内核启动早期的打印
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在移植4.16内核到tiny4412的过程中遇到一个问题，官方的uboot2012引导内核成功卡在staring the kernel没有了下文，看来内核还没有到注册uart驱动就已经挂掉了，尝试打开 early printk ，让内核自解压之后能够及时的打印信息。
说句题外话，内核启动卡在staring the kernel，在2440的时代，未使用设备树，这种情况往往是机器ID设置错误，或者是串口波特率等方面的原因导致，在设备树的时代，机器ID已经被废除，最有可能的问题可能就是出在设备树的身上。
early printk的使用：
1、配置内核
make menuconfig
Kernel hacking --->
[*] Kernel low-level debugging functions (read help!)
Kernel low-level debugging port (Use Samsung S3C UART 0 for low-level debug)
[*] Early printk
2、设置环境变量
Add earlyprintk to your kernel parameters to enable this console
在 bootargs 中添加 earlyprintk
首先来看 bootargs earlyprintk的解析：
arch/arm/kernel/early_printk.c
extern void printascii(const char *);
static void early_write(const char *s, unsigned n)
{
char buf[128];
while (n) {
unsigned l = min(n, sizeof(buf)-1);
memcpy(buf, s, l);
buf[l] = 0;
s += l;
n -= l;
printascii(buf);
}
}
static void early_console_write(struct console *con, const char *s, unsigned n)
{
early_write(s, n);
}
static struct console early_console_dev = {
.name = "earlycon",
.write = early_console_write,
.flags = CON_PRINTBUFFER | CON_BOOT,
.index = -1,
};
static int __init setup_early_printk(char *buf)
{
early_console = &early_console_dev;
register_console(&early_console_dev);
return 0;
}
early_param("earlyprintk", setup_early_printk);35
1
extern void printascii(const char *);2
3
static void early_write(const char *s, unsigned n)4
{5
char buf[128];6
while (n) {7
unsigned l = min(n, sizeof(buf)-1);8
memcpy(buf, s, l);9
buf[l] = 0;10
s += l;11
n -= l;12
printascii(buf);13
}14
}15
16
static void early_console_write(struct console *con, const char *s, unsigned n)17
{18
early_write(s, n);19
}20
21
static struct console early_console_dev = {22
.name = "earlycon",23
.write = early_console_write,24
.flags = CON_PRINTBUFFER | CON_BOOT,25
.index = -1,26
};27
28
static int __init setup_early_printk(char *buf)29
{30
early_console = &early_console_dev;31
register_console(&early_console_dev);32
return 0;33
}34
35
early_param("earlyprintk", setup_early_printk);在内核启动进入C语言阶段,start_kernel->parse_early_param 就会第一时间解析early_param("earlyprintk", setup_early_printk);
然后调用 register_console(&early_console_dev);
可以看到 early_console 的 write 函数最终靠 printascii 来实现，同样，直接调用 earlyprintk 也会调用 printascii
void __init early_print(const char *str, ...)
