Message-ID: <20241112180416.ssyivc53h7i6w2wq@thinkpad>
Date: Tue, 12 Nov 2024 23:34:16 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Hongxing Zhu <hongxing.zhu@....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>,
	Frank Li <frank.li@....com>,
	"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()

On Tue, Nov 12, 2024 at 09:25:57AM +0000, Hongxing Zhu wrote:
> > -----Original Message-----
> > From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > Sent: 2024年11月11日 14:09
> > To: Bjorn Helgaas <helgaas@...nel.org>
> > Cc: Hongxing Zhu <hongxing.zhu@....com>; jingoohan1@...il.com;
> > bhelgaas@...gle.com; lpieralisi@...nel.org; kw@...ux.com;
> > robh@...nel.org; Frank Li <frank.li@....com>; 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 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.
> > 
> I understand Manivannan' s concerns.
> When check the link is up or not before dumping error message, 
> there is another check racy.

Right.

> How about to replace the dev_err() by dev_info(), and no error return?
> Whatever the timeout is caused by no EP connected or something else. Just
> inform user the real stat it is.
> 

But users don't want the timeout message if no EP is connected, that's my point.

- Mani

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

-- 
மணிவண்ணன் சதாசிவம்

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