dm: core: Add documentation on how to debug driver model

Sometimes devices don't appear and it can be confusing. Add a few notes to
help with this situation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fix 2 issues in the doc and include the doc in the index.rst]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

doc/driver-model/debugging.rst

@@ -0,0 +1,62 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. sectionauthor:: Simon Glass <sjg@chromium.org>
+
+Debugging driver model
+======================
+
+This document aims to provide help when you cannot work out why driver model is
+not doing what you expect.
+
+
+Useful techniques in general
+----------------------------
+
+Here are some useful debugging features generally.
+
+   - If you are writing a new feature, consider doing it in sandbox instead of
+     on your board. Sandbox has no limits, allows easy debugging (e.g. gdb) and
+     you can write emulators for most common devices.
+   - Put '#define DEBUG' at the top of a file, to activate all the debug() and
+     log_debug() statements in that file.
+   - Where logging is used, change the logging level, e.g. in SPL with
+     CONFIG_SPL_LOG_MAX_LEVEL=7 (which is LOGL_DEBUG) and
+     CONFIG_LOG_DEFAULT_LEVEL=7
+   - Where logging of return values is implemented with log_msg_ret(), set
+     CONFIG_LOG_ERROR_RETURN=y to see exactly where the error is happening
+   - Make sure you have a debug UART enabled - see CONFIG_DEBUG_UART. With this
+     you can get serial output (printf(), etc.) before the serial driver is
+     running.
+   - Use a JTAG emulator to set breakpoints and single-step through code
+
+Not that most of these increase code/data size somewhat when enabled.
+
+
+Failure to locate a device
+--------------------------
+
+Let's say you have uclass_first_device_err() and it is not finding anything.
+
+If it is returning an error, then that gives you a clue. Look up linux/errno.h
+to see errors. Common ones are:
+
+   - -ENOMEM which indicates that memory is short. If it happens in SPL or
+     before relocation in U-Boot, check CONFIG_SPL_SYS_MALLOC_F_LEN and
+     CONFIG_SYS_MALLOC_F_LEN as they may need to be larger. Add '#define DEBUG'
+     at the very top of malloc_simple.c to get an idea of where your memory is
+     going.
+   - -EINVAL which typically indicates that something was missing or wrong in
+     the device tree node. Check that everything is correct and look at the
+     ofdata_to_platdata() method in the driver.
+
+If there is no error, you should check if the device is actually bound. Call
+dm_dump_all() just before you locate the device to make sure it exists.
+
+If it does not exist, check your device tree compatible strings match up with
+what the driver expects (in the struct udevice_id array).
+
+If you are using of-platdata (e.g. CONFIG_SPL_OF_PLATDATA), check that the
+driver name is the same as the first compatible string in the device tree (with
+invalid-variable characters converted to underscore).
+
+If you are really stuck, #define DEBUG at the top of lists.c should show you
+what is going on.

doc/driver-model/index.rst

@@ -6,6 +6,7 @@ Driver Model
 .. toctree::
    :maxdepth: 2
 
+   debugging
    design
    fdt-fixup
    fs_firmware_loader