Create pi4_cpu_stepping.md

docs/notes/pi4_cpu_stepping.md

@@ -0,0 +1,35 @@
+# BCM2711 SoC Version about stepping
+
+## 2711ZPKFSB06B0T & 2711ZPKFSB06C0T
+
+The new stepping is "**C0T**" than "**B0T**".
+
+Run the command to check:
+
+```
+od -An -tx1 /proc/device-tree/emmc2bus/dma-ranges
+```
+
+It will show different output depending on the stepping:
+
+#### B0
+
+````
+pi@raspberrypi:~$ od -An -tx1 /proc/device-tree/emmc2bus/dma-ranges
+ 00 00 00 00 c0 00 00 00 00 00 00 00 00 00 00 00
+ 40 00 00 00
+````
+
+#### C0
+
+```
+pi@raspberrypi:~$ od -An -tx1 /proc/device-tree/emmc2bus/dma-ranges
+ 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+ fc 00 00 00
+ ```
+ 
+In the case of the BCM2711, it looks like the two main fixes are related to RAM addressing:
+
+- 1.The EMMC2 bus can only directly address the first 1GB.
+- 2.The PCIe interface can only directly address the first 3GB.
+