KGDB internal note

  • kgdboc: kgdb over console

kgdboc init

從 kgdboc 設定 sysfs 碰到 error 開始

$ echo 'ttyS0' > /sys/module/kgdboc/parameters/kgdboc
-bash: echo: write error: No such device

這個 sysfs entry 是一個 kernel module kgdboc 的 parameter.

kgdboc parameter 帶有 setter, 也就是說當設定 sysfs parameter 時會呼叫 setter. kgdboc 的 setter 會呼叫 configure_kgdboc(), 把 parameter 中輸入的 tty driver 設定到 kgdboc 之中.

configure_kgdboc() 有幾個核心: (假設 input parameter 為 ttyS0)

  • 參數如果有 kbd 或 kdb, 使用 register_kbd() 去設定 kdb_poll_funcs

  • tty_find_polling_driver(): 在 tty_drivers 當中搜尋 ttyS0 並回傳. (p.s. tty_driver="ttyS" and tty_line="0")

  • 搜尋 console_drivers 當中有無跟該 tty_driver 同名的 driver, 有的話設定 kgdboc_io_ops.is_console = true.

  • 將 kgdb_tty_driver 設定為 ttyS0, 該 driver 為 struct kgdb_io kgdboc_io_ops 的 backend.

  • 最後呼叫 kgdb_register_io_module() 跟 kgdb_register_nmi_console() 進行註冊. kgdb_register_io_module() 會將 kgdboc 設定為 KGDB 的 IO driver.

在 configure_kgdboc() 當中, kgdb_register_io_module() 之前碰到的錯誤幾乎都會回傳 NODEV.

不過這次的問題是出在 tty_find_polling_driver() 的檢查上:

當 kgdboc 使用的 tty_driver 是以 uart 為 backend 時, uart_driver 需要支援 polled mode. 具體來說是 uart_driver 需要實作 uart_ops->poll_put_char() 跟 poll_get_char() 兩個函式.

trace:

tty_find_polling_driver() => struct tty_operations->poll_init()

in tty_driver serial_core:
  tty_operations->poll_init() == uart_poll_init()

uart_poll_init(): check if struct uart_ops->poll_get_char() and ->poll_put_char() is exist
  => calls custom init function uart_ops->poll_init() if it exist.

# odroid c2/amlogic S905
uart_driver meson_uart doesn't implement poll_get_char() and poll_put_char().

相關文件

分檔:

  • KGDB core and arch support

    • kernel/debug/debug_core.c: kgdb_register_io_module(), struct kgdb_io dbg_io_ops;

    • arch/arm64/kernel/kgdb.c: struct kgdb_arch arch_kgdb_ops;

  • KGDB over console: one implementation of KGDB IO driver

    • drivers/tty/serial/kgdboc.c: implementation of struct kgdb_io kgdboc_io_ops

    • drivers/tty/serial/kgdb_nmi.c: kgdb_register_nmi_console()

  • TTY driver/subsystem

    • drivers/tty/tty_io.c: tty_find_polling_driver(), tty_drivers

    • include/linux/tty_driver.h: struct tty_driver, tty_operations;

  • UART driver/subsystem

    • drivers/tty/serial/serial_core.c: struct tty_operations uart_ops (same meaning as tty_driver serial_core)

    • include/linux/serial_core.h: struct uart_ops, uart_port, uart_driver;

    • fs/proc/proc_tty.c: /proc/tty/drivers, proc/tty/driver/<tty_driver>

  • include/linux/console.h: struct console

  • kernel/printk/printk.c: console_drivers

KGDB

KGDB:

include/linux/kgdb.h
kernel/debug/{debug_core.c,gdbstub.c}

struct kgdb_arch, kgdb_io
arch_kgdb_ops
dbg_io_ops, kgdb_register_io_module()

kgdb_breakpoint() => arch_kgdb_breakpoint()
kgdb_handle_exception()
dbg_{set,get}_reg()

debug_core.c:

有個全域的 struct kgdb_io 變數 dbg_io_ops, kgdb_register_io_module() 便是把一個 kgdb_io 的實作 assign 給 dbg_io_ops.

  • kgdb_register_nmi_console() check

kgdboc.c:

其中一套 struct kgdb_io 的實作, kgdboc_io_ops. kgdboc_io_ops 的實作使用全域的 struct tty_driver 變數 kgdb_tty_driver. get/put_char() 的運算會去使用 kgdb_tty_driver 下面的 tty_operations.

TTY and console

global variable tty_drivers, /proc/tty/drivers, char devices /dev/tty*

/proc/tty/drivers: list all struct tty_driver in (global var) tty_drivers.:

5 columns: driver_name, name, major, minor_start~minor_end, type

# major and minor are char device number, like /dev/ttyS0 or /dev/ttyUSB0
# 主要有 4 個 type: system, serial, pty, console.
# 每個 type 下還有 subtype, 比如說 pty 就有分 pty master/slave.

/proc/tty/driver/<tty_driver>:

show uart_driver->uart_state->uart_port

/proc/tty/driver/meson_uart

many columns: line, uart, mmio, irq, tx, rx, ..., flags

# line: uport_line
# uart: uart_type

tty and uart

uart subsystem 有寫一個 tty_driver serial_core, 該 tty_driver 可以使用 uart subsystem 下的所有 driver 來實作 tty_driver.

UART

note

UART driver is also like network driver. It has send and recieve function, move their data by DMA, recieve data when interrupt trigger them.

  • send function: uart_ops.start_tx()

  • recieve function: interrupt handler => … => dma_rx()

examples:

  • drivers/tty/serial/8250/