Re: [PATCH RFC v2 1/8] bus: mhi: host: add mhi_power_down_no_destroy()

[Manivannan Sadhasivam <mani@kernel.org>]

On Mon, Jan 22, 2024 at 04:09:53PM +0800, Baochen Qiang wrote:
> 
> 
> On 1/22/2024 2:24 PM, Manivannan Sadhasivam wrote:
> > On Thu, Jan 04, 2024 at 11:39:12AM +0530, Manivannan Sadhasivam wrote:
> > 
> > + Can, Qiang
> > 
> > [...]
> > 
> > > > > To me it all sounds like the probe deferral is not handled properly in mac80211
> > > > > stack. As you mentioned in the commit message that the dpm_prepare() blocks
> > > > > probing of devices. It gets unblocked and trigerred in dpm_complete():
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/base/power/main.c#n1131
> > > > > 
> > > > > So if mac80211/ath11k cannot probe the devices at the dpm_complete() stage, then
> > > > > it is definitely an issue that needs to be fixed properly.
> > > > To clarify, ath11k CAN probe the devices at dpm_complete() stage. The
> > > > problem is kernel does not wait for all probes to finish, and in that way we
> > > > will face the issue that user space applications are likely to fail because
> > > > they get thawed BEFORE WLAN is ready.
> > > > 
> > > 
> > > Hmm. Please give me some time to reproduce this issue locally. I will get back
> > > to this thread with my analysis.
> > > 
> > 
> > We reproduced the issue with the help of PCIe team (thanks Can). What we found
> > out was, during the resume from hibernation the faliure happens in
> > ath11k_core_resume(). Precisely here:
> > https://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git/tree/drivers/net/wireless/ath/ath11k/core.c?h=ath11k-hibernation-support#n850
> > 
> > This code waits for the QMI messages to arrive and eventually timesout. But the
> > impression I got from the start was that the mhi_power_up() always fails during
> > resume. In our investigation, we confirmed that the failure is not happening at
> > the MHI level.No, mhi_power_up() never fails as it only downloads PBL,
> > SBL and waits
> for mission mode, no MHI device created hence not affected by the deferred
> probe. However in addition to PBL/SBL, ath11k also needs to download m3.bin,
> borad.bin and regdb.bin. Those files are part of WLAN firmware and are
> downloaded via QMI messages. After mhi_power_up() succeeds
> ath11k_core_resume() waits for QMI downloading those files. As you know QMI
> relies on MHI channels, these channels are managed by qcom_mhi_qrtr_driver.
> Since device probing is deferred, qcom_mhi_qrtr_driver has no chance to run
> at this stage. As a result ath11k_core_resume() times out.
> 

Thanks for the info, this clarifies the issue in detail.

> > 
> > I'm not pointing fingers here, but trying to understand why can't you fix
> > ath11k_core_resume() to not timeout? IMO this timeout should be handled as a
> > deferral case.
> Let's see what happens if we do it in a deferral way:
> 1. In ath11k_core_resume() we returns success directly without waiting for
> QMI downloading other firmware files.
> 2. Kernel unblocks device probe and schedules a work item to trigger all
> deferred probing. As a result MHI devices are probed by qcom_mhi_qrtr_driver
> and finally QMI is online.
> 3. kernel continues to resume and wake up userspace applications.
> 4. ath11k gets the message, either by kernel PM notification or something
> else, that QMI is ready and then downloads other firmware files.
> 
> What happens if userspace applications or network stack immediately initiate
> some WLAN request after resume back? Can ath11k handle such request? The
> answer is, most likely, no. Because there is no guarantee that QMI finishes
> downloading before those request.
> 

What will happen to userspace if ath11k returns an error like -EBUSY or
something? Will the netdev completely go away?

- Mani

> > 
> > - Mani
> > 

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