Kernel network tune-up

I closed two open tickets related to networking in Linux Kernel. So, I'd write it down in order not to forget.

* broadcast ping problem. After Linux kernel 2.6.14+, the kernel wont' respond to broadcast ping by default. You have to "echo 0 > /proc/sys/net/ipv4/ignore_broadcast_icmp" to enable response to broadcast ping. The default behavior has been changed.

* loopback ping > 32K payload. For small system with <= 16M memory, we use loopback UDP for IPC. But we can't receive packets with 32K+ payload. I wrote a test program to verify the problem. After one day's hack, I found that it is due to a limitation in internal buffer size of skbuff (the internal data structure in Linux network stack). "/proc/sys/net/core/rmem_max", "/proc/sys/net/core/rmem_default". We have to increase the limit on those two entries to allow loopback ping with 32K+ payload. There won't any problem with network ping, as the MTU limitation will eventually split the skbuff and you won't need a big skbuff. The loopback has no MTU limitation, thus it matters.

Kernel programming and driver debugging

Don't forget to kill the klogd and syslogd daemons before you start to debug your driver or kernel codes. Otherwise, your printk will be buffered and you won't know the exact point of crash or hang.

"
killall klogd
killall syslogd
"

For interrupt handler, the first thing in your ISR is to disable or mask out the interrupt, otherwise, the interrupt may keep firing and your system may be locked up.
At the end of your ISR, you can re-enable and enable the mask of the interrupt.

To get a list of interrupt and interrupt handler,
"
cat /proc/interrupts
"

godaddy programming tip

to use php5, your script extension should be .php5, instead of php

my .vimrc

" LEO's customization

" version 4.0

set nocompatible

" Broadcom coding style
set shiftwidth=3
set tabstop=3
set expandtab

" Nice search
set incsearch
set ignorecase
set smartcase

set ai
set backspace=2


"set backupdir=~/.backup
"set nobackup

" My shortcut key mapping
map gs :%s/

map <c-j> <c-w>j
map <c-k> <c-w>k
map <c-h> <c-w>h
map <c-l> <c-w>l

map <f2> @q
map <f3> @a
map <f10> :set paste<cr>
map <f11> <c-w>-
map <f12> <c-w>+

map :Q :qa!

" For nicer scroll, who knows
set showcmd
set sm
set ss=1
set siso=9
set so=3

" highlight search result
set hls

" syntax highlight
syntax on
colorscheme darkblue

"copy and paste betweeen different vim sessions
nmap <c-y> :!echo ""> ~/.vi_tmp<cr><cr>:w! ~/.vi_tmp<cr>
vmap <c-y> :w! ~/.vi_tmp<cr>
nmap <c-p> :r ~/.vi_tmp<cr>
vmap <c-p> c<esc>:r ~/.vi_tmp<cr>
nmap :Q :qa

"expand the directory with pwd of file under editing
nmap ,e :e <c-r>=expand("%:p:h") . "/" <cr>
nmap ,n :new <c-r>=expand("%:p:h") . "/" <cr>

" Remember the last edit position
set viminfo='10,\"100,:20,%,n~/.viminfo
au BufReadPost * if line("'\"") > 0|if line("'\"") <= line("$")|exe("norm '\"")|else|exe "norm $"|endif|endif

" Command abbreviation for spell checking
cab aspe :w<cr>:!aspell -e -x -c %<cr>:e<cr><cr>

highlight RedundantSpaces term=standout ctermbg=red guibg=red
match RedundantSpaces /\s\+$\| \+\ze\t/

set ruler
set autoindent
set smartindent

"set spell

"set statusline=%F%m%r%h%w\ [FORMAT=%{&ff}]\ [TYPE=%Y]\ [ASCII=\%03.3b]\ [HEX=\%02.2B]\ [POS=%04l,%04v][%p%%]\ [LEN=%L]
"set statusline=%F%m%r%h%w\ [[TYPE=%Y]\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ [POS=%04l,%04v][%p%%]\ [LEN=%L]
"set laststatus=2


let perl_extended_vars=1
filetype plugin on " load filetype plugins

set visualbell t_vb=
let loaded_matchparen=1

Working on my first driver

I have been working on the touchscreen driver for the past two weeks. In last Friday, it seems that my work had made a milestone. The user land touchscreen calibration and test programs works just fine using tslib.

The touchscreen driver I created conforms to the input layer and event interface of Linux kernel. I have to use "input_register_driver" and "input_register_device" to register my device and the driver. The "probe" function then called.

I have to work out the proc entries and cleanup the code, and write documentation in next week, which should be done without much trouble.

let's discuss Linux kernel programming here

As told by Dave today, the MIPS kernel stack dump doesn't dump the calling stack in order. It just dump all the seemly kernel functions in the stack. They could be functions called before the real BUG happened.

For example, the following call trace dump is not the real calling stack. The bug is in set_mctrl, which called spin_lock_irqsave, but the same lock was acquired by the caller of set_mctrl function already.

------------------
BUG: spinlock recursion on CPU#0, swapper/1
lock: 80302a84, .magic: dead4ead, .owner: swapper/1, .owner_cpu: 0
Call Trace:
[<801f03c8>] _raw_spin_lock+0x4c/0x154
[<802c2ef4>] _spin_lock_irqsave+0x40/0x58
[<80212bfc>] bcm1103serial_set_mctrl+0x70/0xb8
[<8012e934>] printk+0x1c/0x28
[<802123ac>] uart_add_one_port+0x290/0x354
[<80184320>] exact_match+0x0/0x8
[<80184328>] exact_lock+0x0/0x28
[<801ff9d4>] alloc_tty_driver+0x24/0x64
[<80212088>] uart_register_driver+0x194/0x1cc
[<80350000>] keypad_init+0x48/0x14c
[<80351020>] bcm1103serial_init+0x44/0x68
[<80100558>] init+0xc4/0x29c
[<80100558>] init+0xc4/0x29c
[<80109cd4>] kernel_thread_helper+0x10/0x18
[<80109cc4>] kernel_thread_helper+0x0/0x18
-------------------

Remember, the call trace dump of MIPS is not the exact calling trace, you have to find clue in the functions dumped.