Mitsubishi CP-D70 family, now with more Raspberry Pi Goodness!

Back in March, my reverse-engineered processing library for the Mitsubishi CP-D70DW family of printers reached quality pairity with the official Windows/OSX drivers and their SDK. This was a huge milestone; it's now possible to drive these printers using entirely Free Software, with no quality compromises, on non-x86 CPU architectures. To do this, you will need Gutenprint 5.2.13-pre1 or newer, along with an up-to-date lib70x snapshot.

Here are the happy models:

  • Mitsubishi CP-D70DW
  • Mitsubishi CP-D707DW
  • Mitsubishi CP-D80DW
  • Mitsubishi CP-K60DW-S
  • Kodak 305
  • Fujifilm ASK-300 [completely untested]

I held off on announcing this achivement due to one rather annoying problem. It seems most folks interested in using this family of printers wanted to do so using the popular Raspberry Pi systems. Unfortunately, for (at the time) unknown reasons, the CP-D70DW spectacularly failed to print with RPi systems, with the image data failing to transfer to the printer.

To further complicate things, I was unable to recreate the problem on the Kodak 305 (aka a rebadged CP-K60DW-S) and an RPi2. Software-based sniffs were useless, and things stayed at this impasse for quite some time.

What finally broke the logjam was one particularly motivated user running into a similar problem with an RPi3 and a CP-D80DW model -- but only when there was nothing plugged into the ethernet controller. This was a bizarre development, and a working theory that there was something funky going on with the RPi's onboard SMSC USB hub/ethernet chip began to develop.

He convinced Mitsubishi/Europe that it was in their interests to help figure this problem out, and I soon found myself with a CP-D80DW and several boxes of media on my doorstep and a backchannel to their technical team.

Try as I might, I was unable to recreate the problem on a RPi1 or RPi2, but on an RPi3, I was finally able to trigger the failure. Armed with a true USB sniffer and a copy of the USB spec, I was able to rapidly identify what was acutally going on.

The USB2 spec (section 8.5.1) dictates that, when in high-speed mode, a flow control mechanism (NYET/PING/[N]ACK) be used to maximize transfer throughput. NYET is sent by the device after a successful transfer to signal that it can't accept more. The controller then starts polling the device with PING requests, with the printer responding with a NACK until it's ready, at which point it responds with an ACK.

The problem, in a nutshell, is that the RPi's USB controller ends up sending the PING requests so rapidly -- about 400 nanoseconds apart) that it effectively triggers a denial-of-service (DoS) situation in the printer. The funny thing is that the RPi's controller starts with a PING interval of about 1.2 microseconds, and everything is fine until it pulls in the timing, triggering the DoS.

(A regular PC with an xHCI controller demonstrates variable PING timing; it starts at about 400ns, but backs off to about a 4us interval)

If other USB traffic is present, then the bus is too busy to send the PINGs so rapidly, and the printer is fine. Notably, simply establishing a physical link with the onboard (USB-attached) ethernet controller (even if the interface is "down") generates sufficient traffic to mitigate the problem. Plugging in a thumbdrive and dumping data back or forth is also more than adequate.

I've reported this problem to both Mitsubishi and the RPi kernel folks. I don't think one can expect much progress on the root cause of this problem, but the above workarounds are at least adequate to using these printers with RPi systems.

...Enjoy, folks, and happy printing.