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我首先通过调查earlycon的参数找到了这个,但发现控制台的选项看起来几乎相同.两者都在下面,取自 this来源：
从控制台文档我们有：
console= [KNL] Output console device and options.
tty<n> Use the virtual console device <n>.
ttyS<n>[,options]
ttyUSB0[,options]
Use the specified serial port. The options are of
the form "bbbbpnf", where "bbbb" is the baud rate,
"p" is parity ("n", "o", or "e"), "n" is number of
bits, and "f" is flow control ("r" for RTS or
omit it). Default is "9600n8".
See Documentation/serial-console.txt for more
information. See
Documentation/networking/netconsole.txt for an
alternative.
uart[8250],io,<addr>[,options]
uart[8250],mmio,<addr>[,options]
uart[8250],mmio16,<addr>[,options]
uart[8250],mmio32,<addr>[,options]
uart[8250],0x<addr>[,options]
Start an early, polled-mode console on the 8250/16550
UART at the specified I/O port or MMIO address,
switching to the matching ttyS device later.
MMIO inter-register address stride is either 8-bit
(mmio), 16-bit (mmio16), or 32-bit (mmio32).
If none of [io|mmio|mmio16|mmio32], <addr> is assumed
to be equivalent to 'mmio'. 'options' are specified in
the same format described for ttyS above; if unspecified,
the h/w is not re-initialized.
hvc<n> Use the hypervisor console device <n>. This is for
both Xen and PowerPC hypervisors.
If the device connected to the port is not a TTY but a braille
device, prepend "brl," before the device type, for instance
console=brl,ttyS0
For now, only VisioBraille is supported.
从earlycon的文档我们有：
earlycon= [KNL] Output early console device and options.
When used with no options, the early console is
determined by the stdout-path property in device
tree's chosen node.
cdns,<addr>
Start an early, polled-mode console on a cadence serial
port at the specified address. The cadence serial port
must already be setup and configured. Options are not
yet supported.
uart[8250],io,<addr>[,options]
uart[8250],mmio,<addr>[,options]
uart[8250],mmio32,<addr>[,options]
uart[8250],mmio32be,<addr>[,options]
uart[8250],0x<addr>[,options]
Start an early, polled-mode console on the 8250/16550
UART at the specified I/O port or MMIO address.
MMIO inter-register address stride is either 8-bit
(mmio) or 32-bit (mmio32 or mmio32be).
If none of [io|mmio|mmio32|mmio32be], <addr> is assumed
to be equivalent to 'mmio'. 'options' are specified
in the same format described for "console=ttyS<n>"; if
unspecified, the h/w is not initialized.
pl011,<addr>
pl011,mmio32,<addr>
Start an early, polled-mode console on a pl011 serial
port at the specified address. The pl011 serial port
must already be setup and configured. Options are not
yet supported. If 'mmio32' is specified, then only
the driver will use only 32-bit accessors to read/write
the device registers.
meson,<addr>
Start an early, polled-mode console on a meson serial
port at the specified address. The serial port must
already be setup and configured. Options are not yet
supported.
msm_serial,<addr>
Start an early, polled-mode console on an msm serial
port at the specified address. The serial port
must already be setup and configured. Options are not
yet supported.
msm_serial_dm,<addr>
Start an early, polled-mode console on an msm serial
dm port at the specified address. The serial port
must already be setup and configured. Options are not
yet supported.
smh Use ARM semihosting calls for early console.
s3c2410,<addr>
s3c2412,<addr>
s3c2440,<addr>
s3c6400,<addr>
s5pv210,<addr>
exynos4210,<addr>
Use early console provided by serial driver available
on Samsung SoCs, requires selecting proper type and
a correct base address of the selected UART port. The
serial port must already be setup and configured.
Options are not yet supported.
lpuart,<addr>
lpuart32,<addr>
Use early console provided by Freescale LP UART driver
found on Freescale Vybrid and QorIQ LS1021A processors.
A valid base address must be provided, and the serial
port must already be setup and configured.
armada3700_uart,<addr>
Start an early, polled-mode console on the
Armada 3700 serial port at the specified
address. The serial port must already be setup
and configured. Options are not yet supported.
用法的一个例子是：
earlycon = uart8250,0x21c0500
我的问题是：
>为什么有参考8250/16550物理硬件？这个旧的实现是否已成为现代设计的接口规范？也就是说,当这些通信设备在SoC外部时,我们是否还在使用兼容的UART驱动程序？
>如果MMIO是内存映射IO,那么在此上下文中引用的“正常”IO是什么？
>什么是< addr>参数？这是您运行此内核的特定SoC的UART配置寄存器的开头吗？大多数UART配置寄存器是否符合特定的寄存器布局,以便通用UART驱动程序可以适当地配置硬件？
最佳答案
我确信有人仍在这样做,但是在ILO / DRAC等之前的日子里.变得便宜,ubiquitios是在紧急情况下通过串行端口进行“带外”访问控制台的最佳方式.您可以在机架中安装终端服务器,然后将电缆连接到服务器的串行端口.某些BIOS支持将控制台重定向到串行端口(例如,1999年时间帧中的VA Linux和SuperMicro服务器).
8250/16550 UARTS是当时最流行的串口芯片,这意味着它们将是Linux下最好的支持,并且它们都使用8250内核驱动程序(该系列中还有更多的型号都使用过同一个司机).
我怀疑许多旨在被Linux使用的SoC设计构建了8250/16550兼容的UART,因为它是简单的按钮 – 记录良好且经过良好测试的驱动程序.虽然希望他们构建后来的多字节缓冲版本(当然,即使是“慢”处理器,今天的标准也可以比115k串口接收它更频繁地服务串行中断.IIRC Mac有一个用于本地/ Apple Talk的串口(不记得哪个是协议,哪个是硬件)做了248k.但是,当CPU达到60Mhz时,这又回来了.
这可能是MMIO和端口I / O之间差异的最佳答案：https://en.wikipedia.org/wiki/Memory-mapped_I/O我不明白这个级别是否足以将其降低.
上面的链接可能会回答用于这些目的的内容,但基本上它是一个内存地址.
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