title: efi shell command and register R/W
body: <p>I got a basic question of <code>efi mm command</code>.
  I need to control a controller (MAC) inside a SOC, and let it generate MDIO traffic to external PHY chip, to read its ID.
  The instruction told me to do the following under efi shell</p>
  
  <p>Shell> <code>mm xxxxxxx yyyyyyy -w 4 -MEM -n</code></p>
  
  <p>Shell> <code>mm xxxxxxx -w 4 -MEM -n</code>         </p>
  
  <p>I'm wondering what's the mm command do?
  Looks like it write to <code>xxxxxx</code> register the <code>yyyyyy</code> data, and then "mm" this register again?
  Not sure why.</p>
  
  <p>Can anyone help me on this?</p>