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

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Date:   Mon, 19 Apr 2021 21:21:38 +0800
From:   Yicong Yang <yangyicong@...ilicon.com>
To:     Suzuki K Poulose <suzuki.poulose@....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 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/.

> As Alex mentioned, the ETM drivers were initially developed when the AUX
> buffer was not available. The sysfs interface is there only for the backward compatibility and for bring up ( due to the nature of the
> connections between the CoreSight components and sometimes the missing engineering spec).
> 

got it. thanks for the explanation.

Regards,
Yicong

> Suzuki
> 
> .