在kernel booting的早期，肯定是希望尽快把一些信息输出的串口，能够尽早知道kernel boot 到什么阶段。 所以一般在u-boot中，在FVP平台上有如下bootargs
static int __init pl011_early_console_setup(struct earlycon_device *device,
const char *opt)
{
if (!device->port.membase)
return -ENODEV;
device->con->write = pl011_early_write;
device->con->read = pl011_early_read;
return 0;
}
OF_EARLYCON_DECLARE(pl011, "arm,pl011", pl011_early_console_setup);
OF_EARLYCON_DECLARE(pl011, "arm,sbsa-uart", pl011_early_console_setup);
earlycon的IO地址是如何映射的？
在Linux boot 的早期，即启动到start_kernel，这个时候就不能直接使用物理地址访问设备和内存，因为MMU已经enable了。
而Linux只映射了自己image和device tree所在的位置，而这里要尽可能早的能个输出关键boot信息，所以这个串口的IO地址也要在早期给映射好。
在mem_init之前，系统的内存管理并没有准备好，所以并不能直接使用ioremap 来给串口进行映射。所以这里就会使用到fixed map来映射这个早期的map。
也就是 FIX_EARLYCON_MEM_BASE ，这也是在arm64的系统里面，必须要使能： CONFIG_FIX_EARLYCON_MEM 的原因：
static void __iomem * __init earlycon_map(resource_size_t paddr, size_t size)
{
void __iomem *base;
#ifdef CONFIG_FIX_EARLYCON_MEM
set_fixmap_io(FIX_EARLYCON_MEM_BASE, paddr & PAGE_MASK);
base = (void __iomem *)__fix_to_virt(FIX_EARLYCON_MEM_BASE);
base += paddr & ~PAGE_MASK;
#else
base = ioremap(paddr, size);
#endif
if (!base)
pr_err("%s: Couldn't map %pa\n", __func__, &paddr);
return base;
}
如何不使能这个 CONFIG_FIX_EARLYCON_MEM 的话，会出现如下错误：
[ 0.000000] ------------[ cut here ]------------
[ 0.000000] WARNING: CPU: 0 PID: 0 at mm/ioremap.c:23 generic_ioremap_prot+0xf0/0x12c
[ 0.000000] Modules linked in:
[ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.10.0-rc1-00013-g2bfcfd584ff5-dirty #4
[ 0.000000] Hardware name: FVP Base RevC (DT)
[ 0.000000] pstate: 800003c9 (Nzcv DAIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 0.000000] pc : generic_ioremap_prot+0xf0/0x12c
[ 0.000000] lr : generic_ioremap_prot+0x28/0x12c
[ 0.000000] sp : ffff800082623b40
[ 0.000000] x29: ffff800082623b40 x28: 0000000000000000 x27: ffff8000829ab000
[ 0.000000] x26: 0000000000000000 x25: ffff800081e11398 x24: 0000000000000000
[ 0.000000] x23: ffff800081e13608 x22: ffff800080033710 x21: 0068000000000713
[ 0.000000] x20: 000000001c090000 x19: 0000000000000040 x18: fffffffffffe0198
[ 0.000000] x17: 00000000242dc565 x16: 0000000000000000 x15: 0000000000000028
[ 0.000000] x14: ffffffffffffffff x13: 0000000000000007 x12: ffff80008168a740
[ 0.000000] x11: 0101010101010101 x10: ffff800101d38846 x9 : 0fffffffffffffff
[ 0.000000] x8 : 0000000000000010 x7 : 0000000000000018 x6 : ffff800082b20500
[ 0.000000] x5 : ffff800082b20500 x4 : 0000000000000000 x3 : 0000000000000000
[ 0.000000] x2 : 0068000000000713 x1 : 0000000000000040 x0 : 0000000000000000
[ 0.000000] Call trace:
[ 0.000000] generic_ioremap_prot+0xf0/0x12c
[ 0.000000] ioremap_prot+0x50/0x78
[ 0.000000] earlycon_map+0x4c/0x94
[ 0.000000] setup_earlycon+0x254/0x314
[ 0.000000] param_setup_earlycon+0x30/0x50
[ 0.000000] do_early_param+0xb8/0xf8
[ 0.000000] parse_args+0x144/0x3ac
[ 0.000000] parse_early_param+0x60/0x78
