grouper: Suspend fails repeatedly

[Seth Forshee]

Suspend on the nexus 7 frequently fails with the following messages in the kernel:

[15623.213301] host1x host1x: host1x prevented suspend
[15623.213323] host1x host1x: suspend status: -16
[15623.213358] pm_op(): nvhost_pm_suspend+0x0/0x5c returns -16
[15623.213634] PM: Device host1x failed to suspend: error -16
[15623.213777] PM: Some devices failed to suspend

This doesn't happen all of the time, but when it does happen it seems to be stuck in a loop -- try to suspend, abort suspend due to the above errors, resume devices, then try to suspend again. Thus the device never actually enters suspend and drains the battery excessively.

[Seth Forshee (sforshee)]

The host1x device is associated with graphics. It's suspend operation is nvhost_suspend() in drivers/video/tegra/host/dev.c, which calls nvhost_module_suspend(). This function will return -EBUSY (-16) if it times out waiting for the number of references to the device to go to 0. References are taken from quite a few different places in the code, so it isn't immediately clear where the remaining reference(s) come from. It appears that references can even be taken from userspace via an ioctl.

[Seth Forshee (sforshee)]

I upgraded today (saucy-24) and have been trying to reproduce the bug, and so far my device is suspending fine. I'll keep an eye out to see if it reappears.

[Seth Forshee (sforshee)]

I haven't seen this in a while, so marking invalid.