12757

OLPC has identified and fixed a problem with the wireless driver on the XO-1, XO-1.5, XO-1.75 and XO-4 laptops.

Problem

Wireless networks are not shown in network neighbourhood, or take a long time to be shown, or disappear, which prevents use of the network.

On XO-4 (8787), the problem does not occur.

On XO-4 (8686), XO-1.75, XO-1.5, and XO-1 (8388) the problem is more likely as the number of hidden networks increases, becoming especially obvious once there are many more hidden networks than normal networks. A cell phone hosted hidden network carried into a classroom by an adult may induce the symptom.

On XO-1 (8388), the problem is very likely as the number of XO-1 laptops in a room increases, becoming obvious at around 10 laptops.

On OLPC OS 11.3.1 and earlier the problem should not occur.

Solution

Upgrade to 13.2.1.

Or, on unsecure laptops, upgrade just the kernel, Start the Terminal activity and type this:

sudo yum install -y kernel

And then shutdown and reboot.

Workaround

If the solution cannot be applied, there are two alternate workarounds:

• On an affected laptop, repeat the scan using Terminal until the access point appears:

sudo iwlist eth0 scan

• Disable all hidden networks. Disable mesh on all laptops within 300m. This may be impractical for some deployments, since mesh is often useful.

Diagnosis

XO-1, XO-1.5, XO-1.75, and XO-4: hidden network probe responses arrive at the XO during a scan, may cause the loss of scan results for networks that responded later.

XO-1: mesh probe responses that are sent by other XO-1s, and arrive at an XO-1 during a scan, cause the loss of scan results for access points that responded later. As the number of XO-1 laptops increases relative to the number of access points, the probability of failure increases. With one access point and nine XO-1 laptops the probability of failure can be as high as 90%.

The probability of failure also depends on the collision detection backoff, which may depend on the MAC address of the access point.

The Marvell 8388 and 8686 wireless adapters depend on the libertas driver. The Marvell 8787 wireless adapter depends on the mwifiex driver. The problem only occurs in the libertas driver.

Verification

Enable scan debugging and watch the kernel messages:

sudo su
echo 0x80 > /sys/module/libertas/parameters/libertas_debug
cat /proc/kmsg

Perform a scan. Check if the message scan response: invalid IE fmt is shown. The message only appears if the problem occurs.

Credits

• microdeployments in Haiti,
• a deployment in Nepal,
• Terry Gillett of the Village Telco project for trying to figure out which access point models would work, only to find that any access point may not work,
• James Cameron of OLPC, for finding the problem and fixing it,

See Also

• a call for testing,
• ticket #12757,
• the kernel patch for XO-1, XO-1.5, XO-1.75, and XO-4,
• the upstreamed kernel patch merged by Linus Torvalds for 3.14.