devicetree下的earlycon的初始化

最新推荐文章于 2025-06-05 09:11:59 发布

tiantao2012

最新推荐文章于 2025-06-05 09:11:59 发布

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分类专栏： Linux 源码分析

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本文介绍如何通过Device Tree配置Earlycon参数，并详细解析了Linux内核中early_init_dt_scan_chosen_stdout函数的工作流程，包括从Device Tree中读取stdout-path属性、匹配相应的串口设备以及earlycon的初始化过程。

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在ACPI的时候可以通过spcr表来制定earlycon的参数，在devcetree的时候可以通过在chosen这个节点下通过stdout-path来制定earlycon

/ {
   chosen {
       stdout-path = "/serial@f00:115200";
   };

   serial@f00 {
       compatible = "vendor,some-uart";
       reg = <0xf00 0x10>;
   };
};
而解析的stdout-path在code在drivers/fdt.c 中，可见要定义CONFIG_SERIAL_EARLYCON 这个宏才会使能
下面的code
#ifdef CONFIG_SERIAL_EARLYCON

int __init early_init_dt_scan_chosen_stdout(void)
{
   int offset;
   const char *p, *q, *options = NULL;
   int l;
   const struct earlycon_id *match;
   const void *fdt = initial_boot_params;
// 从devicetree中找到chosen节点
   offset = fdt_path_offset(fdt, "/chosen");
   if (offset < 0)
       offset = fdt_path_offset(fdt, "/chosen@0");
   if (offset < 0)
       return -ENOENT;
//从chosen节点下找到stdout-path
   p = fdt_getprop(fdt, offset, "stdout-path", &l);
   if (!p)
       p = fdt_getprop(fdt, offset, "linux,stdout-path", &l);
   if (!p || !l)
       return -ENOENT;

   q = strchrnul(p, ':');
   if (*q != '\0')
       options = q + 1;
   l = q - p;

   /* Get the node specified by stdout-path */
   offset = fdt_path_offset_namelen(fdt, p, l);
   if (offset < 0) {
       pr_warn("earlycon: stdout-path %.*s not found\n", l, p);
       return 0;
   }
//从__earlycon_table 到 __earlycon_table_end 之间找是否有匹配的，本例中匹配的就是”vendor,some-uart“
   for (match = __earlycon_table; match < __earlycon_table_end; match++) {
       if (!match->compatible[0])
           continue;

       if (fdt_node_check_compatible(fdt, offset, match->compatible))
           continue;
//如果找到的话，就调用of_setup_earlycon
       of_setup_earlycon(match, offset, options);
       return 0;
   }
   return -ENODEV;
}
#endif

#ifdef CONFIG_OF_EARLY_FLATTREE

int __init of_setup_earlycon(const struct earlycon_id *match,
                 unsigned long node,
                 const char *options)
{
   int err;
   struct uart_port *port = &early_console_dev.port;
   const __be32 *val;
   bool big_endian;
   u64 addr;

   spin_lock_init(&port->lock);
   port->iotype = UPIO_MEM;
   addr = of_flat_dt_translate_address(node);
   if (addr == OF_BAD_ADDR) {
       pr_warn("[%s] bad address\n", match->name);
       return -ENXIO;
   }
   port->mapbase = addr;
   port->uartclk = BASE_BAUD * 16;
   port->membase = earlycon_map(port->mapbase, SZ_4K);

   val = of_get_flat_dt_prop(node, "reg-offset", NULL);
   if (val)
       port->mapbase += be32_to_cpu(*val);
   val = of_get_flat_dt_prop(node, "reg-shift", NULL);
   if (val)
       port->regshift = be32_to_cpu(*val);
   big_endian = of_get_flat_dt_prop(node, "big-endian", NULL) != NULL ||
       (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN) &&
       of_get_flat_dt_prop(node, "native-endian", NULL) != NULL);
   val = of_get_flat_dt_prop(node, "reg-io-width", NULL);
   if (val) {
       switch (be32_to_cpu(*val)) {
       case 1:
           port->iotype = UPIO_MEM;
           break;
       case 2:
           port->iotype = UPIO_MEM16;
           break;
       case 4:
           port->iotype = (big_endian) ? UPIO_MEM32BE : UPIO_MEM32;
           break;
       default:
           pr_warn("[%s] unsupported reg-io-width\n", match->name);
           return -EINVAL;
       }
   }

   if (options) {
       strlcpy(early_console_dev.options, options,
           sizeof(early_console_dev.options));
   }
//其面都是找参数，从这里到最后就是真正的earlycon的初始化，这部分和ACPI的方式就一模一样了
   earlycon_init(&early_console_dev, match->name);
   err = match->setup(&early_console_dev, options);
   if (err < 0)
       return err;
   if (!early_console_dev.con->write)
       return -ENODEV;

   register_console(early_console_dev.con);
   return 0;
}

#endif /* CONFIG_OF_EARLY_FLATTREE */