[ 0.000000] setup_arch+0x14c/0x60c
[ 0.000000] start_kernel+0x70/0x708
[ 0.000000] __primary_switched+0x80/0x88
[ 0.000000] ---[ end trace 0000000000000000 ]---
[ 0.000000] earlycon: earlycon_map: Couldn't map 0x000000001c090000
[ 0.000000] earlycon: pl11 at MMIO 0x000000001c090000 (options '')
[ 0.000000] Malformed early option 'earlycon'
在运行完函数earlycon_map 之后,即：
#0 register_earlycon(buf = (char*) 0x0, match = <Value optimised away by compiler>) at earlycon.c:151
#1 setup_earlycon(buf = 0xFFFF800081C78847 "0x1c090000") at earlycon.c:221
#2 param_setup_earlycon(buf = <Value not available : Undefined value in stack frame for register X1>) at earlycon.c:245
#3 do_early_param(param = 0xFFFF800081C78838 "earlycon", val = 0xFFFF800081C78841 "pl011,0x1c090000", unused = <Value not available : Undefined value in stack frame for register X2>, arg = <Value not available : Undefined value in stack frame for register X3>) at main.c:753
#4 parse_one(param = <Value optimised away by compiler>, val = <Value optimised away by compiler>, doing = <Value optimised away by compiler>, params = <Value optimised away by compiler>, num_params = <Value optimised away by compiler>, min_level = <Value optimised away by compiler>, max_level = <Value optimised away by compiler>, arg = <Value optimised away by compiler>, handle_unknown = <Value optimised away by compiler>) at params.c:153
#5 parse_args(doing = 0xFFFF8000819075B0 "early options", args = <Value not available : Undefined value in stack frame for register X1>, params = (const struct kernel_param*) 0x0, num = 0, min_level = <Value not available : Undefined value in stack frame for register X4>, max_level = <Value not available : Undefined value in stack frame for register X5>, arg = <Value not available : Undefined value in stack frame for register X6>, unknown = init/main.c#int do_early_param( char* param, char* val, const char* unused, void* arg ) = 0xffff800081b700b8) at params.c:188
#6 parse_early_param() at main.c:779
#7 local_daif_restore(flags = <Value optimised away by compiler>) at setup.c:298
#8 cpu_set_reserved_ttbr0_nosync() at daifflags.h:117
#9 get_boot_config_from_initrd(_size = <Value optimised away by compiler>) at main.c:906
#10 setup_command_line(command_line = <Value optimised away by compiler>) at main.c:643
#11 __primary_switched() at head.S:244
就会发现mmu页表里面就多了串口的映射,即0x000000001C090000 --> 0xFFFFFFFFFF5FD000
>mmu memory-map
Virtual Range | Physical Range | Type | AP | C | S | X
---------------------------------------------------------------------------------------------------------------------------------
EL2N:0x0000000000000000-0x000000008020FFFF | <unmapped> | | | | |
EL2N:0x0000000080210000-0x0000000081E6FFFF | NP:0x0000000080210000-0x0000000081E6FFFF | Normal | RO | True | True | True
EL2N:0x0000000081E70000-0x0000000082C9FFFF | NP:0x0000000081E70000-0x0000000082C9FFFF | Normal | RW | True | True | False
