docs: perf: Update usage for target filter of hisi-pcie-pmu

One of the "port" and "bdf" target filter interface must be set, and
the related events should preferably used in the same group.
Update the usage in the documentation.
Signed-off-by: Junhao He <hejunhao3@huawei.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240223103359.18669-9-yangyicong@huawei.comSigned-off-by: Will Deacon <will@kernel.org>

Documentation/admin-guide/perf/hisi-pcie-pmu.rst

@@ -37,9 +37,20 @@ Example usage of perf::
   hisi_pcie0_core0/rx_mwr_cnt/ [kernel PMU event]
   ------------------------------------------
 
-  $# perf stat -e hisi_pcie0_core0/rx_mwr_latency/
-  $# perf stat -e hisi_pcie0_core0/rx_mwr_cnt/
-  $# perf stat -g -e hisi_pcie0_core0/rx_mwr_latency/ -e hisi_pcie0_core0/rx_mwr_cnt/
+  $# perf stat -e hisi_pcie0_core0/rx_mwr_latency,port=0xffff/
+  $# perf stat -e hisi_pcie0_core0/rx_mwr_cnt,port=0xffff/
+
+The related events usually used to calculate the bandwidth, latency or others.
+They need to start and end counting at the same time, therefore related events
+are best used in the same event group to get the expected value. There are two
+ways to know if they are related events:
+a) By event name, such as the latency events "xxx_latency, xxx_cnt" or
+   bandwidth events "xxx_flux, xxx_time".
+b) By event type, such as "event=0xXXXX, event=0x1XXXX".
+
+Example usage of perf group::
+
+  $# perf stat -e "{hisi_pcie0_core0/rx_mwr_latency,port=0xffff/,hisi_pcie0_core0/rx_mwr_cnt,port=0xffff/}"
 
 The current driver does not support sampling. So "perf record" is unsupported.
 Also attach to a task is unsupported for PCIe PMU.
@@ -51,8 +62,12 @@ Filter options
 
    PMU could only monitor the performance of traffic downstream target Root
    Ports or downstream target Endpoint. PCIe PMU driver support "port" and
-   "bdf" interfaces for users, and these two interfaces aren't supported at the
-   same time.
+   "bdf" interfaces for users.
+   Please notice that, one of these two interfaces must be set, and these two
+   interfaces aren't supported at the same time. If they are both set, only
+   "port" filter is valid.
+   If "port" filter not being set or is set explicitly to zero (default), the
+   "bdf" filter will be in effect, because "bdf=0" meaning 0000:000:00.0.
 
    - port
 
@@ -95,7 +110,7 @@ Filter options
 
    Example usage of perf::
 
-     $# perf stat -e hisi_pcie0_core0/rx_mrd_flux,trig_len=0x4,trig_mode=1/ sleep 5
+     $# perf stat -e hisi_pcie0_core0/rx_mrd_flux,port=0xffff,trig_len=0x4,trig_mode=1/ sleep 5
 
 3. Threshold filter
 
@@ -109,7 +124,7 @@ Filter options
 
    Example usage of perf::
 
-     $# perf stat -e hisi_pcie0_core0/rx_mrd_flux,thr_len=0x4,thr_mode=1/ sleep 5
+     $# perf stat -e hisi_pcie0_core0/rx_mrd_flux,port=0xffff,thr_len=0x4,thr_mode=1/ sleep 5
 
 4. TLP Length filter
 
@@ -127,4 +142,4 @@ Filter options
 
    Example usage of perf::
 
-     $# perf stat -e hisi_pcie0_core0/rx_mrd_flux,len_mode=0x1/ sleep 5
+     $# perf stat -e hisi_pcie0_core0/rx_mrd_flux,port=0xffff,len_mode=0x1/ sleep 5