Do we need a USB MIDI interface to use MIDI with the Raspberry Pi? No!

Is it possible to have MIDI IN into Raspberry Pi's GPIO? Yes!

After all, MIDI is nothing else than serial connection at 31250 baud, so let's use the Pi's serial port.

Here is a working solution:

1. Build this cicuit (you need a 6N138 optoisolator, a few resistors, and a diode 1N4148):

   

2. Connect GND to a GPIO pin #6, connect +3.3V to a GPIO pin #1, connect RXD to the GPIO pin #10!

   

3. Check your Linux kernel version with uname -a. If it's 4.5 or higher, just add these three lines at the end of /boot/config.txt and go to step 4.

   enable_uart=1
   dtoverlay=pi3-miniuart-bt
   dtoverlay=midi-uart0 

   (pi3-miniuart-bt disables the Bluetooth and changes the connection of GPIO pins #14 #15, see here)

   If you have an older version of Linux kernel (4.4 or lower), you have to add bcm2708.uart_clock=3000000 at the end of /boot/cmdline.txt, remove everything in this file related to ttyAMA0 (it would pollute the serial port input with console text), and add this at the end of /boot/config.txt:

   init_uart_clock=2441406
   init_uart_baud=38400

   It's a hack (but it perfectly works!) allowing the Pi's serial port to run at 31250 baud, which would normally not be possible with UART clock of 3Mhz.

4. Reboot and run this Python code:

   import serial
   
   ser = serial.Serial('/dev/ttyAMA0', baudrate=38400)    
   
   message = [0, 0, 0]
   while True:
     i = 0
     while i < 3:
       data = ord(ser.read(1)) # read a byte
       if data >> 7 != 0:  
         i = 0      # status byte!   this is the beginning of a midi message!
       message[i] = data
       i += 1
       if i == 2 and message[0] >> 4 == 12:  # program change: don't wait for a
         message[2] = 0                      # third byte: it has only 2 bytes
         i = 3
   
     messagetype = message[0] >> 4
     messagechannel = (message[0] & 15) + 1
     note = message[1] if len(message) > 1 else None
     velocity = message[2] if len(message) > 2 else None
   
     if messagetype == 9:    # Note on
       print 'Note on'
     elif messagetype == 8:  # Note off
       print 'Note off'            
     elif messagetype == 12: # Program change
       print 'Program change'

5. That's it!

`

Note: the method described here is nothing really new. It's just the sum of various (sometimes difficult to find) informations found in lots of places over the internet, and some personal hacks (the Python code) to make it easy to use.

Note2: thanks to Damien for pointing the new method for Linux kernel >= 4.5.