EL2N:0x0000000082CA0000-0x00000000FD6BDFFF | <unmapped> | | | | |
EL2N:0x00000000FD6BE000-0x00000000FD8BDFFF | NP:0x00000000FD6BE000-0x00000000FD8BDFFF | Normal | RW | True | True | False
EL2N:0x00000000FD8BE000-0x0000FFFFFFFFFFFF | <unmapped> | | | | |
EL2N:0xFFFF000000000000-0xFFFF80008000FFFF | <unmapped> | | | | |
EL2N:0xFFFF800080010000-0xFFFF80008105FFFF | NP:0x0000000080210000-0x000000008125FFFF | Normal | RO | True | True | True
EL2N:0xFFFF800081060000-0xFFFF800081B6FFFF | NP:0x0000000081260000-0x0000000081D6FFFF | Normal | RW | True | True | False
EL2N:0xFFFF800081B70000-0xFFFF800081C6FFFF | NP:0x0000000081D70000-0x0000000081E6FFFF | Normal | RO | True | True | True
EL2N:0xFFFF800081C70000-0xFFFF800082A9FFFF | NP:0x0000000081E70000-0x0000000082C9FFFF | Normal | RW | True | True | False
EL2N:0xFFFF800082AA0000-0xFFFFFFFFFF5FCFFF | <unmapped> | | | | |
EL2N:0xFFFFFFFFFF5FD000-0xFFFFFFFFFF5FDFFF | NP:0x000000001C090000-0x000000001C090FFF | Device-nGnRE | RW | False | False | False
EL2N:0xFFFFFFFFFF5FE000-0xFFFFFFFFFF600FFF | NP:0x00000000FD6BE000-0x00000000FD6C0FFF | Normal | RO | True | True | False
EL2N:0xFFFFFFFFFF601000-0xFFFFFFFFFFFFFFFF | <unmapped> | | | | |
有了这些IO的映射之后，就可以调用pl011_early_console_setup 对PL011串口进行初始化，然后再把这个串口设备通过函数register_console 注册到输出设备上，就可以开始输出信息了。
printk调用PL011驱动函数的调用stack如下：
#0 pl011_early_write(con = (struct console*) 0xFFFF8000828B4878, s = 0xFFFF8000829F3678 "[ 0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd0f0]
", n = 71) at amba-pl011.c:2541
#1 console_emit_next_record(con = <Value optimised away by compiler>, handover = <Value optimised away by compiler>, cookie = <Value optimised away by compiler>) at printk.c:2915
#2 __srcu_read_unlock_nmisafe(ssp = <Value optimised away by compiler>, idx = <Value optimised away by compiler>) at srcu.h:301
#3 console_unlock() at printk.c:3049
#4 vprintk_emit(facility = 0, level = -1, dev_info = (const struct dev_printk_info*) 0x0, fmt = 0xFFFF800081917728 "6printk: %s%sconsole [%s%d] enabled
", args = {__stack = (void*) 0x8, __gr_top = (void*) 0x2, __vr_top = (void*) 0x0, __gr_offs = 0, __vr_offs = 0}) at printk.c:2348
#5 vprintk_default(fmt = <Value not available : Undefined value in stack frame for register X0>, args ERROR(CMD367-TAB183):
! Error computing the variable "args"
! Undefined value in stack frame for register X5
) at printk.c:2364
#6 vprintk(fmt = 0xFFFF800081917728 "6printk: %s%sconsole [%s%d] enabled
", args = {__stack = (void*) 0xFFFF800082533B90, __gr_top = (void*) 0xFFFF800082533B90, __vr_top = (void*) 0xFFFF800082533B50, __gr_offs = -56, __vr_offs = 0}) at printk_safe.c:46
#7 _printk(fmt = <Value not available : Undefined value in stack frame for register X0>) at printk.c:2374
#8 register_console(newcon = (struct console*) 0xFFFF8000828B4878) at printk.c:3570
