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int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)这个函数用于parse bios 传递过来的spcr表，来设置串口输出其源码分析如下：int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console){	static char opts[64];	struct acpi_table_spcr *table;	acpi_status status;	char *uart;	char *iotype;	int baud_rate;	int err;	if (acpi_disabled)		return -ENODEV;	#首先找到SPCR这个表的指针	status = acpi_get_table(ACPI_SIG_SPCR, 0,				(struct acpi_table_header **)&table);	if (ACPI_FAILURE(status))		return -ENOENT;	#如果spcr 版本小于2则退出	if (table->header.revision < 2)		pr_info("SPCR table version %d\n", table->header.revision);	#根据space_id 来决定uart使用的io是16/32 bits		if (table->serial_port.space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY) {		switch (ACPI_ACCESS_BIT_WIDTH((			table->serial_port.access_width))) {		default:			pr_err("Unexpected SPCR Access Width.  Defaulting to byte size\n");			/* fall through */		case 8:			iotype = "mmio";			break;		case 16:			iotype = "mmio16";			break;		case 32:			iotype = "mmio32";			break;		}	} else		iotype = "io";	#设置uart确定的name，这里就是给uart这个字符串赋值	switch (table->interface_type) {	case ACPI_DBG2_ARM_SBSA_32BIT:		iotype = "mmio32";		/* fall through */	case ACPI_DBG2_ARM_PL011:	case ACPI_DBG2_ARM_SBSA_GENERIC:	case ACPI_DBG2_BCM2835:		uart = "pl011";		break;	case ACPI_DBG2_16550_COMPATIBLE:	case ACPI_DBG2_16550_SUBSET:		uart = "uart";		break;	default:		err = -ENOENT;		goto done;	}	#根据spcr 来决定波特率	switch (table->baud_rate) {	case 3:		baud_rate = 9600;		break;	case 4:		baud_rate = 19200;		break;	case 6:		baud_rate = 57600;		break;	case 7:		baud_rate = 115200;		break;	default:		err = -ENOENT;		goto done;	}	 #这两款驱动有bug，这里是修复bug	if (qdf2400_erratum_44_present(&table->header)) {		qdf2400_e44_present = true;		if (enable_earlycon)			uart = "qdf2400_e44";	}	if (xgene_8250_erratum_present(table)) {		iotype = "mmio32";		/* for xgene v1 and v2 we don't know the clock rate of the		 * UART so don't attempt to change to the baud rate state		 * in the table because driver cannot calculate the dividers		 */		snprintf(opts, sizeof(opts), "%s,%s,0x%llx", uart, iotype,			 table->serial_port.address);	} else {		snprintf(opts, sizeof(opts), "%s,%s,0x%llx,%d", uart, iotype,			 table->serial_port.address, baud_rate);	}#从开机log中可以看到console这个子串	pr_info("console: %s\n", opts);#是否使用earlycon	if (enable_earlycon)		setup_earlycon(opts);#添加preferr console	if (enable_console)		err = add_preferred_console(uart, 0, opts + strlen(uart) + 1);	else		err = 0;done:	acpi_put_table((struct acpi_table_header *)table);	return err;}

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