linux-kernel - Re: [PATCH v2 2/3] drivers/perf: add DesignWare PCIe PMU driver

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Message-ID: <2dc023c9-7805-3dfa-f902-31468a2d3f7e@linux.alibaba.com>
Date:   Wed, 19 Apr 2023 09:39:28 +0800
From:   Shuai Xue <xueshuai@...ux.alibaba.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>, helgaas@...nel.org,
        yangyicong@...wei.com, will@...nel.org, Jonathan.Cameron@...wei.com
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, rdunlap@...radead.org,
        robin.murphy@....com, mark.rutland@....com,
        zhuo.song@...ux.alibaba.com
Subject: Re: [PATCH v2 2/3] drivers/perf: add DesignWare PCIe PMU driver



On 2023/4/18 AM9:51, Baolin Wang wrote:
> 
> 
> On 4/17/2023 9:16 AM, Shuai Xue wrote:
> 
> [snip]
> 
>>>> +
>>>> +static void dwc_pcie_pmu_event_update(struct perf_event *event)
>>>> +{
>>>> +    u64 counter;
>>>> +    struct dwc_pcie_pmu *pcie_pmu = to_dwc_pcie_pmu(event->pmu);
>>>> +    struct dwc_pcie_rp_info *rp_info = pmu_to_pcie_info(event->pmu);
>>>> +    struct pci_dev *pdev = rp_info->pdev;
>>>> +    u16 ras_des = rp_info->ras_des;
>>>> +    struct hw_perf_event *hwc = &event->hw;
>>>> +    enum dwc_pcie_event_type type = DWC_PCIE_EVENT_TYPE(event);
>>>> +    u64 delta, prev, now;
>>>> +
>>>> +    do {
>>>> +        prev = local64_read(&hwc->prev_count);
>>>> +
>>>> +        if (type == DWC_PCIE_LANE_EVENT)
>>>> +            dwc_pcie_pmu_read_event_counter(pdev, ras_des, &counter);
>>>> +        else if (type == DWC_PCIE_TIME_BASE_EVENT)
>>>> +            dwc_pcie_pmu_read_base_time_counter(pdev, ras_des,
>>>> +                                &counter);
>>>> +        else
>>>> +            dev_err(pcie_pmu->dev, "invalid event type: 0x%x\n", type);
>>>> +
>>>> +        now = counter;
>>>> +    } while (local64_cmpxchg(&hwc->prev_count, prev, now) != prev);
>>>> +
>>>> +    delta = now - prev;
>>>
>>> This can be overflow? better to add a mask to avoid possible overflow.
>>
>> I think it can not. This Root Complex supports up to PCIe Gen5 (32 GT/s)
>> and one root port support up to x16 lanes, with peek bandwidth 64 GB/s.
>> On Yitian 710, one root port is x4 lane with peak bandwidth 16 GB/s.
>> The counter is 64 bit width with 16 bytes unit.
>>
>>     2^64*16/(64*10^9)/60/60/24/365 = 146 years
>>
>> For x16 root port, it will not overflow within 146 yeasr and for yitian 710,
>> it will never overflow in my life too.
> 
> However the lane event counter is 32bit, so still a maximum counter mask is preferable.

You are right, will mask it in next version.

Thank you.
Best Regards,
Shuai