#9 register_earlycon(buf = (char*) 0x0, match = <Value optimised away by compiler>) at earlycon.c:162
#10 setup_earlycon(buf = 0xFFFF800081C78847 "0x1c090000") at earlycon.c:221
#11 param_setup_earlycon(buf = <Value not available : Undefined value in stack frame for register X1>) at earlycon.c:245
#12 do_early_param(param = 0xFFFF800081C78838 "earlycon", val = 0xFFFF800081C78841 "pl011,0x1c090000", unused = <Value not available : Undefined value in stack frame for register X2>, arg = <Value not available : Undefined value in stack frame for register X3>) at main.c:753
#13 parse_one(param = <Value optimised away by compiler>, val = <Value optimised away by compiler>, doing = <Value optimised away by compiler>, params = <Value optimised away by compiler>, num_params = <Value optimised away by compiler>, min_level = <Value optimised away by compiler>, max_level = <Value optimised away by compiler>, arg = <Value optimised away by compiler>, handle_unknown = <Value optimised away by compiler>) at params.c:153
#14 parse_args(doing = 0xFFFF8000819075B0 "early options", args = <Value not available : Undefined value in stack frame for register X1>, params = (const struct kernel_param*) 0x0, num = 0, min_level = <Value not available : Undefined value in stack frame for register X4>, max_level = <Value not available : Undefined value in stack frame for register X5>, arg = <Value not available : Undefined value in stack frame for register X6>, unknown = init/main.c#int do_early_param( char* param, char* val, const char* unused, void* arg ) = 0xffff800081b700b8) at params.c:188
#15 parse_early_param() at main.c:779
#16 local_daif_restore(flags = <Value optimised away by compiler>) at setup.c:298
#17 cpu_set_reserved_ttbr0_nosync() at daifflags.h:117
#18 get_boot_config_from_initrd(_size = <Value optimised away by compiler>) at main.c:906
#19 setup_command_line(command_line = <Value optimised away by compiler>) at main.c:643
#20 __primary_switched() at head.S:244
在串口初始化之前，也会调用printk，但是这些信息会被buffer住，等到串口设备注册好之后，才会输出到串口。 所以看到如下信息：
[ 0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd0f0]
[ 0.000000] Linux version 6.9.0-dirty (qixxu01@a015921) (aarch64-none-linux-gnu-gcc (Arm GNU Toolchain 13.2.rel1 (Build arm-13.7)) 13.2.1 20231009, GNU ld (Arm GNU Toolchain 13.2.rel1 (Build arm-13.7)) 2.41.0.20231009) #37 SMP PREEMPT Fri May 31 16:16:32 CST 2024
[ 0.000000] KASLR disabled due to lack of seed
[ 0.000000] Machine model: FVP Base RevC
[ 0.000000] earlycon: pl11 at MMIO 0x000000001c090000 (options '')
[ 0.000000] printk: legacy bootconsole [pl11] enabled
earlycon是如何切换到正常串口？
在前面的bootargs里面，这里有给出最后console的device console=ttyAMA0：
bootargs=console=ttyAMA0 earlycon=pl011,0x1c090000 root=/dev/vda1 rw ip=dhcp debug user_debug=31 loglevel=9
这里指定了console对应的device，这个是在函数console_setup 里面进行解析，然后调用__add_preferred_console 来设置正常串口的信息.
static int __init console_setup(char *str)
{
//...
