linux-kernel - Re: [PATCH] Documentation: coresight: Update the generic device names

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Message-ID: <78c98c28-4f3f-825b-18e1-c71fb63a80eb@arm.com>
Date:   Fri, 7 Jun 2019 09:40:48 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     leo.yan@...aro.org
Cc:     mathieu.poirier@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, coresight@...ts.linaro.org,
        corbet@....net
Subject: Re: [PATCH] Documentation: coresight: Update the generic device names

Hi Leo,

>>>>    A Coresight PMU works the same way as any other PMU, i.e the name of the PMU is
>>>>    listed along with configuration options within forward slashes '/'.  Since a
>>>>    Coresight system will typically have more than one sink, the name of the sink to
>>>> -work with needs to be specified as an event option.  Names for sink to choose
>>>> -from are listed in sysFS under ($SYSFS)/bus/coresight/devices:
>>>> +work with needs to be specified as an event option.
>>>> +On newer kernels the available sinks are listed in sysFS under:
>>>> +($SYSFS)/bus/event_source/devices/cs_etm/sinks/
>>>> -	root@...aro-nano:~# ls /sys/bus/coresight/devices/
>>>> -		20010000.etf   20040000.funnel  20100000.stm  22040000.etm
>>>> -		22140000.etm  230c0000.funnel  23240000.etm 20030000.tpiu
>>>> -		20070000.etr     20120000.replicator  220c0000.funnel
>>>> -		23040000.etm  23140000.etm     23340000.etm
>>>> +	root@...alhost:/sys/bus/event_source/devices/cs_etm/sinks# ls
>>>> +	tmc_etf0  tmc_etr0  tpiu0
>>>> -	root@...aro-nano:~# perf record -e cs_etm/@...70000.etr/u --per-thread program
>>>> +On older kernels, this may need to be found from the list of coresight devices,
>>>> +available under ($SYSFS)/bus/coresight/devices/:
>>>> +
>>>> +	root@...alhost:/sys/bus/coresight/devices# ls
>>>> +	etm0  etm1  etm2  etm3  etm4  etm5  funnel0  funnel1  funnel2  replicator0  stm0 tmc_etf0  tmc_etr0  tpiu0
>>>> +
>>>> +	root@...aro-nano:~# perf record -e cs_etm/@..._etr0/u --per-thread program
>>>
>>> On the "older" kernels you are referring to one would find the original naming
>>> convention.  Everything else looks good to me.
>>
>> True, but do we care what we see there ? All we care about is the location,
>> where to find them. I could fix it, if you think thats needed.
> 
> IIUC, either the old kernel or newer kernel, both we can find the event
> from ($SYSFS)/bus/event_source/devices/cs_etm/sinks/; the only
> difference between them is the naming convention.

The cs_etm/sinks was only added with the CPU-wide trace support. So, if someone
refers to this document alone and then tries to do something on on older kernel,
which is quite possible for a production device running a stable kernel, {s,}he
might be surprised.

> 
> So the doc can use the same location to find event for both new and
> old kernel, and explain the naming convention difference?

My question is really, does the naming convention matter ? What you see
under the directory is the name. But yes, I am open to add a section to
explain the fact that we changed the naming scheme, if everyone agrees
to it.

Cheers
Suzuki