doc: describe the analysis of crash dumps

Provide an overview of the analysis of U-Boot crash dumps.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/develop/crash_dumps.rst

@@ -0,0 +1,122 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (c) 2020 Heinrich Schuchardt
+
+Analyzing crash dumps
+=====================
+
+When the CPU detects an instruction that it cannot execute it raises an
+interrupt. U-Boot than writes a crash dump. This chapter describes how such
+dump can be analyzed.
+
+Creating a crash dump voluntarily
+---------------------------------
+
+For describing the analysis of a crash dump we need an example. U-Boot comes
+with a command 'exception' that comes in handy here. The command is enabled
+by::
+
+    CONFIG_CMD_EXCEPTION=y
+
+The example output below was recorded when running qemu\_arm64\_defconfig on
+QEMU::
+
+    => exception undefined
+    "Synchronous Abort" handler, esr 0x02000000
+    elr: 00000000000101fc lr : 00000000000214ec (reloc)
+    elr: 000000007ff291fc lr : 000000007ff3a4ec
+    x0 : 000000007ffbd7f8 x1 : 0000000000000000
+    x2 : 0000000000000001 x3 : 000000007eedce18
+    x4 : 000000007ff291fc x5 : 000000007eedce50
+    x6 : 0000000000000064 x7 : 000000007eedce10
+    x8 : 0000000000000000 x9 : 0000000000000004
+    x10: 6db6db6db6db6db7 x11: 000000000000000d
+    x12: 0000000000000006 x13: 000000000001869f
+    x14: 000000007edd7dc0 x15: 0000000000000002
+    x16: 000000007ff291fc x17: 0000000000000000
+    x18: 000000007eed8dc0 x19: 0000000000000000
+    x20: 000000007ffbd7f8 x21: 0000000000000000
+    x22: 000000007eedce10 x23: 0000000000000002
+    x24: 000000007ffd4c80 x25: 0000000000000000
+    x26: 0000000000000000 x27: 0000000000000000
+    x28: 000000007eedce70 x29: 000000007edd7b40
+
+    Code: b00003c0 912ad000 940029d6 17ffff52 (e7f7defb)
+    Resetting CPU ...
+
+    resetting ...
+
+The first line provides us with the type of interrupt that occurred.
+(On ARMv8 a synchronous abort is an exception where the return address stored
+in the ESR register indicates the instruction that caused the exception.)
+
+The second line provides the contents of the elr and the lr register after
+subtracting the relocation offset. - U-Boot relocates itself after being
+loaded. - The relocation offset can also be displayed using the bdinfo command.
+
+After the contents of the registers we get a line indicating the machine
+code of the instructions preceding the crash and in parentheses the instruction
+leading to the dump.
+
+Analyzing the code location
+---------------------------
+
+We can convert the instructions in the line starting with 'Code:' into mnemonics
+using the objdump command. To make things easier scripts/decodecode is
+supplied::
+
+    $echo 'Code: b00003c0 912ad000 940029d6 17ffff52 (e7f7defb)' | \
+      CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 scripts/decodecode
+    Code: b00003c0 912ad000 940029d6 17ffff52 (e7f7defb)
+    All code
+    ========
+       0:   b00003c0     adrp   x0, 0x79000
+       4:   912ad000     add    x0, x0, #0xab4
+       8:   940029d6     bl     0xa760
+       c:   17ffff52     b      0xfffffffffffffd54
+      10:*  e7f7defb     .inst  0xe7f7defb ; undefined <-- trapping instruction
+
+    Code starting with the faulting instruction
+    ===========================================
+       0:   e7f7defb     .inst  0xe7f7defb ; undefined
+
+Now lets use the locations provided by the elr and lr registers after
+subtracting the relocation offset to find out where in the code the crash
+occurred and from where it was invoked.
+
+File u-boot.map contains the memory layout of the U-Boot binary. Here we find
+these lines::
+
+   .text.do_undefined
+                  0x00000000000101fc        0xc cmd/built-in.o
+   .text.exception_complete
+                  0x0000000000010208       0x90 cmd/built-in.o
+   ...
+   .text.cmd_process
+                  0x00000000000213b8      0x164 common/built-in.o
+                  0x00000000000213b8                cmd_process
+   .text.cmd_process_error
+                  0x000000000002151c       0x40 common/built-in.o
+                  0x000000000002151c                cmd_process_error
+
+So the error occurred at the start of function do\_undefined() and this
+function was invoked from somewhere inside function cmd\_process().
+
+If we want to dive deeper, we can disassemble the U-Boot binary::
+
+    $ aarch64-linux-gnu-objdump -S -D u-boot | less
+
+    00000000000101fc <do_undefined>:
+    {
+            /*
+             * 0xe7f...f.   is undefined in ARM mode
+             * 0xde..       is undefined in Thumb mode
+            */
+            asm volatile (".word 0xe7f7defb\n");
+       101fc:       e7f7defb        .inst   0xe7f7defb ; undefined
+            return CMD_RET_FAILURE;
+    }
+    10200:       52800020        mov     w0, #0x1        // #1
+    10204:       d65f03c0        ret
+
+This example is based on the ARMv8 architecture but the same procedures can be
+used on other architectures as well.

doc/develop/index.rst

@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Develop U-Boot
+==============
+
+
+.. toctree::
+   :maxdepth: 2
+
+   crash_dumps

doc/index.rst

@@ -26,6 +26,17 @@ trying to get it to work optimally on a given system.
 
    build/index
 
+Developer-oriented documentation
+--------------------------------
+
+The following manuals are written for *developers* of the U-Boot - those who
+want to contribute to U-Boot.
+
+.. toctree::
+   :maxdepth: 2
+
+   develop/index
+
 Unified Extensible Firmware (UEFI)
 ----------------------------------