Re: [PATCH v2 2/3] net: qrtr: support suspend/hibernation

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Tue, Feb 27, 2024 at 04:39:41PM +0800, Baochen Qiang wrote:
> 
> 
> On 2/27/2024 3:15 PM, Manivannan Sadhasivam wrote:
> > On Tue, Feb 27, 2024 at 02:36:12PM +0800, Baochen Qiang wrote:
> > > MHI devices may not be destroyed during suspend/hibernation, so need
> > > to unprepare/prepare MHI channels throughout the transition, this is
> > > done by adding suspend/resume callbacks.
> > > 
> > > The suspend callback is called in the late suspend stage, this means
> > > MHI channels are still alive at suspend stage, and that makes it
> > > possible for an MHI controller driver to communicate with others over
> > > those channels at suspend stage. While the resume callback is called
> > > in the early resume stage, for a similar reason.
> > > 
> > > Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.30
> > > 
> > > Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
> > 
> > Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > 
> > Could you please confirm if this patch can be applied separately and won't cause
> > regression?
> Just searched the kernel and found another two drivers using IPCR channels,
> one is pci_epf_mhi_driver in drivers/pci/endpoint/functions/pci-epf-mhi.c
> and the other is mhi_pci_driver in drivers/bus/mhi/host/pci_generic.c.
> 
> For pci_epf_mhi_driver, this is not impacted because the devices for those
> channels are attached to mhi_ep_bus_type while QRTR MHI driver attached to
> mhi_bus_type.
> 
> For mhi_pci_driver, I am afraid there would be regressions caused by this
> patch. The regression is because when system suspends, mhi_pci_suspend() is
> called and then qcom_mhi_qrtr_pm_suspend_late() called, however the latter
> would fail because MHI is moved to M3 state by call mhi_pm_suspend() in
> mhi_pci_suspend(). To address this, there might be two options: the first is
> to move mhi_pci_suspend() to a late suspend stage which would be called
> after qcom_mhi_qrtr_pm_suspend_late(). and the second is to avoid whole QRTR
> suspend operation in such cases. I prefer the second one, because if MHI is
> going to suspend, instead of power down, it is pointless to unprepare MHI
> channels and re-prepare them after resume back. We can achieve this purpose
> by adding a status_cb() to QRTR MHI driver which would be called when MHI
> goes to low power mode. And then QRTR MHI driver could decide not to
> suspend/resume if low power mode is notified.
> 
> Your thoughts?
> 

I'd rather query the MHI state of the device in suspend/resume callback using
mhi_get_mhi_state(mhi_dev->mhi_cntrl) and if the device is found to be in M3
(suspend) state, then I'd skip prepare/unprepare part.

Because this makes it clear that, if the device is in suspend state, then no
need for the client drivers to unprepare/prepare the channels.

- Mani

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