realtek: rtl930x: add memory mapping documentation

No code changes. Just some explanation how these devices
work with physical and logical memory.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22054
Signed-off-by: Robert Marko <robimarko@gmail.com>

diff --git a/target/linux/realtek/files-6.12/arch/mips/rtl-otto/prom.c b/target/linux/realtek/files-6.12/arch/mips/rtl-otto/prom.c
index 22b8030e4e8b7563dc1a3c5ee664765b6f2e727c..047ea74c69ceca7ff718c0db823d0ec28d491422 100644 (file)
--- a/target/linux/realtek/files-6.12/arch/mips/rtl-otto/prom.c
+++ b/target/linux/realtek/files-6.12/arch/mips/rtl-otto/prom.c
@@ -286,12 +286,32 @@ static void prepare_highmem(void)
                return;
 
        /*
-        * To use highmem on RTL930x, SRAM must be deactivated and the highmem mapping
-        * registers must be setup properly. The hardcoded 0x70000000 might be strange
-        * but at least it conforms somehow to the RTL931x devices.
+        * The RTL930x provides 3 logical adressing zones that can be configured individually
+        * and offer an additional memory access indirection. Memory is accessed via the OCP
+        * bus that checks these regions and maps logical addresses to physical ones. They are
         *
-        * - RTL930x: highmem start 0x20000000 + offset 0x70000000 = 0x90000000
-        * - RTL931x: highmem start 0x90000000 + no offset at all  = 0x90000000
+        * zone 1: logical address 0x00000000-0x0fffffff (256 MB) - map register 0xb8004200
+        * zone 2: logical address 0x10000000-0x13ffffff (64 MB)  - map register 0xb8004210
+        * zone 3: logical address 0x20000000-0x9fffffff (2 GB)   - map register 0xb8004220
+        *
+        * Whenever CPU accesses memory the normal MIPS translation is applied and afterwards
+        * the bus adds the zone mapping. E.g. a read to 0x81230000 is converted to an cached
+        * memory access to logical address 0x01230000. It is issued to the OCP bus and the 
+        * mapping from zone 1 register is added. That allows for two memory topologies:
+        *
+        * Linear memory with a maximum of 320 MB:
+        *
+        * Zone | map content | logical               | physical
+        * -------------------+-----------------------+-----------------------
+        *    1 | 0x00000000  | 0x00000000-0x0fffffff | 0x00000000-0x0fffffff
+        *    2 | 0x00000000  | 0x10000000-0x13ffffff | 0x10000000-0x13ffffff
+        *
+        * 256MB low memory plus up to 2GB high memory:
+        *
+        * Zone | map content | logical               | physical
+        * -------------------+-----------------------+-----------------------
+        *    1 | 0x00000000  | 0x00000000-0x0fffffff | 0x00000000-0x0fffffff
+        *    3 | 0x70000000  | 0x20000000-0x9fffffff | 0x10000000-0x7fffffff
         */
 
        pr_info("highmem kernel on RTL930x with > 256 MB RAM, adapt SoC memory mapping\n");