{
extern void printascii(const char *);
char buf[256];
va_list ap;
va_start(ap, str);
vsnprintf(buf, sizeof(buf), str, ap);
va_end(ap);
#ifdef CONFIG_DEBUG_LL
printascii(buf);
#endif
printk("%s", buf);
}15
1
void __init early_print(const char *str, ...)2
{3
extern void printascii(const char *);4
char buf[256];5
va_list ap;6
7
va_start(ap, str);8
vsnprintf(buf, sizeof(buf), str, ap);9
va_end(ap);10
11
12
printascii(buf);13
14
printk("%s", buf);15
}printascii：arch/arm/kernel/debug.S
ENTRY(printascii)
addruart_current r3, r1, r2
1: teq r0, #0
ldrneb r1, [r0], #1
teqne r1, #0
reteq lr
2: teq r1, #‘
‘
bne 3f
mov r1, #‘
‘
waituart r2, r3
senduart r1, r3
busyuart r2, r3
mov r1, #‘
‘
3: waituart r2, r3
senduart r1, r3
busyuart r2, r3
b 1b
ENDPROC(printascii)18
1
ENTRY(printascii)2
addruart_current r3, r1, r23
1: teq r0, #04
ldrneb r1, [r0], #15
teqne r1, #06
reteq lr7
2: teq r1, #‘
‘8
bne 3f9
mov r1, #‘
‘10
waituart r2, r311
senduart r1, r312
busyuart r2, r313
mov r1, #‘
‘14
3: waituart r2, r315
senduart r1, r316
busyuart r2, r317
b 1b18
ENDPROC(printascii) .macro addruart_current, rx, tmp1, tmp2
addruart mp1, mp2,
x
mrc p15, 0,
x, c1, c0
tst
x, #1
moveq
x, mp1
movne
x, mp2
.endm7
1
.macro addruart_current, rx, tmp1, tmp22
addruart mp1, mp2,
x3
mrc p15, 0,
x, c1, c04
tst
x, #15
moveq
x, mp16
movne
x, mp27
.endmaddruart_current 和 addruart 都是宏定义
各个平台实现自己的 addruart 宏：4412的位于：arch/arm/include/debug/exynos.S
23 .macro addruart, rp, rv, tmp
24 mrc p15, 0, mp, c0, c0, 0
25 and mp, mp, #0xf0
26 teq mp, #0xf0 @@ A15
27 beq 100f
28 mrc p15, 0, mp, c0, c0, 5
29 and mp, mp, #0xf00
30 teq mp, #0x100 @@ A15 + A7 but boot to A7
31 100: ldreq
p, =EXYNOS5_PA_UART
32 movne
p, #EXYNOS4_PA_UART @@ EXYNOS4
33 ldr
v, =S3C_VA_UART
34 #if CONFIG_DEBUG_S3C_UART != 0
35 add
p,
p, #(0x10000 * CONFIG_DEBUG_S3C_UART)
36 add
v,
v, #(0x10000 * CONFIG_DEBUG_S3C_UART)
37 #endif
38 .endm
17
1
23 .macro addruart, rp, rv, tmp2
24 mrc p15, 0, mp, c0, c0, 03
25 and mp, mp, #0xf04
26 teq mp, #0xf0 @@ A155
27 beq 100f6
28 mrc p15, 0, mp, c0, c0, 57
29 and mp, mp, #0xf008
30 teq mp, #0x100 @@ A15 + A7 but boot to A79
31 100: ldreq
p, =EXYNOS5_PA_UART10
32 movne
p, #EXYNOS4_PA_UART @@ EXYNOS411
33 ldr
v, =S3C_VA_UART12
34 #if CONFIG_DEBUG_S3C_UART != 013
35 add
p,
p, #(0x10000 * CONFIG_DEBUG_S3C_UART)14
36 add
v,
v, #(0x10000 * CONFIG_DEBUG_S3C_UART)15
37 #endif16
38 .endm17
这里返回的是 uart0 的基地址，虚拟地址和物理地址，EXYNOS4_PA_UART=0x13800000
有了基地址还不够，能需要UTXHn的偏移地址才行
arch/arm/include/debug/samsung.S
51 .macro senduart,rd,rx
52 strb
d, [
x, # S3C2410_UTXH]
53 .endm3
1
51 .macro senduart,rd,rx2
52 strb
d, [
x, # S3C2410_UTXH]3
53 .endminclude/linux/serial_s3c.h:33:#define S3C2410_UTXH (0x20)
因此，可以得知，内核earlyprintk不会去初始化串口，直接使用Uboot初始化ok的，这是个好消息~
C语言阶段，可以调用early_printk，那么在自解压之后，C语言之前的汇编阶段，如何打印呢？
参考：
__error_p:
#ifdef CONFIG_DEBUG_LL
adr r0, str_p1
bl printascii
mov r0, r9
bl printhex8
adr r0, str_p2
bl printascii
b __error
str_p1: .asciz "
Error: unrecognized/unsupported processor variant (0x"
str_p2: .asciz ").
