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

[Baolin Wang]

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.