Re: [PATCH v1] PCI: dwc: Clean up some unnecessary codes in dw_pcie_suspend_noirq()

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

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.

- Mani

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