Re: [PATCH] PCI: Add missing link delays required by the PCIe spec

[Mika Westerberg]

On Fri, Aug 23, 2019 at 09:12:54PM -0500, Bjorn Helgaas wrote:
> Hi Mika,

Hi,

> I'm trying to figure out specifically why we need this and where it
> should go.  Questions below.

Thanks for looking at this.

> On Wed, Aug 21, 2019 at 03:45:19PM +0300, Mika Westerberg wrote:
> > Currently Linux does not follow PCIe spec regarding the required delays
> > after reset. A concrete example is a Thunderbolt add-in-card that
> > consists of a PCIe switch and two PCIe endpoints:
> > 
> >   +-1b.0-[01-6b]----00.0-[02-6b]--+-00.0-[03]----00.0 TBT controller
> >                                   +-01.0-[04-36]-- DS hotplug port
> >                                   +-02.0-[37]----00.0 xHCI controller
> >                                   \-04.0-[38-6b]-- DS hotplug port
> > 
> > The root port (1b.0) and the PCIe switch downstream ports are all PCIe
> > gen3 so they support 8GT/s link speeds.
> > 
> > We wait for the PCIe hierarchy to enter D3cold (runtime):
> > 
> >   pcieport 0000:00:1b.0: power state changed by ACPI to D3cold
> > 
> > When it wakes up from D3cold, according to the PCIe 4.0 section 5.8 the
> > PCIe switch is put to reset and its power is re-applied. This means that
> > we must follow the rules in PCIe 4.0 section 6.6.1.
> > 
> > For the PCIe gen3 ports we are dealing with here, the following applies:
> > 
> >   With a Downstream Port that supports Link speeds greater than 5.0
> >   GT/s, software must wait a minimum of 100 ms after Link training
> >   completes before sending a Configuration Request to the device
> >   immediately below that Port. Software can determine when Link training
> >   completes by polling the Data Link Layer Link Active bit or by setting
> >   up an associated interrupt (see Section 6.7.3.3).
> > 
> > Translating this into the above topology we would need to do this (DLLLA
> > stands for Data Link Layer Link Active):
> > 
> >   pcieport 0000:00:1b.0: wait for 100ms after DLLLA is set before access to 0000:01:00.0
> >   pcieport 0000:02:00.0: wait for 100ms after DLLLA is set before access to 0000:03:00.0
> >   pcieport 0000:02:02.0: wait for 100ms after DLLLA is set before access to 0000:37:00.0
> > 
> > I've instrumented the kernel with additional logging so we can see the
> > actual delays the kernel performs:
> > 
> >   pcieport 0000:00:1b.0: power state changed by ACPI to D0
> >   pcieport 0000:00:1b.0: waiting for D3cold delay of 100 ms
> >   pcieport 0000:00:1b.0: waking up bus
> >   pcieport 0000:00:1b.0: waiting for D3hot delay of 10 ms
> >   pcieport 0000:00:1b.0: restoring config space at offset 0x2c (was 0x60, writing 0x60)
> >   ...
> >   pcieport 0000:00:1b.0: PME# disabled
> >   pcieport 0000:01:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
> >   ...
> >   pcieport 0000:01:00.0: PME# disabled
> >   pcieport 0000:02:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
> >   ...
> >   pcieport 0000:02:00.0: PME# disabled
> >   pcieport 0000:02:01.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
> >   ...
> >   pcieport 0000:02:01.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
> >   pcieport 0000:02:01.0: PME# disabled
> >   pcieport 0000:02:02.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
> >   ...
> >   pcieport 0000:02:02.0: PME# disabled
> >   pcieport 0000:02:04.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
> >   ...
> >   pcieport 0000:02:04.0: PME# disabled
> >   pcieport 0000:02:01.0: PME# enabled
> >   pcieport 0000:02:01.0: waiting for D3hot delay of 10 ms
> >   pcieport 0000:02:04.0: PME# enabled
> >   pcieport 0000:02:04.0: waiting for D3hot delay of 10 ms
> >   thunderbolt 0000:03:00.0: restoring config space at offset 0x14 (was 0x0, writing 0x8a040000)
> >   ...
> >   thunderbolt 0000:03:00.0: PME# disabled
> >   xhci_hcd 0000:37:00.0: restoring config space at offset 0x10 (was 0x0, writing 0x73f00000)
> >   ...
> >   xhci_hcd 0000:37:00.0: PME# disabled
> > 
> > For the switch upstream port (01:00.0) we wait for 100ms but not taking
> > into account the DLLLA requirement. We then wait 10ms for D3hot -> D0
> > transition of the root port and the two downstream hotplug ports. This
> > means that we deviate from what the spec requires.
> > 
> > Performing the same check for system sleep (s2idle) transitions we can
> > see following when resuming from s2idle:
> > 
> >   pcieport 0000:00:1b.0: power state changed by ACPI to D0
> >   pcieport 0000:00:1b.0: restoring config space at offset 0x2c (was 0x60, writing 0x60)
> >   ...
> >   pcieport 0000:01:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
> >   ...
> 
> I think the important thing in all the above logging is that it
> doesn't show any delay, right?

