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 03:29:18AM +0000, Hongxing Zhu wrote:
> > -----Original Message-----
> > From: Krishna Chaitanya Chundru <quic_krichai@quicinc.com>
> > Sent: 2024年11月10日 8:10
> > To: Bjorn Helgaas <helgaas@kernel.org>; Manivannan Sadhasivam
> > <manivannan.sadhasivam@linaro.org>
> > Cc: Hongxing Zhu <hongxing.zhu@nxp.com>; jingoohan1@gmail.com;
> > bhelgaas@google.com; lpieralisi@kernel.org; kw@linux.com;
> > robh@kernel.org; Frank Li <frank.li@nxp.com>; imx@lists.linux.dev;
> > kernel@pengutronix.de; linux-pci@vger.kernel.org;
> > linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH v1] PCI: dwc: Clean up some unnecessary codes in
> > dw_pcie_suspend_noirq()
> > 
> > 
> > 
> > On 11/8/2024 5:54 AM, 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."
> > >
> > Hi Bjorn,
> > 
> > I agree that link-up check is racy but once link is up and link has gone down
> > due to some reason the ltssm state will not move detect quiet or detect act, it
> > will go to pre detect quiet (i.e value 0f 0x5).
> > we can assume if the link is up LTSSM state will greater than detect act even if
> > the link was down.
> > 
> > - Krishna Chaitanya.
> > >>> 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.
> > >
> > > 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).
> Thanks for the comments.
> So, the L2 poll is better kept, since PCIe r6.0, sec 5.3.3.2.1 also recommends
>  1ms to 10ms timeout to check L2 ready or not.
> The v2 of this patch would only remove the LTSSM stat check when issue
>  the PME_TURN_OFF message if there are no further comments.
> 

If you unconditionally send PME_Turn_Off message, then you'd end up polling for
L23 Ready, which may result in a timeout and users will see the error message.
This is my concern.

- Mani

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