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

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Message-ID: <529defac-cd67-3a7b-91d3-76eb0bb9dc6c@arm.com>
Date:   Mon, 19 Apr 2021 12:17:41 +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 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.

Also, please move your driver to 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).

Suzuki