Category Archives: AR5210

Making the AR5210 NIC work in the office..

I'm quite happy that FreeBSD's ath(4) driver supports almost all of the PCI and PCIe devices that Atheros has made. Once I find a way to open source this AR9380 HAL I've constructed, we'll actually support them all. However, there are a few little niggling things that have been bugging me. Today I addressed one of those.

The AR5210. It's their first 11a-only NIC. It does up to 54MBit OFDM 802.11a; it doesn't do QoS/WME (as it only has one data queue); it "may" go up to 72MBit if I hack on some magic extensions. And in open mode, it works great.

But it didn't work in the office or at home. All of which are 802.11n APs with WPA2 authentication and AES-CCMP encryption.

Now, the AR5210 only does open and WEP encryption. It doesn't do TKIP or AES-CCMP. So the encryption has to happen in software. The NIC was associating fine, but when wpa_supplicant went to program in the AES-CCMP encryption keys, the HAL simply refused.

What I discovered was this.

The driver keycache was also trying to allocate keycache slots for the AR5210, where it only supports the 4 WEP keys.  This is a big no-no. So once I mapped them to all be slot 0, I made a little progress.

The net80211 layer was trying to program in an AES-CCMP key, which the driver was dutifully passing to the HAL. The AR5210 HAL doesn't support anything but WEP or open, so the encryption key type was "clear". Now, "clear" means "for this MAC address, don't try decrypting anything." But the AR5210 HAL code rejected it - as I said, it doesn't do that.

Ok, so I ignored that entirely. I mapped all of the software encrypted key entries to slot 0 and just didn't program the hardware. So now the HAL didn't reject things. But it wasn't working. The received frames were being corrupted somehow and failed the CCMP MIC integrity check. I took at look at the frames being received (which should've been "clear" versus what was going on in the air - luckily, this laptop has an AR9280 inside so I could put it into monitor mode and sniff things. The packets just didn't add up. I was confused.

Then after discussing this with my flatmate, I idly wondered if the hardware was decrypting the traffic anyway. And, well, it was. Encrypted frames have the WEP bit set in the 802.11 header - whether they're WEP, TKIP, AES-CCMP. The AR5210 didn't know it wasn't WEP, so it tried decoding the frames itself. And corrupting them.

So after finding a PCU control register (hi AR_DIAG_SW) that lets me disable encryption/decryption, I was able to pass through the encrypted traffic fine and everything just plain worked. It's odd seeing an 11a, non-QoS station on my 11n AP, but that just goes to show that backwards interoperability is still useful.

And yes, I did take the AR5210 into the office and I did sit in a meeting with it and use it to work from. It let me onto the corporate wireless just fine, thankyou.

So now the FreeBSD AR5210 support doesn't do any hardware encryption. You can turn it on again if you'd like. Why? Because I don't want the headache of someone coming to me and asking why a dual-VAP AP with WEP and CCMP is failing. The hardware can only do _either_ WEP/open with hardware encryption, _or_ it can do everything without hardware encryption. So I decided to just disable it for now.

There's also a problem with how encryption is specified to net80211. It's done at startup time, when the driver attaches. Anything that isn't specified as being done in hardware is done in software. There is currently no clean way to dynamically change that configuration. So, if I have WEP encryption in hardware but CCMP/TKIP in software, I have to dynamically flip on/off the hardware encryption _AND_ I have to enforce that WEP and CCMP doesn't get configured at the same time.

The cleaner solution would be to:
  • Create a new driver attribute, which indicates the hardware can do WEP and CCMP at the same time - make sure it's off for the AR5210;
  • Add a HAL call to enable/disable hardware encryption;
  • If a user wants to do WEP or open - enable hardware encryption;
  • If a user wants to do CCMP/TKIP/etc - disable hardware encryption;
  • Complain if the user wants to create a VAP with CCMP/TKIP and WEP.
 If someone wants a mini-project - and they have an AR5210 - I'm all for it. But at the moment, this'll just have to do.