Linux driver: How to enable dynamic debug at booting time for built-in driver.

 Dynamic debug is useful for debug driver, and can be enable by:

1. Mount debug fs

#>mount -t debugfs none /sys/kernel/debug

2. Enable dynamic debug for specifiy funcition

 echo 'file virtio_pci_modern.c +p' > /sys/kernel/debug/dynamic_debug/control

However somtimes we need to debug built-in driver and need enable dynamic debug at boot time. then we can enable by passing dyndbg to Kernel command line like

dyndbg="'file virtio_pci_modern.c +p"

with QEMU we can use -append to attach dyndbg

qemu-system-arm -kernel kernel-qemu -cpu arm1176 -m 256 -append "dyndbg="file virtio_pci_modern.c +p""

Ref:

  Kernel document/dynamic-debug-howto

 https://www.kernel.org/doc/html/v4.11/admin-guide/dynamic-debug-howto.html

Hexdump: Covert file content to hex array.

 Hexdump can dump file content with hex format,  with parameter -e can format displaying data and it's useful in writing shell.

Example usage:

#> hexdump -v FILE -e '1/1 "0x%02X "'

Test result:

Ref:

  Hexdump man page

  https://man7.org/linux/man-pages/man1/hexdump.1.html

Performance tuning- Leveraging modern CPU branch predict mechanism.

In modern CPU the branch predictor is complicated and the deeper pipeline causing the cost of miss branch prediction higher.

How can we leveraging the modern CPU branch predictor, In [1] example code, the branch miss rate can  be reduce by just sorting input data before exciting original algorithm. 

Conclusion 

1. Adding pattern to your algorithm(sorted data).

2. Using likely()/unlikely() marco to help branch prediction more accuracy.

Reference:

1. Performance analysis in Linux

2. Code DetectiveHow to investigate LinuxPerformance Issues

3. Linux likely() unlikely() MARCO.

Linux kernel module: Add entry to debugfs and a read/write file.

Goal

This module will create Ray_DBG directory in debugfs and create a file REG for read/write. it's similar to previous module(create /proc/Ray), but easier to use and need less line of code.

 

Code

Makefile

Usage

1. Insert module 

$>insmod test_module.ko
$>dmesg | tail
[7501255.251763] Create Ray_DBG !
[7501255.251776] Create REG debug !

2. Mount debugfs 

$>mount -t debugfs none /sys/kernel/debug

3. Read data

$>cat /sys/kernel/debug/Ray_DBG/REG
$>0x000000aa

4. Write data 

$>echo 0xff>  /sys/kernel/debug/Ray_DBG/REG 
$>cat /sys/kernel/debug/Ray_DBG/REG 
$>0x000000ff

Reference

1. Kernel document: debugfs
2. Debugfs

Linux kernel module: Add entry in /proc and passing args while insmod

Goal 

Implement a kernel module that can passing parameter when insert module, and add an entry to /proc that can write/read data. In embedded linux often using similar facility to help debug.

Code:

Makefile:

Usage:

1. Insert module and make /proc/Ray entry

 #>sudo insmod test_module.ko entry="Ray" mode=1238

 

2. Write data 

#>echo 123 > /proc/Ray

3. Read data

 #>cat /proc/Ray

 

Reference:

1. Linux Device Drivers Development: Develop customized drivers for embedded Linux
2. LINUX KERNEL DEVELOPMENT – CREATING A PROC FILE AND INTERFACING WITH USER SPACE

Conference note: Making C Less Dangerous in the Linux kernel - Kees Cook

In this conference discuss below topic about unsafely C usage, and how Linux kernel has remove or add  facility to detect such condition.

1. Variable Length array

• Using compiler option to Detect VLA: gcc -W vla
• Using guard page tor prevent stack overflow. VLA is needed lots of instruction compared to the fixed-size array.

2. Switch case break or non-break

• Mark all non-breaks with a “fall through” to whether programmer intent to fall through or it's a bug.
• Compiler support this feature: -Wimplicit-fallthrough

3. Arithmetic overflow detection

• Using compiler option to detection overflow in compile time
• Support different warning label: ignore or take as warring

4. Compare different API for string copy

• The safer string copy function: strscpy().

5. Safe stack - Shadow stack

• Separate the local variable stack and return address stack
• Support by hardware:
• ARM pointer authentication (Sign the return address for distinguish between a local variable and return address 

Reference :

1. Making C Less Dangerous in the Linux kernel - Kees Cook

Conference note: Safety vs Security: A Tale of Two Updates - Jeremy Rosen, Smile.fr

Safety and Security are different aspects of a system, briefly discuss the differences between them and challenge to build a system with both concerns.

• Safety: Proof and make it simple for reliable

1. Completely define spec.
2. Proven: Check every state of the system is meet the design.
3. Change as less as possible: If a bug can solve by period reboot machine, just period reboot machine rather than update the software.

• Security: Prevent system been using in unwanted way

1. Fasting iteration: Change the old encryption algorithm to a new one.
2. Preventive: Need update the spec to face the new challenge.

These two aspects often neglect in embedded system nowadays, especially in customer market. but with more and more device connected to the internet and responsible for critical tasks like healthcare. Designer need to take care of these two fields, Include them in the beginning of designing a system and try to make an optimized combination of them to meet the system standards.

Reference : 

        1. Safety vs Security: A Tale of Two Updates - Jeremy Rosen, Smile.fr

Conference note: Linux I2C in the 21st Century - Wolfram Sang, Consultant / Renesas

This conference is intent to brief overview what't new in Linux I2C subsystem.
I found some interesting part of the I2C subsystem of the latest linux kernel:

• The API i2c_dummy_device() for I2C device have more than one slave address.

Declare a dummy I2C deivce share same device but different slave address

• Recommend new API to create I2C device: i2c_new_ancillary_device()

• Dynamic address assign : In same I2C bus and dynamic detect what address should use.

I2C has been widely used in industry for decades and has some enhance feature like have multi slave address or dynamic assign address of slave in the same bus,

 bring lots of challenge for developer and Linux kernel provide some new API for more generic driver development.

Further speaking:

I3C is the next generation serial bus to replace I2C but I didn't saw much application  right now.

Reference : 

1.  Linux I2C in the 21st Century - Wolfram Sang, Consultant / Renesas
2.  I2C and SMBus Subsystem
3.  I3C 

How to get main() function return value in Linux terminal.

Sometimes we want to get the exit value of main() function, using the "echo $? " can get the most recently program exit value.

Example program:

Result:

Reference:
  1. UNIX Shell Programming 

C : strrchr() analyze and example get self executable directory function.

strrchr() function will return a pointer to the last occurrence.
Another example is modify from Advanced Linux Programming chapter 11.We can get a sub-sting without the occurrence with sub two pointer.





Ref:
   1. http://www.cplusplus.com/reference/cstring/strrchr/
   2. Advanced Linux Programming by Mark Mitchell, Jeffrey Oldham, and Alex Samuel www.newriders.com