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Date: Tue, 12 Nov 2024 09:15:25 +0000
From: Hongxing Zhu <hongxing.zhu@....com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>, Frank Li
	<frank.li@....com>
CC: Bjorn Helgaas <helgaas@...nel.org>, "jingoohan1@...il.com"
	<jingoohan1@...il.com>, "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"lpieralisi@...nel.org" <lpieralisi@...nel.org>, "kw@...ux.com"
	<kw@...ux.com>, "robh@...nel.org" <robh@...nel.org>, "imx@...ts.linux.dev"
	<imx@...ts.linux.dev>, "kernel@...gutronix.de" <kernel@...gutronix.de>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v1] PCI: dwc: Clean up some unnecessary codes in
 dw_pcie_suspend_noirq()

> -----Original Message-----
> From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> Sent: 2024年11月12日 16:02
> To: Frank Li <frank.li@....com>
> Cc: Bjorn Helgaas <helgaas@...nel.org>; Hongxing Zhu
> <hongxing.zhu@....com>; jingoohan1@...il.com; bhelgaas@...gle.com;
> lpieralisi@...nel.org; kw@...ux.com; robh@...nel.org; imx@...ts.linux.dev;
> kernel@...gutronix.de; linux-pci@...r.kernel.org;
> linux-kernel@...r.kernel.org
> Subject: Re: [PATCH v1] PCI: dwc: Clean up some unnecessary codes in
> dw_pcie_suspend_noirq()
> 
> On Mon, Nov 11, 2024 at 12:42:50PM -0500, Frank Li wrote:
> > On Mon, Nov 11, 2024 at 11:39:02AM +0530, Manivannan Sadhasivam
> wrote:
> > > On Thu, Nov 07, 2024 at 06:24:25PM -0600, Bjorn Helgaas wrote:
> > > > On Thu, Nov 07, 2024 at 11:13:34AM +0000, Manivannan Sadhasivam
> wrote:
> > > > > On Thu, Nov 07, 2024 at 04:44:55PM +0800, Richard Zhu wrote:
> > > > > > Before sending PME_TURN_OFF, don't test the LTSSM stat. Since
> > > > > > it's safe to send PME_TURN_OFF message regardless of whether
> > > > > > the link is up or down. So, there would be no need to test the
> > > > > > LTSSM stat before sending PME_TURN_OFF message.
> > > > >
> > > > > What is the incentive to send PME_Turn_Off when link is not up?
> > > >
> > > > There's no need to send PME_Turn_Off when link is not up.
> > > >
> > > > But a link-up check is inherently racy because the link may go
> > > > down between the check and the PME_Turn_Off.  Since it's
> > > > impossible for software to guarantee the link is up, the Root Port
> > > > should be able to tolerate attempts to send PME_Turn_Off when the link
> is down.
> > > >
> > > > So IMO there's no need to check whether the link is up, and
> > > > checking gives the misleading impression that "we know the link is
> > > > up and therefore sending PME_Turn_Off is safe."
> > > >
> > >
> > > I agree that the check is racy (not sure if there is a better way to
> > > avoid that), but if you send the PME_Turn_Off unconditionally, then
> > > it will result in
> > > L23 Ready timeout and users will see the error message.
> > >
> > > > > > Remove the L2 poll too, after the PME_TURN_OFF message is sent
> > > > > > out.  Because the re-initialization would be done in
> > > > > > dw_pcie_resume_noirq().
> > > > >
> > > > > As Krishna explained, host needs to wait until the endpoint acks
> > > > > the message (just to give it some time to do cleanups). Then
> > > > > only the host can initiate D3Cold. It matters when the device supports
> L2.
> > > >
> > > > The important thing here is to be clear about the *reason* to poll
> > > > for
> > > > L2 and the *event* that must wait for L2.
> > > >
> > > > I don't have any DesignWare specs, but when
> > > > dw_pcie_suspend_noirq() waits for DW_PCIE_LTSSM_L2_IDLE, I think
> > > > what we're doing is waiting for the link to be in the L2/L3 Ready
> > > > pseudo-state (PCIe r6.0, sec 5.2, fig 5-1).
> > > >
> > > > L2 and L3 are states where main power to the downstream component
> > > > is off, i.e., the component is in D3cold (r6.0, sec 5.3.2), so
> > > > there is no link in those states.
> > > >
> > > > The PME_Turn_Off handshake is part of the process to put the
> > > > downstream component in D3cold.  I think the reason for this
> > > > handshake is to allow an orderly shutdown of that component before
> > > > main power is removed.
> > > >
> > > > When the downstream component receives PME_Turn_Off, it will stop
> > > > scheduling new TLPs, but it may already have TLPs scheduled but
> > > > not yet sent.  If power were removed immediately, they would be
> > > > lost.  My understanding is that the link will not enter L2/L3
> > > > Ready until the components on both ends have completed whatever
> > > > needs to be done with those TLPs.  (This is based on the L2/L3
> > > > discussion in the Mindshare PCIe book; I haven't found clear spec
> > > > citations for all of it.)
> > > >
> > > > I think waiting for L2/L3 Ready is to keep us from turning off
> > > > main power when the components are still trying to dispose of those
> TLPs.
> > > >
> > >
> > > Not just disposing TLPs as per the spec, most endpoints also need to
> > > reset their state machine as well (if there is a way for the
> > > endpoint sw to delay sending
> > > L23 Ready).
> > >
> > > > So I think every controller that turns off main power needs to
> > > > wait for L2/L3 Ready.
> > > >
> > > > There's also a requirement that software wait at least 100 ns
> > > > after
> > > > L2/L3 Ready before turning off refclock and main power (sec
> > > > 5.3.3.2.1).
> > > >
> > >
> > > Right. Usually, the delay after PERST# assert would make sure this,
> > > but in layerscape driver (user of dw_pcie_suspend_noirq) I don't see
> > > power/refclk removal.
> > >
> > > Richard Zhu/Frank, thoughts?
> >
> > Generally, power/refclk remove when system enter sleep state. There is
> > signal "suspend_request_b", which connect to PMIC. After CPU trigger
> > this signnal, PMIC will turn off (pre fused) some power rail.
> >
> > Refclk(come from SOC chip), OSC will be shutdown when send out
> > "suspend_request_b".
> >
> 
> Thanks for clarifying! Then it would be better to add the 100ns delay after
> receiving the L23 Ready message from endpoint.
Okay.
How about the following changes?
- Before dump error message, make sure link is up.
- Add 1us delay after L2/L3 Ready is received.

--- a/drivers/pci/controller/dwc/pcie-designware-host.c
+++ b/drivers/pci/controller/dwc/pcie-designware-host.c
@@ -940,9 +940,16 @@ int dw_pcie_suspend_noirq(struct dw_pcie *pci)
                ret = read_poll_timeout(dw_pcie_get_ltssm, val, val == DW_PCIE_LTSSM_L2_IDLE,
                                        PCIE_PME_TO_L2_TIMEOUT_US/10,
                                        PCIE_PME_TO_L2_TIMEOUT_US, false, pci);
-               if (ret) {
+               if (ret && dw_pcie_link_up(pci)) {
                        dev_err(pci->dev, "Timeout waiting for L2 entry! LTSSM: 0x%x\n", val);
                        return ret;
+               } else {
+                       /*
+                        * Refer to r6.0, sec 5.3.3.2.1, software should wait at
+                        * least 100ns after L2/L3 Ready before turning off
+                        * refclock and main power.
+                        */
+                       udelay(1);

Best Regards
Richard Zhu
> 
> - Mani
> 
> --
> மணிவண்ணன் சதாசிவம்

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