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========================================Debugging advice for driver development========================================
This document serves as a general starting point and lookup for debuggingdevice drivers.While this guide focuses on debugging that requires re-compiling themodule/kernel, the :doc:`userspace debugging guide</process/debugging/userspace_debugging_guide>` will guideyou through tools like dynamic debug, ftrace and other tools useful fordebugging issues and behavior.For general debugging advice, see the :doc:`general advice document</process/debugging/index>`.
.. contents:: :depth: 3
The following sections show you the available tools.
printk() & friends------------------
These are derivatives of printf() with varying destinations and support forbeing dynamically turned on or off, or lack thereof.
Simple printk()~~~~~~~~~~~~~~~
The classic, can be used to great effect for quick and dirty developmentof new modules or to extract arbitrary necessary data for troubleshooting.
Prerequisite: ``CONFIG_PRINTK`` (usually enabled by default)
**Pros**:
- No need to learn anything, simple to use- Easy to modify exactly to your needs (formatting of the data (See: :doc:`/core-api/printk-formats`), visibility in the log)- Can cause delays in the execution of the code (beneficial to confirm whether timing is a factor)
**Cons**:
- Requires rebuilding the kernel/module- Can cause delays in the execution of the code (which can cause issues to be not reproducible)
For the full documentation see :doc:`/core-api/printk-basics`
Trace_printk~~~~~~~~~~~~
Prerequisite: ``CONFIG_DYNAMIC_FTRACE`` & ``#include <linux/ftrace.h>``
It is a tiny bit less comfortable to use than printk(), because you will haveto read the messages from the trace file (See: :ref:`read_ftrace_log`instead of from the kernel log, but very useful when printk() adds unwanteddelays into the code execution, causing issues to be flaky or hidden.)
If the processing of this still causes timing issues then you can trytrace_puts().
For the full Documentation see trace_printk()
dev_dbg~~~~~~~
Print statement, which can be targeted by:ref:`process/debugging/userspace_debugging_guide:dynamic debug` that containsadditional information about the device used within the context.
**When is it appropriate to leave a debug print in the code?**
Permanent debug statements have to be useful for a developer to troubleshootdriver misbehavior. Judging that is a bit more of an art than a science, butsome guidelines are in the :ref:`Coding style guidelines<process/coding-style:13) printing kernel messages>`. In almost all cases thedebug statements shouldn't be upstreamed, as a working driver is supposed to besilent.
Custom printk~~~~~~~~~~~~~
Example::
#define core_dbg(fmt, arg...) do { \ if (core_debug) \ printk(KERN_DEBUG pr_fmt("core: " fmt), ## arg); \ } while (0)
**When should you do this?**
It is better to just use a pr_debug(), which can later be turned on/off withdynamic debug. Additionally, a lot of drivers activate these prints via avariable like ``core_debug`` set by a module parameter. However, Moduleparameters `are not recommended anymore<https://lore.kernel.org/all/2024032757-surcharge-grime-d3dd@gregkh>`_.
Ftrace------
Creating a custom Ftrace tracepoint~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A tracepoint adds a hook into your code that will be called and logged when thetracepoint is enabled. This can be used, for example, to trace hitting aconditional branch or to dump the internal state at specific points of the codeflow during a debugging session.
Here is a basic description of :ref:`how to implement new tracepoints<trace/tracepoints:usage>`.
For the full event tracing documentation see :doc:`/trace/events`
For the full Ftrace documentation see :doc:`/trace/ftrace`
DebugFS-------
Prerequisite: ``CONFIG_DEBUG_FS` & `#include <linux/debugfs.h>``
DebugFS differs from the other approaches of debugging, as it doesn't writemessages to the kernel log nor add traces to the code. Instead it allows thedeveloper to handle a set of files.With these files you can either store values of variables or makeregister/memory dumps or you can make these files writable and modifyvalues/settings in the driver.
Possible use-cases among others:
- Store register values- Keep track of variables- Store errors- Store settings- Toggle a setting like debug on/off- Error injection
This is especially useful, when the size of a data dump would be hard to digestas part of the general kernel log (for example when dumping raw bitstream data)or when you are not interested in all the values all the time, but with thepossibility to inspect them.
The general idea is:
- Create a directory during probe (``struct dentry *parent = debugfs_create_dir("my_driver", NULL);``)- Create a file (``debugfs_create_u32("my_value", 444, parent, &my_variable);``)
- In this example the file is found in ``/sys/kernel/debug/my_driver/my_value`` (with read permissions for user/group/all) - any read of the file will return the current contents of the variable ``my_variable``
- Clean up the directory when removing the device (``debugfs_remove(parent);``)
For the full documentation see :doc:`/filesystems/debugfs`.
KASAN, UBSAN, lockdep and other error checkers----------------------------------------------
KASAN (Kernel Address Sanitizer)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prerequisite: ``CONFIG_KASAN``
KASAN is a dynamic memory error detector that helps to find use-after-free andout-of-bounds bugs. It uses compile-time instrumentation to check every memoryaccess.
For the full documentation see :doc:`/dev-tools/kasan`.
UBSAN (Undefined Behavior Sanitizer)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prerequisite: ``CONFIG_UBSAN``
UBSAN relies on compiler instrumentation and runtime checks to detect undefinedbehavior. It is designed to find a variety of issues, including signed integeroverflow, array index out of bounds, and more.
For the full documentation see :doc:`/dev-tools/ubsan`
lockdep (Lock Dependency Validator)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prerequisite: ``CONFIG_DEBUG_LOCKDEP``
lockdep is a runtime lock dependency validator that detects potential deadlocksand other locking-related issues in the kernel.It tracks lock acquisitions and releases, building a dependency graph that isanalyzed for potential deadlocks.lockdep is especially useful for validating the correctness of lock ordering inthe kernel.
PSI (Pressure stall information tracking)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prerequisite: ``CONFIG_PSI``
PSI is a measurement tool to identify excessive overcommits on hardwareresources, that can cause performance disruptions or even OOM kills.
device coredump---------------
Prerequisite: ``CONFIG_DEV_COREDUMP`` & ``#include <linux/devcoredump.h>``
Provides the infrastructure for a driver to provide arbitrary data to userland.It is most often used in conjunction with udev or similar userland applicationto listen for kernel uevents, which indicate that the dump is ready. Udev hasrules to copy that file somewhere for long-term storage and analysis, as bydefault, the data for the dump is automatically cleaned up after a default5 minutes. That data is analyzed with driver-specific tools or GDB.
A device coredump can be created with a vmalloc area, with read/freemethods, or as a scatter/gather list.
You can find an example implementation at:`drivers/media/platform/qcom/venus/core.c<https://elixir.bootlin.com/linux/v6.11.6/source/drivers/media/platform/qcom/venus/core.c#L30>`__,in the Bluetooth HCI layer, in several wireless drivers, and in severalDRM drivers.
devcoredump interfaces~~~~~~~~~~~~~~~~~~~~~~
.. kernel-doc:: include/linux/devcoredump.h
.. kernel-doc:: drivers/base/devcoredump.c
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