Right.

> If that's the case, you can just say
> that in one line; I trust you even without 40 lines of config space
> restore debug output :)

OK :)

> >   xhci_hcd 0000:37:00.0: restoring config space at offset 0x10 (was 0x0, writing 0x73f00000)
> >   ...
> >   thunderbolt 0000:03:00.0: restoring config space at offset 0x14 (was 0x0, writing 0x8a040000)
> > 
> > This is even worse. None of the mandatory delays are performed. If this
> > would be S3 instead of s2idle then according to PCI FW spec 3.2 section
> > 4.6.8.  there is a specific _DSM that allows the OS to skip the delays
> > but this platform does not provide the _DSM and does not go to S3 anyway
> > so no firmware is involved that could already handle these delays.
> > 
> > In this particular Intel Coffee Lake platform these delays are not
> > actually needed because there is an additional delay as part of the ACPI
> > power resource that is used to turn on power to the hierarchy but since
> > that additional delay is not required by any of standards (PCIe, ACPI)
> 
> So it sounds like this Coffee Lake accidentally works because of
> unrelated firmware delay that's not actually required, or at least not
> related to the delay required by PCIe?

Correct. The root port ACPI Power Resource includes quite long delay
which allows the links to be trained before the _ON method is finished.

> I did notice that we don't implement all of _DSM function 9 and the
> parts we're missing look like they could be relevant.

AFAICT that _DSM is only for lowering the delays, not increasing them.
If the platform does not provide it the OS must adhere to all the timing
requirements of the PCIe spec (PCI FW 3.2 section 4.6.9). Here we are
actually trying to do just that :)

> > it is not present in the Intel Ice Lake, for example where missing the
> > mandatory delays causes pciehp to start tearing down the stack too early
> > (links are not yet trained).
> 
> I'm guessing the Coffee Lake/Ice Lake distinction is not really
> relevant and the important thing is that something about Ice Lake is
> faster and reduces the accidental delay to the point that things stop
> working.
> 
> So probably the same thing could happen on Coffee Lake or any other
> system if it had different firmware.

Indeed.

> It *would* be interesting to mention what the pciehp/link training
> issue looks like to a user, just to help people who see it find this
> fix.

OK.

> > There is also one reported case (see the bugzilla link below) where the
> > missing delay causes xHCI on a Titan Ridge controller fail to runtime
> > resume when USB-C dock is plugged.
> > 
> > For this reason, introduce a new function pcie_wait_downstream_accessible()
> > that is called on PCI core resume and runtime resume paths accordingly
> > if downstream/root port with device below entered D3cold.
> > 
> > This is second attempt to add the missing delays. The previous solution
> > in commit c2bf1fc212f7 ("PCI: Add missing link delays required by the
> > PCIe spec") was reverted because of two issues it caused:
> 
> c2bf1fc212f7 was merged for v5.3 (it appeared in v5.3-rc1).  I *guess*
> it addressed https://bugzilla.kernel.org/show_bug.cgi?id=203885, which
> was reported on v5.2-rc4, though c2bf1fc212f7 doesn't actually mention
> the bugzilla?

I think c2bf1fc212f7 was merged before I was aware of that bug (or I
just missed it).

> 0617bdede511 ("Revert "PCI: Add missing link delays required by the
> PCIe spec"") appeared in v5.3-rc4.  If c2bf1fc212f7 was supposed to
> address BZ 203885, I'm confused about why you asked Kai-Heng to test
> v5.3-rc4, where c2bf1fc212f7 had already been reverted.

I was hoping that the revert in v5.3-rc4 would help here as well but
after futher investigation it turned to be the exact opposite.

> Or maybe c2bf1fc212f7 wasn't connected with BZ 203885 in the first
> place?
> 
> The net result is that I think v5.3-rc4 is equivalent to v5.2 with
> respect to this issue.  It *is* a pretty serious usability issue, no
> question, but it's not completely obvious that this fix needs to be in
> v5.3 since it's not something we broke during the v5.3 merge window.
> So if we *do* want it in v5.3, we need to think about how to justify
> that, e.g., if this issue affects shipping systems that are likely to
> run an upstream kernel, etc.

