Message-ID: <38cfc6fa-ad39-86e6-3904-45f5fc937609@linux.intel.com>
Date: Fri, 26 Apr 2024 11:42:23 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
cc: linux-pci@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
    Bjorn Helgaas <helgaas@...nel.org>, 
    Mahesh J Salgaonkar <mahesh@...ux.ibm.com>, Lukas Wunner <lukas@...ner.de>, 
    Yazen Ghannam <yazen.ghannam@....com>, Bowman Terry <terry.bowman@....com>, 
    Hagan Billy <billy.hagan@....com>, Simon Guinot <simon.guinot@...gate.com>, 
    "Maciej W . Rozycki" <macro@...am.me.uk>, 
    Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>, 
    jon.grimm@....com, pradeepvineshreddy.kodamati@....com
Subject: Re: [PATCH] PCI: pciehp: Clear LBMS on hot-remove to prevent link
 speed reduction

On Thu, 25 Apr 2024, Smita Koralahalli wrote:
> On 4/24/2024 2:32 AM, Ilpo Järvinen wrote:
> > On Wed, 24 Apr 2024, Smita Koralahalli wrote:
> > 
> > > Clear Link Bandwidth Management Status (LBMS) if set, on a hot-remove
> > > event.
> > > 
> > > The hot-remove event could result in target link speed reduction if LBMS
> > > is set, due to a delay in Presence Detect State Change (PDSC) happening
> > > after a Data Link Layer State Change event (DLLSC).
> > > 
> > > In reality, PDSC and DLLSC events rarely come in simultaneously. Delay in
> > > PDSC can sometimes be too late and the slot could have already been
> > > powered down just by a DLLSC event. And the delayed PDSC could falsely be
> > > interpreted as an interrupt raised to turn the slot on. This false process
> > > of powering the slot on, without a link forces the kernel to retrain the
> > > link if LBMS is set, to a lower speed to restablish the link thereby
> > > bringing down the link speeds [2].
> > > 
> > > According to PCIe r6.2 sec 7.5.3.8 [1], it is derived that, LBMS cannot
> > > be set for an unconnected link and if set, it serves the purpose of
> > > indicating that there is actually a device down an inactive link.
> > > However, hardware could have already set LBMS when the device was
> > > connected to the port i.e when the state was DL_Up or DL_Active. Some
> > > hardwares would have even attempted retrain going into recovery mode,
> > > just before transitioning to DL_Down.
> > > 
> > > Thus the set LBMS is never cleared and might force software to cause link
> > > speed drops when there is no link [2].
> > > 
> > > Dmesg before:
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): Link Down
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): Card present
> > > 	pcieport 0000:20:01.1: broken device, retraining non-functional
> > > downstream link at 2.5GT/s
> > > 	pcieport 0000:20:01.1: retraining failed
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): No link
> > > 
> > > Dmesg after:
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): Link Down
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): Card present
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): No link
> > > 
> > > [1] PCI Express Base Specification Revision 6.2, Jan 25 2024.
> > >      https://members.pcisig.com/wg/PCI-SIG/document/20590
> > > [2] Commit a89c82249c37 ("PCI: Work around PCIe link training failures")
> > > 
> > > Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
> > > ---
> > > 1. Should be based on top of fixes for link retrain status in
> > > pcie_wait_for_link_delay()
> > > https://patchwork.kernel.org/project/linux-pci/list/?series=824858
> > > https://lore.kernel.org/linux-pci/53b2239b-4a23-a948-a422-4005cbf76148@linux.intel.com/
> > > 
> > > Without the fixes patch output would be:
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): Link Down
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): Card present
> > > 	pcieport 0000:20:01.1: broken device, retraining non-functional
> > > downstream link at 2.5GT/s
> > > 	pcieport 0000:20:01.1: retraining failed
> > > 	pcieport 0000:20:01.1: pciehp: Slot(59): No device found.
> > 
> > Did you hit the 60 sec delay issue without series 824858? If you've tested
> > them and the fixes helped your case, could you perhaps give Tested-by for
> > that series too (in the relevant thread)?
> 
> I'm assuming the 60s delay issue is from pci_dev_wait()?

Yes.
 
> Correct me if I'm wrong.
> I think series 824858 potentially fixes the bug at two different places. What
> you are seeing is at suspend/resume operation called from the calls below.
>
> pci_pm_runtime_resume()
>     pci_pm_bridge_power_up_actions()
>       pci_bridge_wait_for_secondary_bus()
>         pcie_wait_for_link_delay()
>           pcie_failed_link_retrain()
>         pci_dev_wait()
> 
> But series 824858 helped me in properly returning an error code from
> pcie_wait_for_link_delay() and also avoiding the 100ms delay inside
> pcie_wait_for_link_delay() and probably the timeout in
> pcie_wait_for_presence()..
> 
> The sequence of operations which I'm looking at is after an PDSC event as
> below:
> pciehp_handle_presence_or_link_change()
>   pciehp_enable_slot()
>     __pciehp_enable_slot()
>       board_added()
>         pciehp_check_link_status()
>           pcie_wait_for_link()
>             pcie_wait_for_link_delay()
>               pcie_failed_link_retrain()
> 
> pcie_failed_link_retrain() would initially return false on a "failed link
> retrain" attempt which would make pcie_wait_for_link_delay() and
> pcie_wait_for_link() to erroneously succeed thereby unnecessarily proceeding
> with other checks in pciehp_check_link_status().
> 
> Series 824858 fixes the bug by properly returning an error code.

