docs: perf: Add description for Alibaba's T-Head PMU driver

Alibaba's T-Head SoC implements uncore PMU for performance and functional
debugging to facilitate system maintenance. Document it to provide guidance
on how to use it.
Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220914022326.88550-2-xueshuai@linux.alibaba.comSigned-off-by: Will Deacon <will@kernel.org>

Documentation/admin-guide/perf/alibaba_pmu.rst

+=============================================================
+Alibaba's T-Head SoC Uncore Performance Monitoring Unit (PMU)
+=============================================================
+
+The Yitian 710, custom-built by Alibaba Group's chip development business,
+T-Head, implements uncore PMU for performance and functional debugging to
+facilitate system maintenance.
+
+DDR Sub-System Driveway (DRW) PMU Driver
+=========================================
+
+Yitian 710 employs eight DDR5/4 channels, four on each die. Each DDR5 channel
+is independent of others to service system memory requests. And one DDR5
+channel is split into two independent sub-channels. The DDR Sub-System Driveway
+implements separate PMUs for each sub-channel to monitor various performance
+metrics.
+
+The Driveway PMU devices are named as ali_drw_<sys_base_addr> with perf.
+For example, ali_drw_21000 and ali_drw_21080 are two PMU devices for two
+sub-channels of the same channel in die 0. And the PMU device of die 1 is
+prefixed with ali_drw_400XXXXX, e.g. ali_drw_40021000.
+
+Each sub-channel has 36 PMU counters in total, which is classified into
+four groups:
+
+- Group 0: PMU Cycle Counter. This group has one pair of counters
+  pmu_cycle_cnt_low and pmu_cycle_cnt_high, that is used as the cycle count
+  based on DDRC core clock.
+
+- Group 1: PMU Bandwidth Counters. This group has 8 counters that are used
+  to count the total access number of either the eight bank groups in a
+  selected rank, or four ranks separately in the first 4 counters. The base
+  transfer unit is 64B.
+
+- Group 2: PMU Retry Counters. This group has 10 counters, that intend to
+  count the total retry number of each type of uncorrectable error.
+
+- Group 3: PMU Common Counters. This group has 16 counters, that are used
+  to count the common events.
+
+For now, the Driveway PMU driver only uses counters in group 0 and group 3.
+
+The DDR Controller (DDRCTL) and DDR PHY combine to create a complete solution
+for connecting an SoC application bus to DDR memory devices. The DDRCTL
+receives transactions Host Interface (HIF) which is custom-defined by Synopsys.
+These transactions are queued internally and scheduled for access while
+satisfying the SDRAM protocol timing requirements, transaction priorities, and
+dependencies between the transactions. The DDRCTL in turn issues commands on
+the DDR PHY Interface (DFI) to the PHY module, which launches and captures data
+to and from the SDRAM. The driveway PMUs have hardware logic to gather
+statistics and performance logging signals on HIF, DFI, etc.
+
+By counting the READ, WRITE and RMW commands sent to the DDRC through the HIF
+interface, we could calculate the bandwidth. Example usage of counting memory
+data bandwidth::
+
+  perf stat \
+    -e ali_drw_21000/hif_wr/ \
+    -e ali_drw_21000/hif_rd/ \
+    -e ali_drw_21000/hif_rmw/ \
+    -e ali_drw_21000/cycle/ \
+    -e ali_drw_21080/hif_wr/ \
+    -e ali_drw_21080/hif_rd/ \
+    -e ali_drw_21080/hif_rmw/ \
+    -e ali_drw_21080/cycle/ \
+    -e ali_drw_23000/hif_wr/ \
+    -e ali_drw_23000/hif_rd/ \
+    -e ali_drw_23000/hif_rmw/ \
+    -e ali_drw_23000/cycle/ \
+    -e ali_drw_23080/hif_wr/ \
+    -e ali_drw_23080/hif_rd/ \
+    -e ali_drw_23080/hif_rmw/ \
+    -e ali_drw_23080/cycle/ \
+    -e ali_drw_25000/hif_wr/ \
+    -e ali_drw_25000/hif_rd/ \
+    -e ali_drw_25000/hif_rmw/ \
+    -e ali_drw_25000/cycle/ \
+    -e ali_drw_25080/hif_wr/ \
+    -e ali_drw_25080/hif_rd/ \
+    -e ali_drw_25080/hif_rmw/ \
+    -e ali_drw_25080/cycle/ \
+    -e ali_drw_27000/hif_wr/ \
+    -e ali_drw_27000/hif_rd/ \
+    -e ali_drw_27000/hif_rmw/ \
+    -e ali_drw_27000/cycle/ \
+    -e ali_drw_27080/hif_wr/ \
+    -e ali_drw_27080/hif_rd/ \
+    -e ali_drw_27080/hif_rmw/ \
+    -e ali_drw_27080/cycle/ -- sleep 10
+
+The average DRAM bandwidth can be calculated as follows:
+
+- Read Bandwidth =  perf_hif_rd * DDRC_WIDTH * DDRC_Freq / DDRC_Cycle
+- Write Bandwidth = (perf_hif_wr + perf_hif_rmw) * DDRC_WIDTH * DDRC_Freq / DDRC_Cycle
+
+Here, DDRC_WIDTH = 64 bytes.
+
+The current driver does not support sampling. So "perf record" is
+unsupported.  Also attach to a task is unsupported as the events are all
+uncore.

Documentation/admin-guide/perf/index.rst

@@ -18,3 +18,4 @@ Performance monitor support
    xgene-pmu
    arm_dsu_pmu
    thunderx2-pmu
+   alibaba_pmu