I don't think it needs to be in v5.3. I was thinking more like v5.4 so
we get some more testing coverage.

> >   1. One system become unresponsive after S3 resume due to PME service
> >      spinning in pcie_pme_work_fn(). The root port in question reports
> >      that the xHCI sent PME but the xHCI device itself does not have PME
> >      status set. The PME status bit is never cleared in the root port
> >      resulting the indefinite loop in pcie_pme_work_fn().
> 
> I don't see the connection between PME and either c2bf1fc212f7 or this
> patch.  Is there a fix for this pcie_pme_work_fn() infinite loop?

No.

> I do see that BZ 203885 mentions a flood of "PME: Spurious native
> interrupt!" messages, but I don't see how we've fixed that.

It was not completely root caused yet. To me it looks like the root port
keeps the PME status and pending bits set and that causes
pcie_pme_work_fn() spin forever. From the debugging output logs I got
from Matthias, it is the xHCI that seems to send the PME but it does not
have PME status set for some reason.

> >   2. Slows down resume if the root/downstream port does not support
> >      Data Link Layer Active Reporting because pcie_wait_for_link_delay()
> >      waits 1100ms in that case.
> 
> I don't see the slowdown mentioned in BZ 203885; is there another link
> to these reports?

Nicholas for example reported it here:

  https://lore.kernel.org/linux-pci/SL2P216MB01878BBCD75F21D882AEEA2880C60@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

I should have added that link to the changelog.

> > This version should avoid the above issues because we restrict the delay
> > to happen only if the port went into D3cold (so it goes through reset)
> > and only when there is no firmware involved on resume path (so the
> > kernel is responsible for all the delays).
> > 
> > Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203885
> > Reported-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
> > Tested-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
> > Signed-off-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> > ---
> > Hi all,
> > 
> > As the changelog says this is reworked version that tries to avoid reported
> > issues while at the same time fix the missing delays so we can get ICL
> > systems and at least the one system with Titan Ridge controller working
> > properly.
> > 
> > @Matthias, @Paul and @Nicholas: it would be great if you could try the
> > patch on top of v5.4-rc5+ and verify that it does not cause any issues on
> > your systems.
> > 
> >  drivers/pci/pci-driver.c |  19 ++++++
> >  drivers/pci/pci.c        | 127 ++++++++++++++++++++++++++++++++++++---
> >  drivers/pci/pci.h        |   1 +
> >  3 files changed, 137 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> > index a8124e47bf6e..9aec78ed8907 100644
> > --- a/drivers/pci/pci-driver.c
> > +++ b/drivers/pci/pci-driver.c
> > @@ -918,6 +918,7 @@ static int pci_pm_resume_noirq(struct device *dev)
> >  {
> >  	struct pci_dev *pci_dev = to_pci_dev(dev);
> >  	struct device_driver *drv = dev->driver;
> > +	pci_power_t state = pci_dev->current_state;
> >  	int error = 0;
> >  
> >  	if (dev_pm_may_skip_resume(dev))
> > @@ -947,6 +948,15 @@ static int pci_pm_resume_noirq(struct device *dev)
> >  
> >  	pcie_pme_root_status_cleanup(pci_dev);
> >  
> > +	/*
> > +	 * If resume involves firmware assume it takes care of any delays
> > +	 * for now. For suspend-to-idle case we need to do that here before
> > +	 * resuming PCIe port services to keep pciehp from tearing down the
> > +	 * downstream devices too early.
> > +	 */
> > +	if (state == PCI_D3cold && pm_suspend_no_platform())
> > +		pcie_wait_downstream_accessible(pci_dev);
> 
> Aren't these paths used for Conventional PCI devices as well as PCIe
> devices?

Indeed they are.

> I think the D3cold and pm_suspend_no_platform() checks should move
> inside the new interface, whatever it's called.  I'm not sure what
> that means for the fact that you don't check pm_suspend_no_platform()
> in the runtime-resume path; maybe it needs a flag or something.

It is not needed for runtime-resume path since we are not entering
system suspend. For runtime-resume path we need to perform the delays
always.

Here we just assume the firmware actually handles the delays which is
not entirely accurate. According to PCI FW 3.2 section 4.6.8 there is a
_DSM that the firmware can use to indicate to the OS that it can skip
the reset delays. So I think more correct would be to check for that in
resume path. However, since it looks like that _DSM is not implemented
in too many systems I've seen, it may slow down resume times
unnecessarily if the firmware actually handles the delays.

> But the "wait downstream" part seems a little too specific to be at
> the .resume_noirq and .runtime_resume level.
> 
> Do we descend the hierarchy and call .resume_noirq and .runtime_resume
> for the children of the bridge, too?