"
.align
#endif
ENDPROC(__error_p)14
1
__error_p:2
3
adr r0, str_p14
bl printascii5
mov r0, r96
bl printhex87
adr r0, str_p28
bl printascii9
b __error10
str_p1: .asciz "
Error: unrecognized/unsupported processor variant (0x"11
str_p2: .asciz ").
"12
.align13
14
ENDPROC(__error_p)注意：汇编阶段调用函数，一定记得保护现场，否则会破坏原有寄存器，引入不必要的麻烦
自解压阶段的打印：
内核自解压阶段也可以调用早期的打印函数，需要针对自己的平台稍加修改（addruart 宏定义），这个阶段可能出现的问题，内核自解压把设备树覆盖了，导致卡在 staring the kernel 我这里就是如此~~，修改Uboot设备树的重定位地址即可。
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -24,6 +24,7 @@
* 100% relocatable. Any attempt to do so will result in a crash.
* Please select one of the following when turning on debugging.
*/
+#define DEBUG
#ifdef DEBUG
#if defined(CONFIG_DEBUG_ICEDCC)
@@ -67,7 +68,7 @@
.endm
#else
.macro loadsp, rb, tmp
- addruart
b, mp
+ addruart
b, mp, mp
.endm
#endif
#endif
@@ -554,6 +555,12 @@ not_relocated: mov r0, #0
* r7 = architecture ID
* r8 = atags pointer
*/
+ stmfd sp!, {r0-r3, r10-r12, lr}
+ mov r0, r8
+ bl memdump
+ kputc #‘
‘
+ ldmfd sp!, {r0-r3, r10-r12, lr}
+
mov r0, r4
mov r1, sp @ malloc space above stack
add r2, sp, #0x10000 @ 64k max
@@ -563,6 +570,12 @@ not_relocated: mov r0, #0
bl cache_off
mov r1, r7 @ restore architecture number
mov r2, r8 @ restore atags pointer
+
+ stmfd sp!, {r0-r3, r10-r12, lr}
+ mov r0, r8
+ bl memdump
+ kputc #‘
‘
+ ldmfd sp!, {r0-r3, r10-r12, lr}
#ifdef CONFIG_ARM_VIRT_EXT
45
1
--- a/arch/arm/boot/compressed/head.S2
+++ b/arch/arm/boot/compressed/head.S3
@@ -24,6 +24,7 @@4
* 100% relocatable. Any attempt to do so will result in a crash.5
* Please select one of the following when turning on debugging.6
*/7
+#define DEBUG8
9
10
11
@@ -67,7 +68,7 @@12
.endm13
14
.macro loadsp, rb, tmp15
- addruart
b, mp16
+ addruart
b, mp, mp17
.endm18
19
20
@@ -554,6 +555,12 @@ not_relocated: mov r0, #021
* r7 = architecture ID22
* r8 = atags pointer23
*/24
+ stmfd sp!, {r0-r3, r10-r12, lr}25
+ mov r0, r826
+ bl memdump27
+ kputc #‘
‘28
+ ldmfd sp!, {r0-r3, r10-r12, lr}29
+30
mov r0, r431
mov r1, sp @ malloc space above stack32
add r2, sp, #0x10000 @ 64k max33
@@ -563,6 +570,12 @@ not_relocated: mov r0, #034
bl cache_off35
mov r1, r7 @ restore architecture number36
mov r2, r8 @ restore atags pointer37
+38
+ stmfd sp!, {r0-r3, r10-r12, lr}39
+ mov r0, r840
+ bl memdump41
+ kputc #‘
‘42
+ ldmfd sp!, {r0-r3, r10-r12, lr}43
44
45
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