__add_preferred_console(buf, idx, options, brl_options, true);
return 1;
}
__setup("console=", console_setup);
参考调用栈：
#0 console_setup(str = 0xFFFF800081D4DF28 "ￃﾢￃﾍￂﾁ") at printk.c:2488
#1 obsolete_checksetup(line = <Value optimised away by compiler>) at main.c:215
#2 unknown_bootoption(param = 0xFFFF00087F801140 "console=ttyAMA0", val = 0xFFFF00087F801148 "ttyAMA0", unused = <Value not available : Undefined value in stack frame for register X2>, arg = <Value not available : Undefined value in stack frame for register X3>) at main.c:561
#3 parse_one(param = <Value optimised away by compiler>, val = <Value optimised away by compiler>, doing = <Value optimised away by compiler>, params = <Value optimised away by compiler>, num_params = <Value optimised away by compiler>, min_level = <Value optimised away by compiler>, max_level = <Value optimised away by compiler>, arg = <Value optimised away by compiler>, handle_unknown = <Value optimised away by compiler>) at params.c:153
#4 parse_args(doing = 0xFFFF800081907650 "Booting kernel", args = <Value not available : Undefined value in stack frame for register X1>, params = (const struct kernel_param*) 0xFFFF800081B4E1B8, num = 477, min_level = <Value not available : Undefined value in stack frame for register X4>, max_level = <Value not available : Undefined value in stack frame for register X5>, arg = <Value not available : Undefined value in stack frame for register X6>, unknown = init/main.c#int unknown_bootoption( char* param, char* val, const char* unused, void* arg ) = 0xffff800081b703f8) at params.c:188
#5 start_kernel() at main.c:917
#6 __primary_switched() at head.S:244
而在device tree， 中，有：
v2m_serial0: serial@90000 {
compatible = "arm,pl011", "arm,primecell";
reg = <0x090000 0x1000>;
interrupts = <5>;
clocks = <&v2m_clk24mhz>, <&v2m_clk24mhz>;
clock-names = "uartclk", "apb_pclk";
};
当Linux解析device tree 的时候，会触发PL011的driver：
static struct console amba_console = {
.name = "ttyAMA",
.write = pl011_console_write,
.device = uart_console_device,
.setup = pl011_console_setup,
.match = pl011_console_match,
.flags = CON_PRINTBUFFER | CON_ANYTIME,
.index = -1,
.data = &amba_reg,
};
当解析到上面的device tree 会调用pl011_probe ，在这个函数中，又会调用register_console 来注册新的console来替换boot阶段的console， 调用栈：
#8 register_console(newcon = (struct console*) 0xFFFF8000828B4B10) at printk.c:3572
#9 uart_configure_port(drv = <Value optimised away by compiler>, state = <Value optimised away by compiler>, port = <Value optimised away by compiler>) at serial_core.c:2668
#10 serial_core_add_one_port(drv = <Value optimised away by compiler>, uport = <Value optimised away by compiler>) at serial_core.c:3229
#11 serial_ctrl_register_port(drv = <Value not available : Undefined value in stack frame for register X0>, port = <Value not available : Undefined value in stack frame for register X1>) at serial_ctrl.c:42
#12 uart_add_one_port(drv = <Value not available : Undefined value in stack frame for register X0>, port = <Value not available : Undefined value in stack frame for register X1>) at serial_port.c:136
#13 pl011_register_port(uap = (struct uart_amba_port*) 0xFFFF000800A41480) at amba-pl011.c:2764
#14 pl011_probe(dev = (struct amba_device*) 0xFFFF000800A41000, id = <Value not available : Undefined value in stack frame for register X1>) at amba-pl011.c:2829
#15 amba_probe(dev = (struct device*) 0xFFFF000800A41000) at bus.c:308
#16 call_driver_probe(dev = <Value optimised away by compiler>, drv = <Value optimised away by compiler>) at dd.c:578
在函数register_console 就完成了从earlycon到正式的console的切换，这个时候串口会有如下输出：
[ 0.196646] Serial: AMBA PL011 UART driver
[ 0.236878] 1c090000.serial: ttyAMA0 at MMIO 0x1c090000 (irq = 16, base_baud = 0) is a PL011 rev2
[ 0.237034] printk: legacy console [ttyAMA0] enabled
[ 0.237034] printk: legacy console [ttyAMA0] enabled
[ 0.237160] printk: legacy bootconsole [pl11] disabled
[ 0.237160] printk: legacy bootconsole [pl11] disabled
这里的log打印两次是因为这个时候老的还没有解注册，所以就会有两个console存在，会打印两次。
运行到register_console的最后，earlycon也会被unregister_console_locked给删除掉。 这样就完成了从earycon到正式的console的切换。
Note：别的系统可能会在start_kernel-> console_init里面完成console的转换。参考：
console_initcall(con_init);