ath11k resume fails due to kernel blocks probing MHI virtual devices

[Baochen Qiang]

Hi Rafael and Pavel,

Currently I am facing an ath11k (a kernel WLAN driver) resume issue 
related with kernel PM framework and MHI module.

Before introducing the issue details, I'd like to summarize how ath11k 
interacts with MHI stack to download WLAN firmware to hardware target:
1. when booting/restarting, ath11k powers on MHI module and waits for 
MHI channels to be ready.
2. When power on, MHI stack creates some virtual MHI devices, which 
represents MHI hardware channels, and adds them to MHI bus. This 
triggers MHI client driver, named QRTR, to get matched and probe those 
MHI devices. In probe, QRTR initializes MHI channels and finally move 
them to ready state.
3. Once MHI channels ready, ath11k downloads WLAN firmware to hardware 
target, then WLAN is working.

Such an flow works well in general, but introduces issues in hibernation 
cycle: when preparing for hibernation, ath11k powers down MHI, this 
results in MHI devices being destroyed thus QRTR resets MHI channels. 
When resuming back from hibernation, ath11k powers on MHI and waits for 
MHI channels to be ready in its resume callback. As said above, MHI 
creates and adds MHI devices to MHI bus, but they can't be probed at 
that time because device probe is prohibited in device_block_probing(), 
finally this results in ath11k resume timeout.

Now there is an potential fix to this issue which would needs changes in 
MHI stack, i.e., don't destroy MHI devices while hibernating. And we 
have had a plenty talk with MHI community regarding this change, see [1] 
and [2].

However Mani (the MHI maintainer) doesn't think it's right to fix it in 
MHI stack. Instead, he thought we might need to add a new PM callback 
which will be called after device probe is unblocked. By registering 
such a callback ath11k can wait the dependency driver, i.e., QRTR, to 
probe and initialize those MHI devices.

Your thoughts?


[1] https://lists.infradead.org/pipermail/ath11k/2023-December/005098.html
[2] https://lists.infradead.org/pipermail/ath11k/2024-January/005205.html