"I wish my Raspberry Pi had a 360K floppy drive"

-- Something nobody said, ever. (except smbaker)

Scott Baker http://www.smbaker.com/

Links:

• Blog Post: https://www.smbaker.com/raspberry-pi-floppy-controller-board
• Youtube Video: https://www.youtube.com/watch?v=I6E7oM7EDcc
• smb-pi-lib library, containing the floppy driver: https://github.com/sbelectronics/smb-pi-lib

Performance tuning

See https://www.raspberrypi.org/forums/viewtopic.php?t=228727 for background.

Raspbian is not a real-time OS, and the FDC is absolutely intolerant of bytes not being serviced during read or write. If a context switch occurs, then a buffer overrun WILL occur. There are a few tricks that can mitigate this.

• The --realtime option will set the process to SCHED_FIFO with maximum priority. This should keep other processes from interrupting it. It also writes -1 to /proc/sys/kernel/sched_rt_runtime_us, which will prevent Linux's mandatory interruption of the process for 50ms every second.

• The --pincpu option will pin the process to a particular CPU. In and of itself this isn't useful, but it makes a difference when combined with the isolcpus kernel option.

For maximum performance:

• Add isolcpus=3 to /boot/cmdline.txt
• Use the --realtime and ``--pincpu 3` options.

It's still not perfect, and writing a floppy driver as a user-mode python program was a pretty silly thing to do, but you can get a failure rate down to a couple of percent, and the automatic retry will take care of that.