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

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Message-ID: <ZLln6GDX/u3pdEQ6@lizhi-Precision-Tower-5810>
Date:   Thu, 20 Jul 2023 12:59:20 -0400
From:   Frank Li <Frank.li@....com>
To:     Manivannan Sadhasivam <mani@...nel.org>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        helgaas@...nel.org, imx@...ts.linux.dev, bhelgaas@...gle.com,
        devicetree@...r.kernel.org, gustavo.pimentel@...opsys.com,
        kw@...ux.com, leoyang.li@....com,
        linux-arm-kernel@...ts.infradead.org, linux-imx@....com,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        lorenzo.pieralisi@....com, minghuan.lian@....com,
        mingkai.hu@....com, robh+dt@...nel.org, roy.zang@....com,
        shawnguo@...nel.org, zhiqiang.hou@....com
Subject: Re: [PATCH v3 1/2] PCI: dwc: Implement general suspend/resume
 functionality for L2/L3 transitions

On Thu, Jul 20, 2023 at 10:13:18PM +0530, Manivannan Sadhasivam wrote:
> On Thu, Jul 20, 2023 at 12:26:38PM -0400, Frank Li 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 loose 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).
> > 
> > I supposed this should work. but I have not hardware to test it now. NMVE already
> > sent shut down command to SSD, which can safely turn off. after resume, that most
> > likely a cold reset.
> > 
> 
> NO, it won't work and that's the reason the Qcom platforms are not transitioning
> the link to L2/L3 state during suspend. This applies to other platforms
> including layerscape as well.
> 
> > > 
> > > 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.
> > 
> > This function is not called defaultly and need platform driver to call it as need.
> > Actually, I think PCI framework should handle L1.2 and L2 case, some devices
> > or user case want to L1.2 to get better resume latency, some devices want to
> > L2 to get better power saving, which out of scope of this patches.
> > 
> 
> I'm referring to the platform where these helper functions are being used which
> is layerscape. It doesn't matter whether you test this series with NVMe or not,
> it will not work unless you disable ASPM.

I tested with NVNE. You are right, ASPM is disabled at layerscape platform.

> 
> > This patch just handle L2 case, I remember L1.2 don't expect send PME at all.
> > 
> > > 
> > > 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.
> > 
> > Platform should choose call or no call this function according to their
> > user case. So far, I have not found a good mathod to let ASPM to affect
> > suspend/resume. 
> > 
> 
> You are missing my point here. If any platform decides to use these helper
> functions, they will face problems with NVMe. So please add a check for ASPM
> state before doing any L2/L3 handling.
> 
> I agree that it may not be optimal w.r.t power savings, but the PCIe controller
> driver should work for all devices.

I can add aspm check here. but I hope there are a flags in future, which can
show if expect pci controller enter L2.

Frank

> 
> - Mani
> 
> > > 
> > > - Mani
> > > 
> > > > This API help remove duplicate codes and it can be improved gradually.
> > > > 
> > > > 
> > > > > 
> > > > > - Mani
> > > > > 
> > > > > 
> > > > > -- 
> > > > > மணிவண்ணன் சதாசிவம்
> > > 
> > > -- 
> > > மணிவண்ணன் சதாசிவம்
> 
> -- 
> மணிவண்ணன் சதாசிவம்