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 at gmx.de>
---
 doc/develop/crash_dumps.rst | 122 ++++++++++++++++++++++++++++++++++++
 doc/develop/index.rst       |  10 +++
 doc/index.rst               |  11 ++++
 3 files changed, 143 insertions(+)
 create mode 100644 doc/develop/crash_dumps.rst
 create mode 100644 doc/develop/index.rst

--
2.26.2

Thanks Heinrich!

On Fri, Apr 24, 2020 at 11:31:20PM +0200, Heinrich Schuchardt wrote:
> Provide an overview of the analysis of U-Boot crash dumps.
> 
> Signed-off-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
> ---
>  doc/develop/crash_dumps.rst | 122 ++++++++++++++++++++++++++++++++++++
>  doc/develop/index.rst       |  10 +++
>  doc/index.rst               |  11 ++++
>  3 files changed, 143 insertions(+)
>  create mode 100644 doc/develop/crash_dumps.rst
>  create mode 100644 doc/develop/index.rst
> 
> diff --git a/doc/develop/crash_dumps.rst b/doc/develop/crash_dumps.rst
> new file mode 100644
> index 0000000000..5aede4a9af
> --- /dev/null
> +++ b/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.
> diff --git a/doc/develop/index.rst b/doc/develop/index.rst
> new file mode 100644
> index 0000000000..072db63b5c
> --- /dev/null
> +++ b/doc/develop/index.rst
> @@ -0,0 +1,10 @@
> +.. SPDX-License-Identifier: GPL-2.0+
> +
> +Develop U-Boot
> +==============
> +
> +
> +.. toctree::
> +   :maxdepth: 2
> +
> +   crash_dumps
> diff --git a/doc/index.rst b/doc/index.rst
> index cd98be6cc5..fd9f10f28e 100644
> --- a/doc/index.rst
> +++ b/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)
>  ----------------------------------
> 
> --
> 2.26.2
> 

Acked-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>

On Fri, Apr 24, 2020 at 11:31:20PM +0200, Heinrich Schuchardt wrote:

> Provide an overview of the analysis of U-Boot crash dumps.
> 
> Signed-off-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
> Acked-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>

Applied to u-boot/master, thanks!