linux-kernel - Re: [PATCH v7 3/4] drivers/perf: add DesignWare PCIe PMU driver

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Message-ID: <20231013163025.GA1116248@bhelgaas>
Date:   Fri, 13 Oct 2023 11:30:25 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Shuai Xue <xueshuai@...ux.alibaba.com>
Cc:     chengyou@...ux.alibaba.com, kaishen@...ux.alibaba.com,
        yangyicong@...wei.com, will@...nel.org,
        Jonathan.Cameron@...wei.com, baolin.wang@...ux.alibaba.com,
        robin.murphy@....com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
        rdunlap@...radead.org, mark.rutland@....com,
        zhuo.song@...ux.alibaba.com, renyu.zj@...ux.alibaba.com
Subject: Re: [PATCH v7 3/4] drivers/perf: add DesignWare PCIe PMU driver

On Fri, Oct 13, 2023 at 11:46:44AM +0800, Shuai Xue wrote:
> 
> 
> On 2023/10/13 00:25, Bjorn Helgaas wrote:
> > On Thu, Oct 12, 2023 at 11:28:55AM +0800, Shuai Xue wrote:
> >> This commit adds the PCIe Performance Monitoring Unit (PMU) driver support
> >> for T-Head Yitian SoC chip. Yitian is based on the Synopsys PCI Express
> >> Core controller IP which provides statistics feature. The PMU is not a PCIe
> >> Root Complex integrated End Point(RCiEP) device but only register counters
> >> provided by each PCIe Root Port.

IIUC, the PMU is directly integrated into the Root Port: it's
discovered and operated via the Root Port config space.  If so, I
wouldn't bother mentioning RCiEP because there's no need to list all
the things it's *not*.

> >> To facilitate collection of statistics the controller provides the
> >> following two features for each Root Port:
> >>
> >> - Time Based Analysis (RX/TX data throughput and time spent in each
> >>   low-power LTSSM state)
> >> - Event counters (Error and Non-Error for lanes)
> >>
> >> Note, only one counter for each type and does not overflow interrupt.
> > 
> > Not sure what "does not overflow interrupt" means.  Does it mean
> > there's no interrupt generated when the counter overflows?
> 
> Yes, exactly. The rootport does NOT generate interrupt when the
> couter overflows.  I think the assumption hidden in this design is
> 64-bit counter will not overflow within observable time.
> 
> PCIe 5.0 slots can now reach anywhere between ~4GB/sec for a x1 slot
> up to ~64GB/sec for a x16 slot. The unit of counter is 16 byte.
> 
> 	2^64/(64/16*10^9)/60/60/24/365=146 years
> 
> so, the counter will not overflow within 146 years.

Certainly a reasonable assumption :)

But I'm confused about how many counters there are.  Clearly there are
two features ((1) time-based analysis and (2) event counters).

"One counter for each type" suggests there's one counter for
time-based analysis and a second counter for event counting, but from
dwc_pcie_pmu_event_add(), it looks like each Root Port might have a
single counter, and you can decide whether that counter is used for
time-based analysis or event counting, but you can't do both at the
same time?  And the event counting is for a single lane, not for the
link as a whole?

If so, I might word this as:

  Each Root Port contains one counter that can be used for either:

    - Time-Based Analysis (RX/TX data throughput and time spent in
      each low-power LTSSM state) or

    - Event counting (error and non-error events for a specified lane)

  There is no interrupt for counter overflow.

> >> +	  Enable perf support for Synopsys DesignWare PCIe PMU Performance
> >> +	  monitoring event on platform including the Yitian 710.
> > 
> > Should this mention Alibaba or T-Head?  I don't know how
> > Alibaba/T-Head/Yitian are all related.
> 
> The server chips, named Yitian 710, are custom-built by Alibaba Group's chip
> development business, T-Head.
> 
> 	  Enable perf support for Synopsys DesignWare PCIe PMU Performance
> 	  monitoring event on platform including the Alibaba Yitian 710.
> 
> Is this okay?

Perfect :)

Bjorn