We do but at that time it is too late as we have already resumed pciehp
of the parent downstream port and it may have already started tearing
down the device stack below.

I'm open to any better ideas where this delay could be added, though :)

> 
> >  	if (drv && drv->pm && drv->pm->resume_noirq)
> >  		error = drv->pm->resume_noirq(dev);
> >  
> > @@ -1329,6 +1339,7 @@ static int pci_pm_runtime_resume(struct device *dev)
> >  	int rc = 0;
> >  	struct pci_dev *pci_dev = to_pci_dev(dev);
> >  	const struct dev_pm_ops *pm = dev->driver ? dev->driver->pm : NULL;
> > +	pci_power_t state = pci_dev->current_state;
> >  
> >  	/*
> >  	 * Restoring config space is necessary even if the device is not bound
> > @@ -1344,6 +1355,14 @@ static int pci_pm_runtime_resume(struct device *dev)
> >  	pci_enable_wake(pci_dev, PCI_D0, false);
> >  	pci_fixup_device(pci_fixup_resume, pci_dev);
> >  
> > +	/*
> > +	 * If the hierarcy went into D3cold wait for the link to be
> > +	 * reactivated before resuming PCIe port services to keep pciehp
> > +	 * from tearing down the downstream devices too early.
> 
> s/hierarcy/hierarchy/

OK.

> > +	 */
> > +	if (state == PCI_D3cold)
> > +		pcie_wait_downstream_accessible(pci_dev);
> > +
> >  	if (pm && pm->runtime_resume)
> >  		rc = pm->runtime_resume(dev);
> >  
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 1b27b5af3d55..9ac50710f1d4 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -1025,15 +1025,11 @@ static void __pci_start_power_transition(struct pci_dev *dev, pci_power_t state)
> >  	if (state == PCI_D0) {
> >  		pci_platform_power_transition(dev, PCI_D0);
> >  		/*
> > -		 * Mandatory power management transition delays, see
> > -		 * PCI Express Base Specification Revision 2.0 Section
> > -		 * 6.6.1: Conventional Reset.  Do not delay for
> > -		 * devices powered on/off by corresponding bridge,
> > -		 * because have already delayed for the bridge.
> > +		 * Mandatory power management transition delays are handled
> > +		 * in pci_pm_runtime_resume() of the corresponding
> > +		 * downstream/root port.
> >  		 */
> >  		if (dev->runtime_d3cold) {
> > -			if (dev->d3cold_delay && !dev->imm_ready)
> > -				msleep(dev->d3cold_delay);
> >  			/*
> >  			 * When powering on a bridge from D3cold, the
> >  			 * whole hierarchy may be powered on into
> > @@ -4607,14 +4603,17 @@ static int pci_pm_reset(struct pci_dev *dev, int probe)
> >  
> >  	return pci_dev_wait(dev, "PM D3->D0", PCIE_RESET_READY_POLL_MS);
> >  }
> > +
> >  /**
> > - * pcie_wait_for_link - Wait until link is active or inactive
> > + * pcie_wait_for_link_delay - Wait until link is active or inactive
> >   * @pdev: Bridge device
> >   * @active: waiting for active or inactive?
> > + * @delay: Delay to wait after link has become active (in ms)
> >   *
> >   * Use this to wait till link becomes active or inactive.
> >   */
> > -bool pcie_wait_for_link(struct pci_dev *pdev, bool active)
> > +static bool pcie_wait_for_link_delay(struct pci_dev *pdev, bool active,
> > +				     int delay)
> >  {
> >  	int timeout = 1000;
> >  	bool ret;
> > @@ -4651,13 +4650,121 @@ bool pcie_wait_for_link(struct pci_dev *pdev, bool active)
> >  		timeout -= 10;
> >  	}
> >  	if (active && ret)
> > -		msleep(100);
> > +		msleep(delay);
> >  	else if (ret != active)
> >  		pci_info(pdev, "Data Link Layer Link Active not %s in 1000 msec\n",
> >  			active ? "set" : "cleared");
> >  	return ret == active;
> >  }
> >  
> > +/**
> > + * pcie_wait_for_link - Wait until link is active or inactive
> > + * @pdev: Bridge device
> > + * @active: waiting for active or inactive?
> > + *
> > + * Use this to wait till link becomes active or inactive.
> > + */
> > +bool pcie_wait_for_link(struct pci_dev *pdev, bool active)
> > +{
> > +	return pcie_wait_for_link_delay(pdev, active, 100);
> > +}
> 
> This part (adding pcie_wait_for_link_delay() to make the delay time
> configurable) could be a separate patch to reduce the size of this
> patch and make the important pieces a little more obvious.

