title: Enable serial ports ttyS2 and ttyS3 in Linux
body: <p>I'm developing serial app for Linux, mips. I'm running the QEMU with Linux for mips inside and want to use all the available serial ports. According to QEMU docs, it supports up to 4 serial ports only and in order to us them I have to add the "--serial" option to to the QEMU run command for any additional port I want to use.  </p>
  
  <p>However, when the qemu is up, it looks like only ttyS0 and ttyS1 are actually available for use. when I'm writing data to ttyS2 I'm getting stuck and to ttyS3 I got: "standard output: Input/output error".  </p>
  
  <p>Under /dev/ there are ttyS2 and ttyS3 FDs. </p>
  
  <p>when I'm <code>"dmesg | grep tty"</code> command I get the following output:</p>
  
  <pre><code>Kernel command line: root=/dev/sda clocksource=GIC console=ttyS0
  console [ttyS0] disabled
  serial8250.0: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
  console [ttyS0] enabled
  serial8250.0: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
  serial8250.0: ttyS2 at MMIO 0x1f000900 (irq = 20, base_baud = 230400) is a 16550A
  </code></pre>
  
  <p>if you look at it you can see that ttyS3 doesn't appear and ttyS2 exists but with another baudRate than ttyS1&amp;0. When I'm trying to change the BR to ttyS2 with sty command, nothing happened. </p>
  
  <p>Can anyone please help me to understand what i'm missing in order to activate ttyS2 and ttyS3?</p>