Re: [PATCH v3 1/2] PCI: dwc: Implement general suspend/resume functionality for L2/L3 transitions

[Manivannan Sadhasivam]

On Thu, Jul 20, 2023 at 11:20:27AM -0500, Bjorn Helgaas wrote:
> On Thu, Jul 20, 2023 at 09:37:38PM +0530, Manivannan Sadhasivam wrote:
> > On Thu, Jul 20, 2023 at 10:37:36AM -0400, Frank Li wrote:
> > > On Thu, Jul 20, 2023 at 07:55:09PM +0530, Manivannan Sadhasivam wrote:
> > > > On Tue, Jul 18, 2023 at 03:34:26PM +0530, Manivannan Sadhasivam wrote:
> > > > > On Mon, Jul 17, 2023 at 02:36:19PM -0400, Frank Li wrote:
> > > > > > On Mon, Jul 17, 2023 at 10:15:26PM +0530, Manivannan Sadhasivam wrote:
> > > > > > > On Wed, Apr 19, 2023 at 12:41:17PM -0400, Frank Li wrote:
> > > > > > > > Introduced helper function dw_pcie_get_ltssm to retrieve SMLH_LTSS_STATE.
> > > > > > > > Added API pme_turn_off and exit_from_l2 for managing L2/L3 state transitions.
> > > > > > > > 
> > > > > > > > Typical L2 entry workflow:
> > > > > > > > 
> > > > > > > > 1. Transmit PME turn off signal to PCI devices.
> > > > > > > > 2. Await link entering L2_IDLE state.
> > > > > > > 
> > > > > > > AFAIK, typical workflow is to wait for PME_To_Ack.
> > > > > > 
> > > > > > 1 Already wait for PME_to_ACK,  2, just wait for link actual enter L2.
> > > > > > I think PCI RC needs some time to set link enter L2 after get ACK from
> > > > > > PME.
> > > > 
> > > > One more comment. If you transition the device to L2/L3, then it
> > > > can lose power if Vaux was not provided. In that case, can all
> > > > the devices work after resume?  Most notably NVMe?
> > > 
> > > I have not hardware to do such test, NVMe driver will reinit
> > > everything after resume if no L1.1\L1.2 support. If there are
> > > L1.1\L1.2, NVME expect it leave at L1.2 at suspend to get better
> > > resume latency.
> > 
> > To be precise, NVMe driver will shutdown the device if there is no
> > ASPM support and keep it in low power mode otherwise (there are
> > other cases as well but we do not need to worry).
> > 
> > But here you are not checking for ASPM state in the suspend path,
> > and just forcing the link to be in L2/L3 (thereby D3Cold) even
> > though NVMe driver may expect it to be in low power state like
> > ASPM/APST.
> > 
> > So you should only put the link to L2/L3 if there is no ASPM
> > support. Otherwise, you'll ending up with bug reports when users
> > connect NVMe to it.
> 
> Can you point me to the NVMe code that shuts down the device if
> there's no ASPM support?  That sounds interesting and of interest to
> other drivers that want to do suspend.
> 

drivers/nvme/host/pci.c #3185

Note that, with ACPI based systems and for a few SSDs the behavior may change
(check NVME_QUIRK_SIMPLE_SUSPEND flag).

- Mani

> Bjorn

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