linux-kernel - Re: [PATCH RESEND 0/4] Add support for HiSilicon PCIe Tune and Trace device

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Message-ID: <e98fdb87-1834-c5eb-70be-4e739ff258bd@arm.com>
Date:   Mon, 19 Apr 2021 17:11:46 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Yicong Yang <yangyicong@...ilicon.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        coresight@...ts.linaro.org, linux-pci@...r.kernel.org
Cc:     alexander.shishkin@...ux.intel.com, helgaas@...nel.org,
        gregkh@...uxfoundation.org, lorenzo.pieralisi@....com,
        will@...nel.org, mark.rutland@....com, mathieu.poirier@...aro.org,
        mike.leach@...aro.org, leo.yan@...aro.org,
        jonathan.cameron@...wei.com, song.bao.hua@...ilicon.com,
        john.garry@...wei.com, prime.zeng@...wei.com, liuqi115@...wei.com,
        zhangshaokun@...ilicon.com, linuxarm@...wei.com
Subject: Re: [PATCH RESEND 0/4] Add support for HiSilicon PCIe Tune and Trace
 device

On 19/04/2021 14:21, Yicong Yang wrote:
> On 2021/4/19 19:17, Suzuki K Poulose wrote:
>> On 17/04/2021 11:17, Yicong Yang wrote:
>>> [RESEND with perf and coresight folks Cc'ed]
>>>
>>> HiSilicon PCIe tune and trace device (PTT) is a PCIe Root Complex
>>> integrated Endpoint (RCiEP) device, providing the capability
>>> to dynamically monitor and tune the PCIe traffic (tune),
>>> and trace the TLP headers (trace).
>>>
>>> PTT tune is designed for monitoring and adjusting PCIe link parameters.
>>> We provide several parameters of the PCIe link. Through the driver,
>>> user can adjust the value of certain parameter to affect the PCIe link
>>> for the purpose of enhancing the performance in certian situation.
>>
>> ...
>>
>>>
>>> The reason for not using perf is because there is no current support
>>> for uncore tracing in the perf facilities. We have our own format
>>> of data and don't need perf doing the parsing. The setting through
>>> perf tools doesn't seem to be friendly as well. For example,
>>> we cannot count on perf to decode the usual format BDF number like
>>> <domain>:<bus>:<dev>.<fn>, which user can use to filter the TLP
>>> headers through the PTT device.
>>>
>>> A similar approach for implementing this function is ETM, which use
>>> sysfs for configuring and a character device for dumping data.
>>>
>>> Greg has some comments on our implementation and doesn't advocate
>>> to build driver on debugfs [1]. So I resend this series to
>>> collect more feedbacks on the implementation of this driver.
>>>
>>> Hi perf and ETM related experts, is it suggested to adapt this driver
>>> to perf? Or is the debugfs approach acceptable? Otherwise use
>>> sysfs + character device like ETM and use perf tools for decoding it?
>>> Any comments is welcomed.
>>
>> Please use perf. Debugfs / sysfs is not the right place for these things.
>>
> 
> ok.
> 
>> Also, please move your driver to drivers/perf/
>>
> 
> Does it make sense as it's a tuning and tracing device, and doesn't have counters
> nor do the sampling like usual PMU device under drivers/perf/.

It doesn't matter. As long as you can drive it via the perf interface,
it can live there. The CoreSight was added way before there was kind
of a suitable place like the above. You could find other uncore PMUs
under drivers/perf.

Suzuki