drm/amdgpu/powerplay: add some documentation about memory clock

We expose the actual memory controller clock rate in Linux,
not the effective memory clock of the DRAMs.  To translate
it, it follows the following formula:

Clock conversion (Mhz):
HBM: effective_memory_clock = memory_controller_clock * 1
G5:  effective_memory_clock = memory_controller_clock * 1
G6:  effective_memory_clock = memory_controller_clock * 2

DRAM data rate (MT/s):
HBM: effective_memory_clock * 2 = data_rate
G5:  effective_memory_clock * 4 = data_rate
G6:  effective_memory_clock * 8 = data_rate

Bandwidth (MB/s):
data_rate * vram_bit_width / 8 = memory_bandwidth

Some examples:
G5 on RX460:
memory_controller_clock = 1750 Mhz
effective_memory_clock = 1750 Mhz * 1 = 1750 Mhz
data rate = 1750 * 4 = 7000 MT/s
memory_bandwidth = 7000 * 128 bits / 8 = 112000 MB/s

G6 on RX5600:
memory_controller_clock = 900 Mhz
effective_memory_clock = 900 Mhz * 2 = 1800 Mhz
data rate = 1800 * 8 = 14400 MT/s
memory_bandwidth = 14400 * 192 bits / 8 = 345600 MB/s
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c

@@ -694,6 +694,52 @@ static ssize_t amdgpu_set_pp_table(struct device *dev,
  * in each power level within a power state.  The pp_od_clk_voltage is used for
  * this.
  *
+ * Note that the actual memory controller clock rate are exposed, not
+ * the effective memory clock of the DRAMs. To translate it, use the
+ * following formula:
+ *
+ * Clock conversion (Mhz):
+ *
+ * HBM: effective_memory_clock = memory_controller_clock * 1
+ *
+ * G5: effective_memory_clock = memory_controller_clock * 1
+ *
+ * G6: effective_memory_clock = memory_controller_clock * 2
+ *
+ * DRAM data rate (MT/s):
+ *
+ * HBM: effective_memory_clock * 2 = data_rate
+ *
+ * G5: effective_memory_clock * 4 = data_rate
+ *
+ * G6: effective_memory_clock * 8 = data_rate
+ *
+ * Bandwidth (MB/s):
+ *
+ * data_rate * vram_bit_width / 8 = memory_bandwidth
+ *
+ * Some examples:
+ *
+ * G5 on RX460:
+ *
+ * memory_controller_clock = 1750 Mhz
+ *
+ * effective_memory_clock = 1750 Mhz * 1 = 1750 Mhz
+ *
+ * data rate = 1750 * 4 = 7000 MT/s
+ *
+ * memory_bandwidth = 7000 * 128 bits / 8 = 112000 MB/s
+ *
+ * G6 on RX5700:
+ *
+ * memory_controller_clock = 875 Mhz
+ *
+ * effective_memory_clock = 875 Mhz * 2 = 1750 Mhz
+ *
+ * data rate = 1750 * 8 = 14000 MT/s
+ *
+ * memory_bandwidth = 14000 * 256 bits / 8 = 448000 MB/s
+ *
  * < For Vega10 and previous ASICs >
  *
  * Reading the file will display: