1. 函數調用路徑
首先看到uart驅動probe的過程：
[ 0.675729] Serial: AMBA PL011 UART driver
[ 0.735090] 9000000.pl011: ttyAMA0 at MMIO 0x9000000 (irq = 39, base_baud = 0) is a PL011 rev1
[ 0.736770] [VUART_DBG] register_console:2656 ttyAMA
[ 0.737602] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc4-00002-g84594dce3dfa-dirty #20
[ 0.739146] Hardware name: linux,dummy-virt (DT)
[ 0.739867] Call trace:
[ 0.740335] dump_backtrace+0x0/0x1a0
[ 0.740922] show_stack+0x14/0x20
[ 0.741514] dump_stack+0xbc/0x104
[ 0.742046] register_console+0x38/0x3e0
[ 0.742707] uart_add_one_port+0x42c/0x500
[ 0.743016] pl011_register_port+0x68/0xd0
[ 0.743496] pl011_probe+0x144/0x198
[ 0.743730] amba_probe+0xbc/0x158
[ 0.743957] really_probe+0x108/0x360
[ 0.744292] driver_probe_device+0x58/0x100
[ 0.744578] __device_attach_driver+0x90/0xb0
[ 0.744866] bus_for_each_drv+0x64/0xc8
[ 0.745165] __device_attach+0xd8/0x138
[ 0.745864] device_initial_probe+0x10/0x18
[ 0.745995] bus_probe_device+0x90/0x98
[ 0.746178] device_add+0x4c4/0x770
[ 0.746288] amba_device_try_add+0x1b8/0x360
[ 0.746416] amba_device_add+0x18/0xd8
[ 0.751260] of_platform_bus_create+0x308/0x3b8
[ 0.753329] of_platform_populate+0x7c/0x108
[ 0.753473] of_platform_default_populate_init+0xb8/0xd4
[ 0.753638] do_one_initcall+0x5c/0x1b0
[ 0.753762] kernel_init_freeable+0x19c/0x204
[ 0.753902] kernel_init+0x10/0x108
[ 0.754016] ret_from_fork+0x10/0x18
[ 0.755131] printk: console [ttyAMA0] enabled
[ 0.755131] printk: console [ttyAMA0] enabled
[ 0.757799] printk: bootconsole [pl11] disabled
[ 0.757799] printk: bootconsole [pl11] disabled
在kernel_init中of_platform_default_populate_init展開初始化過程，amba總線開始遍歷其驅動以及設備進行探測初始化，從而調用pl011_probe對設備進行初始化，對應的dts配置如下：
pl011@9000000 {
clock-names = "uartclk", "apb_pclk";
clocks = <0x8000 0x8000>;
interrupts = <0x0 0x1 0x4>;
reg = <0x0 0x9000000 0x0 0x1000>;
compatible = "arm,pl011", "arm,primecell";
};
而對應的console在chosen節點定義如下：
chosen {
linux,initrd-end = "H h";
linux,initrd-start = <0x48000000>;
bootargs = "root=/dev/ram rdinit=sbin/init console=ttyAMA0 ignore_loglevel earlycon";
stdout-path = "/pl011@9000000";
};
那麼console=ttyAMBA0以及pl011的定義以及driver的註冊探測過程之間的關係如何呢？
2. 驅動pl011註冊過程
在driver/tty/serial下的amba-pl011.c文件中定義了驅動模塊：
static struct amba_driver pl011_driver = {
.drv = {
.name = "uart-pl011",
.pm = &pl011_dev_pm_ops,
.suppress_bind_attrs = IS_BUILTIN(CONFIG_SERIAL_AMBA_PL011),
},
.id_table = pl011_ids,
.probe = pl011_probe,
.remove = pl011_remove,
};
static int __init pl011_init(void)
{
printk(KERN_INFO "Serial: AMBA PL011 UART driver\n");
if (platform_driver_register(&arm_sbsa_uart_platform_driver))
pr_warn("could not register SBSA UART platform driver\n");
return amba_driver_register(&pl011_driver);
}
static void __exit pl011_exit(void)
{
platform_driver_unregister(&arm_sbsa_uart_platform_driver);
amba_driver_unregister(&pl011_driver);
}
/*
* While this can be a module, if builtin it's most likely the console
* So let's leave module_exit but move module_init to an earlier place
*/
arch_initcall(pl011_init);
module_exit(pl011_exit);
這裏驅動模塊採用了arch_initcall，相對module_init較早加載，在amba_driver_register函數中對pl011_driver.drv->bus賦值：
drv->drv.bus = &amba_bustype;
而在probe階段需要做的match由name決定，上面定義中設定爲"uart-pl011"，從而amba_bustype上面存在名爲uart-pl011的驅動。
3. 設備節點的初始化
首先直接來看到dts中設備節點的解析過程中最重要的一箇中間函數：
/**
* of_platform_bus_create() - Create a device for a node and its children.
* @bus: device node of the bus to instantiate
* @matches: match table for bus nodes
* @lookup: auxdata table for matching id and platform_data with device nodes
* @parent: parent for new device, or NULL for top level.
* @strict: require compatible property
*
* Creates a platform_device for the provided device_node, and optionally
* recursively create devices for all the child nodes.
*/
static int of_platform_bus_create(struct device_node *bus,
const struct of_device_id *matches,
const struct of_dev_auxdata *lookup,
struct device *parent, bool strict)
{
const struct of_dev_auxdata *auxdata;
struct device_node *child;
struct platform_device *dev;
const char *bus_id = NULL;
void *platform_data = NULL;
int rc = 0;
/* Make sure it has a compatible property */
if (strict && (!of_get_property(bus, "compatible", NULL))) {
pr_debug("%s() - skipping %pOF, no compatible prop\n",
__func__, bus);
return 0;
}
/* Skip nodes for which we don't want to create devices */
if (unlikely(of_match_node(of_skipped_node_table, bus))) {
pr_debug("%s() - skipping %pOF node\n", __func__, bus);
return 0;
}
if (of_node_check_flag(bus, OF_POPULATED_BUS)) {
pr_debug("%s() - skipping %pOF, already populated\n",
__func__, bus);
return 0;
}
auxdata = of_dev_lookup(lookup, bus);
if (auxdata) {
bus_id = auxdata->name;
platform_data = auxdata->platform_data;
}
if (of_device_is_compatible(bus, "arm,primecell")) {
/*
* Don't return an error here to keep compatibility with older
* device tree files.
*/
of_amba_device_create(bus, bus_id, platform_data, parent);
return 0;
}
dev = of_platform_device_create_pdata(bus, bus_id, platform_data, parent);
if (!dev || !of_match_node(matches, bus))
return 0;
for_each_child_of_node(bus, child) {
pr_debug(" create child: %pOF\n", child);
rc = of_platform_bus_create(child, matches, lookup, &dev->dev, strict);
if (rc) {
of_node_put(child);
break;
}
}
of_node_set_flag(bus, OF_POPULATED_BUS);
return rc;
}
在這裏給每個節點及其所有子節點創建device，而其中amba對應的部分爲經過of_device_is_compatible(bus, "arm,primecell")匹配之後調用的of_amba_device_create(bus, bus_id, platform_data, parent);而對amba設備的操作過程調用路徑如下：
of_amba_device_create->amba_device_add->amba_device_try_add->device_add->bus_probe_device->device_initial_probe->__device_attach->bus_for_each_drv(dev->bus, NULL, &data, __device_attach_driver)->driver_probe_device->really_probe->dev->bus->probe(dev)->amba_probe-> 調用to_amba_driver(dev->driver)->probe函數，->pl011_probe;
最終調用到設備驅動的probe函數pl011_probe進入到前面提到的設備驅動模塊。
4. 從AMBA設備驅動到console設備
同樣首先看函數調用過程：
uart_configure_port->pl011_register_port->uart_add_one_port->uart_configure_port->register_console
最終將設備註冊到console_drivers鏈表中，下面分析爲什麼它被設置爲默認的console設備。
5. 默認console設置
在dts配置中chosen節點我們配置了bootargs，其中有console配置信息，其解析過程定義在printk.c文件中（kernel/printk路徑）：
static int __init console_setup(char *str)
{
char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
char *s, *options, *brl_options = NULL;
int idx;
if (_braille_console_setup(&str, &brl_options))
return 1;
/*
* Decode str into name, index, options.
*/
if (str[0] >= '0' && str[0] <= '9') {
strcpy(buf, "ttyS");
strncpy(buf + 4, str, sizeof(buf) - 5);
} else {
strncpy(buf, str, sizeof(buf) - 1);
}
buf[sizeof(buf) - 1] = 0;
options = strchr(str, ',');
if (options)
*(options++) = 0;
#ifdef __sparc__
if (!strcmp(str, "ttya"))
strcpy(buf, "ttyS0");
if (!strcmp(str, "ttyb"))
strcpy(buf, "ttyS1");
#endif
for (s = buf; *s; s++)
if (isdigit(*s) || *s == ',')
break;
idx = simple_strtoul(s, NULL, 10);
*s = 0;
__add_preferred_console(buf, idx, options, brl_options);
console_set_on_cmdline = 1;
return 1;
}
__setup("console=", console_setup);
可以看到以內核__setup的方式定義其解析函數，在內核啓動參數解析過程中調用解析函數console_setup設置console。其主要過程爲調用__add_preferred_console函數，對console_drivers遍歷，將其設備名稱與命令行配置的設備名稱進行匹配，如果匹配到則說明該console設備已經註冊，將其標註爲默認的console，標註方法是將其序號賦值給preferred_console。而如果遍歷當前已經註冊的console鏈表未找到匹配的設備且還未超出最大可允許註冊的console數目，則將鏈表下一個元素標記爲默認，並將命令行參數對應設備名稱賦值到該鏈表元素中，在設備註冊的時候完成對該元素的完整初始化即可，詳情如下：
static int __add_preferred_console(char *name, int idx, char *options,
char *brl_options)
{
struct console_cmdline *c;
int i;
/*
* See if this tty is not yet registered, and
* if we have a slot free.
*/
for (i = 0, c = console_cmdline;
i < MAX_CMDLINECONSOLES && c->name[0];
i++, c++) {
if (strcmp(c->name, name) == 0 && c->index == idx) {
if (!brl_options)
preferred_console = i;
return 0;
}
}
if (i == MAX_CMDLINECONSOLES)
return -E2BIG;
if (!brl_options)
preferred_console = i;
strlcpy(c->name, name, sizeof(c->name));
c->options = options;
braille_set_options(c, brl_options);
c->index = idx;
return 0;
}
至此，我們可以看到三者的關聯，從總線驅動註冊到設備在總線上被探測和初始化，然後註冊對應的console設備，而命令行參數解析過程結合起來將之後確定了該設備爲默認console。