is
>>>> active on the TRBE.
>>>>
>>>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>>>> Cc: Mike Leach <mike.leach@...aro.org>
>>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>>> Cc: Leo Yan <leo.yan@...aro.org>
>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>>> ---
>>>>    drivers/hwtracing/coresight/coresight-trbe.c | 12 ++++++++++--
>>>>    1 file changed, 10 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
>>>> index 983dd5039e52..797d978f9fa7 100644
>>>> --- a/drivers/hwtracing/coresight/coresight-trbe.c
>>>> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
>>>> @@ -268,6 +268,15 @@ static unsigned long trbe_snapshot_offset(struct perf_output_handle *handle)
>>>>    	return buf->nr_pages * PAGE_SIZE;
>>>>    }
>>>> +static inline struct trbe_cpudata *
>>>> +trbe_handle_to_cpudata(struct perf_output_handle *handle)
>>>> +{
>>>> +	struct trbe_buf *buf = etm_perf_sink_config(handle);
>>>> +
>>>> +	BUG_ON(!buf || !buf->cpudata);
>>>> +	return buf->cpudata;
>>>> +}
>>>> +
>>>>    /*
>>>>     * TRBE Limit Calculation
>>>>     *
>>>> @@ -533,8 +542,7 @@ static enum trbe_fault_action trbe_get_fault_act(struct perf_output_handle *hand
>>>>    {
>>>>    	int ec = get_trbe_ec(trbsr);
>>>>    	int bsc = get_trbe_bsc(trbsr);
>>>> -	struct trbe_buf *buf = etm_perf_sink_config(handle);
>>>> -	struct trbe_cpudata *cpudata = buf->cpudata;
>>>> +	struct trbe_cpudata *cpudata = trbe_handle_to_cpudata(handle);
>>>
>>> There is two other places where this pattern is present:  is_perf_trbe() and
>>> __trbe_normal_offset().
>>
>> I skipped them, as they have to get access to the "trbe_buf" anyways.
>> So the step by step, made sense. But I could replace them too to make it
>> transparent.
>>
>> What do you think ?
> 
> Humm...  I don't think there is a right way or a wrong way here.  If we move
> forward with this patchset we have two ways of getting to buf->cpudata.  One
> using trbe_handle_to_cpudata() and another one as laid out in is_perf_trbe() and
> __trbe_normal_offset(), each with an equal number of occurences (2 for each).
> 
> I am usually not fond of small functions like trbe_handle_to_cpudata() and to me
> keeping the current heuristic in trbe_get_fault_act() would have been just fine.

There is another user introduced in the work around patch. But, yes, I
agree, we could open code it, rather than having it inconsistent across
the driver.

> I agree with the argument that trbe_handle_to_cpudata() provides more checks but
> is it really worth it if they aren't done everywhere?
> 
> In short I would get rid of trbe_handle_to_cpudata() entirely and live without
> the extra checks... But I'm not strongly opinionated on this either.

Ok, I will remove this then. Thanks for the feedback.

Suzuki