Sunday, October 13, 2013

So, FreeBSD on the AR9344? What happened?

I committed a bunch of code a while ago to FreeBSD-HEAD to at least start booting on the AR934x SoCs. The AR934x SoC is a MIPS74k core - a dual-issue superscalar 11-stage pipeline MIPS32r2 CPU. It's slightly different to the existing MIPS24k stuff (which is a single 8-stage pipeline.)

So - first step - it booted up a little, then hit a machine check. At that point the FreeBSD MIPS peeps believed there was hilarity in the TLB exception handling code, so we put it to sleep for a while and I went back to real work.

Then a few weeks ago I decided to finish it off. I brought my developer board to Eurobsdcon in Malta and sat down with Warner Losh, who also has said developer board. We spent a bunch of time going over the TLB code and realised that FreeBSD's instruction/execution hazards are all.. just wrong. Then, on a whim, I read up some more about MIPS32r2 and superscalar stuff and discovered that the correct hazard instruction isn't NOPs or SSNOPs - it's EHB (execution hazard barrier.) It's 'SLL $0, $0, 3' in MIPS parlance which on older CPUs is just a NOP (since register 0 is always 'zero'.) So, this fixed the TLB management and the boot proceeded quite a bit further.

Next - bringing up ethernet and the switch PHY. I was seeing totally crappy and invalid register values when reading/writing the attached switch chips. Even probing didn't work reliably - in fact, I got to the point where I was reading the value I'd expect from the previous register read. So, I wondered if this was another out-of-order behaviour from the MIPS74k superscalar architecture.

After digging into the MIPS bus space code, I found two things:

  1. The MIPS driver(s) don't call bus barrier functions at all - so there's no driver enforced access ordering. It was all assuming that the CPU doesn't re-order things; and
  2. The bus barrier code for MIPS was a no-op. It just plainly wasn't defined.
So, I added read/write memory barriers to the MIPS bus barrier routines and I modified the ethernet driver to use barriers. For good measure, I also added barriers to the SPI driver code as that also has a bunch of register accesses that require ordering.

And with that, the switch PHY probe/attached fine, the SPI driver worked fine and the device started booting userland off of SPI connected NOR flash.

Then, it hung. I dug into that a bit and wondered what the hell was going on. Then after a day of poking, I discovered that the interrupt acknowledgement was not working. It's a quirky thing that I should really fix in the atheros platform support - the AR71xx chips don't require the CPU peripheral interrupts to be ack'ed (eg the uart) but later chips do. I added the AR934x to the list of SoCs that need interrupts to be ack'ed and the system kept booting, all the way to userland.

Next - I haven't yet written the AR8327 support but I started fleshing out the AR934x on-board switch support. I got it probing, attaching.. but not passing any traffic. After more digging, I realised my mistake - I was writing some registers incorrectly. I would mask out the right bits to set, but then I'd always set bit 0. Sigh. So, that came up and things worked.

Then I decided to do the wifi part. This was pretty damned simple. The HAL from Qualcomm Atheros already has support for the AR934x in it and I had already modified it to work for the AR933x SoC (which just required me to 'teach' it the FreeBSD way of exposing the calibration/configuration data from on-board flash.) So, all I had to do was this:

  1. Add the device to the kernel configuration;
  2. Add a hint pointing out where the device is mapped in IO space;
  3. Add a hint pointing out where the calibration data is in the NOR flash;
  4. Reboot.
That's it. No weeks of merging code in from Linux or the internal Qualcomm Atheros driver into the FreeBSD driver. No real debugging required. Just enable it, point it at the right place in memory/flash and .. boot it. I think this again vindicates my efforts to open source the Qualcomm Atheros HAL - I just inherit this working code for free. I don't have to try and merge it into anything.

So, I have a port that's dirty and working. There's a lot of infrastructure changes I need to commit before I can commit this port - lots of new clocking options (there's now variations on the clock rate that the MDIO bus (the MII bus connecting the ethernet port(s) to a PHY or switch), there's lots of new configuration options for how the on-chip ethernet port(s) map to external ports and a bunch of other ancillary stuff that's not really worth mentioning. But it's going to show up in FreeBSD-HEAD soon.