On Mon, Nov 11, 2024 at 03:29:18AM +0000, Hongxing Zhu wrote: > > -----Original Message----- > > From: Krishna Chaitanya Chundru <quic_krichai@xxxxxxxxxxx> > > Sent: 2024年11月10日 8:10 > > To: Bjorn Helgaas <helgaas@xxxxxxxxxx>; Manivannan Sadhasivam > > <manivannan.sadhasivam@xxxxxxxxxx> > > Cc: Hongxing Zhu <hongxing.zhu@xxxxxxx>; jingoohan1@xxxxxxxxx; > > bhelgaas@xxxxxxxxxx; lpieralisi@xxxxxxxxxx; kw@xxxxxxxxx; > > robh@xxxxxxxxxx; Frank Li <frank.li@xxxxxxx>; imx@xxxxxxxxxxxxxxx; > > kernel@xxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; > > linux-kernel@xxxxxxxxxxxxxxx > > 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 -- மணிவண்ணன் சதாசிவம்