Yes, that series also fixes other bugs too. I wasn't aware how the
incorrect return value impacted hotplug too.

> However, I had missed looking at your patchset when I initially wrote
> this. From the patch below I see you have addressed clearing LBMS as well but
> at a different place. But I didn't understand why was it dropped.
> 
> https://lore.kernel.org/all/20240129112710.2852-2-ilpo.jarvinen@linux.intel.com/

That change wasn't perfect solution given how the target speed quirk 
works. Unconditional clearing might have broken the quirk so localizing 
the LBMS clearing into the cases where the device is actually gone would 
be the best approach.

For normal hotplug, it's relatively easy to clear LBMS as demonstrated by 
your patch (I have consider putting it into somewhere in remove.c if the 
device is in disconnected state while removing the function 0 but roughly 
the same idea as in your patch).

The additional problem with suspend/resume cycle is that the device can 
disappear while suspended and we only notice the disconnection after 
hotplug has resumed. This occurs only after full portdrv resume because it 
requires interrupt which is too late to avoid the 60secs delay. Series 
824858 avoids the 60secs delay even for suspend/resume but if LBMS could 
be cleared in time, it would help things further because the quirk 
wouldn't even trigger, or at minimum, the wait delays could be 
short-circuited if device disconnection is noticed while the wait is in 
progress.

> From what I understood while experimenting is, tracking the set/unset behavior
> of LBMS is hard, as HW has the right to attempt retrain at any point except
> when the status is DL_Down as per the statement from the the SPEC: "This bit
> is Set by hardware to indicate that either of the following has occurred
> without the Port transitioning through DL_Down status".

It's actually not that hard. On medium term, I'm trying to get PCIe BW 
controller into kernel (about to send a new version of it, probably next 
week), which will keep clearing LBMS by enabling BW notifications that are 
the SW/OS way to track LBMS assertions.

> The set LBMS when the port was not in DL_Down, is never unset after the Port
> is transitioned to DL_Down. So, I think clearing it after the port status is
> DL_Down (which I assume happens after DLLSC interrupt fires to bring the slot
> down) makes it remain cleared only until the port remains at DL_Down state. As
> soon as the port transitions to other states (I don't know how SW could track
> the states) there is no guarantee that the bit is still clear as HW might have
> attempted retrain.
> 
> The only way would be to track the DL_Down and Active/Up states. I at the
> moment don't know how to do it or if it is possible to do it in SW in the
> first place. Hence, I'm more inclined pushing [2] below as a fix  for link
> speed drop. However, that has some complexities as well :(
> 
> After talking to HW folks one place where our HW sets LBMS is:
> 
> Device is removed.
>   DL_Up doesn't immediately change.
>     HW will just see errors on the receiver and doesn't automatically
>     know that it was because of a hot remove, and tries to recover the
>     link.
>     LBMS gets set here as rate change is attempted.

That's interesting. I have earlier thought it's only set when the rate 
change was actually successful but it seems that isn't even meant to be 
the case here when I now reread the spec.

>     DL_Down occurs after this.
> 
> Once DL_Down occurs nobody is supposed to set the bit. But the set bit is
> never cleared and is creating all issues.
>
> HW mostly considers other parameters and LTSSM behaviors in transitioning
> between Active/Up to Down states which I'm not sure at the moment how much of
> it is transparent to OS. :/
>
> > > @@ -134,4 +134,10 @@ void pciehp_unconfigure_device(struct controller
> > > *ctrl, bool presence)
> > >   	}
> > >     	pci_unlock_rescan_remove();
> > > +
> > > +	/* Clear LBMS on removal */
> > > +	pcie_capability_read_word(ctrl->pcie->port, PCI_EXP_LNKSTA, &lnksta);
> > > +	if (lnksta & PCI_EXP_LNKSTA_LBMS)
> > > +		pcie_capability_write_word(ctrl->pcie->port, PCI_EXP_LNKSTA,
> > > +					   PCI_EXP_LNKSTA_LBMS);
> > 
> > It's enough to unconditionally write PCI_EXP_LNKSTA_LBMS, no need to
> > check first. The comment is just spelling out what can already be read
> > from the code so I'd drop the comment.
> 
> Sure, I will make changes once I send v2 and if we consider to address it this
> way.. :)
> 
> > 
> > I agree it makes sense to clear the LBMS when device is removed.
> 
> Thank you!

As I said, I think it's correct thing to do when we know the device is 
gone and shouldn't interfere with the target speed quirk unlike my 
clear-LBMS-on-resume attempt.

This won't help the disconnected while suspended case for now but it's 
still a step into correct direction even for that.

-- 
 i.

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