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    SubjectRe: [PATCH v6 2/2] drivers/perf: hisi: Add driver for HiSilicon PCIe PMU
    Hi,

    On Wed, Jun 16, 2021 at 09:54:23AM +0800, liuqi (BA) wrote:
    > On 2021/6/15 17:35, Will Deacon wrote:
    > > On Tue, Jun 15, 2021 at 04:57:09PM +0800, liuqi (BA) wrote:
    > > > On 2021/6/12 0:23, Will Deacon wrote:
    > > > > On Mon, May 31, 2021 at 09:32:31PM +0800, Qi Liu wrote:
    > > > > > + /* Process data to set unit of latency as "us". */
    > > > > > + if (is_latency_event(idx))
    > > > > > + return div64_u64(data * us_per_cycle, data_ext);
    > > > > > +
    > > > > > + if (is_bus_util_event(idx))
    > > > > > + return div64_u64(data * us_per_cycle, data_ext);
    > > > > > +
    > > > > > + if (is_buf_util_event(idx))
    > > > > > + return div64_u64(data, data_ext * us_per_cycle);
    > > > >
    > > > > Why do we need to do all this division in the kernel? Can't we just expose
    > > > > the underlying values and let userspace figure out what it wants to do with
    > > > > the numbers?
    > > > >
    > > > Our PMU hardware support 8 sets of counters to count bandwidth, latency and
    > > > utilization events.
    > > >
    > > > For example, when users set latency event, common counter will count delay
    > > > cycles, and extern counter count number of PCIe packets automaticly. And we
    > > > do not have a event number for counting number of PCIe packets.
    > > >
    > > > So this division cannot move to userspace tool.
    > >
    > > Why can't you expose the packet counter as an extra event to userspace?
    > >
    > Maybe I didn’t express it clearly.
    >
    > As there is no hardware event number for PCIe packets counting, extern
    > counter count packets *automaticly* when latency events is selected by
    > users.
    >
    > This means users cannot set "config=0xXX" to start packets counting event.
    > So we can only get the value of counter and extern counter in driver and do
    > the division, then pass the result to userspace.

    I still think it would be ideal if we could expose both values to userspace
    rather than combine them somehow. Hmm. Anyway...

    I struggled to figure out exactly what's being counted from the
    documentation patch (please update that). Please can you explain exactly
    what appears in the HISI_PCIE_CNT and HISI_PCIE_EXT_CNT registers for the
    different modes of operation? Without that, the ratios you've chosen to
    report seem rather arbitrary.

    I also couldn't figure out how the latency event works. For example, I was
    assuming it would be a filter (a bit like the length), so you could say
    things like "I'm only interested in packets with a latency higher than x"
    but it doesn't look like it works that way.

    Thanks,

    Will

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