Indeed, will split it into a separate patch.

> 
> > +static int pcie_get_downstream_delay(struct pci_bus *bus)
> > +{
> > +	struct pci_dev *pdev;
> > +	int min_delay = 100;
> > +	int max_delay = 0;
> > +
> > +	list_for_each_entry(pdev, &bus->devices, bus_list) {
> > +		if (pdev->imm_ready)
> > +			min_delay = 0;
> > +		else if (pdev->d3cold_delay < min_delay)
> > +			min_delay = pdev->d3cold_delay;
> > +		if (pdev->d3cold_delay > max_delay)
> > +			max_delay = pdev->d3cold_delay;
> > +	}
> > +
> > +	return max(min_delay, max_delay);
> > +}
> > +
> > +/**
> > + * pcie_wait_downstream_accessible - Wait downstream device to be accessible
> > + * @pdev: PCIe port whose downstream device is waited
> > + *
> > + * Handle delays according to PCIe 4.0 section 6.6.1 before configuration
> > + * access to the downstream device is permitted. If the port does not have
> > + * any devices connected, does nothing.
> > + *
> > + * This is needed if the hierarchy below @pdev went through reset (after
> > + * exit from D3cold back to D0uninitialized).
> 
> D3cold -> D0 is a cold reset by definition, isn't it?

Yes.

> > + */
> > +void pcie_wait_downstream_accessible(struct pci_dev *pdev)
> > +{
> > +	struct pci_dev *child;
> > +	struct pci_bus *bus;
> > +	int delay;
> 
> IIUC pdev is not guaranteed to be a PCIe device here.

Good point, here we should check for pci_is_pcie() first.

> Do we need to observe the Trhfa requirements for Conventional PCI and
> PCI-X devices here?  If we don't do it here, where is it observed?

We probably should but I intented this to be PCIe specific since there
we have issues. For conventional PCI/PCI-X things "seem" to work and we
don't power manage those bridges anyway.

I'm not aware if Trhfa is handled in anywhere in the PCI stack
currently.

> > +	if (pci_pcie_type(pdev) != PCI_EXP_TYPE_ROOT_PORT &&
> > +	    pci_pcie_type(pdev) != PCI_EXP_TYPE_DOWNSTREAM)
> > +		return;
> > +
> > +	if (pci_dev_is_disconnected(pdev))
> > +		return;
> > +
> > +	if (!pdev->bridge_d3)
> > +		return;
> > +
> > +	bus = pdev->subordinate;
> > +	if (!bus)
> > +		return;
> > +
> > +	child = list_first_entry_or_null(&bus->devices, struct pci_dev,
> > +					 bus_list);
> > +	if (!child)
> > +		return;
> 
> I'm not convinced yet about skipping this if there's no subordinate
> bus or no children.  Don't we have to assume that the caller may
> immediately *probe* for children as soon as we return?

Yes, you are right. I was trying to avoid any unnecessary delays but
looks like we can't avoid them here.

> > +	delay = pcie_get_downstream_delay(bus);
> > +	if (!delay)
> > +		return;
> 
> I'm not sold on the idea that this delay depends on what's *below* the
> bridge.  We're using sec 6.6.1 to justify the delay, and that section
> doesn't say anything about downstream devices.

6.6.1 indeed talks about Downstream Ports and devices immediately below
them.

> If we call .resume_noirq/.runtime_resume for the downstream devices
> themselves, could we use d3cold_delay *there*?

Otherwise probably but since we already have resumed the downstream port
itself including pciehp it may notice that the link is not up anymore
and remove the device below.

> > +	/*
> > +	 * If downstream port does not support speeds greater than 5 GT/s
> > +	 * need to wait minimum 100ms. For higher speeds (gen3) we need to
> > +	 * wait first for the data link layer to become active.
> > +	 *
> > +	 * However, 100ms is the minimum and the spec says that the
> > +	 * software must allow at least 1s before it can determine that the
> > +	 * device that did not respond is a broken device. Also there is
> > +	 * evidence that the 100ms is not always enough so what we do here
> 
> This sort of "there is evidence" statement needs a specific reference.
> Otherwise it ends up meaning "we don't know why it matters and we
> can't test any changes in this area".

Sure I can refer to that bugzilla entry where the xHCI seems to require
more than 100ms.

> > +	 * is that we wait for the minimum 100ms (possibly after link
> > +	 * training completes) and then probe for the device presence once.
> > +	 * If we still don't get response we wait for another 100ms just to
> > +	 * give it some additional time to complete its initialization.
> > +	 */
> > +	if (pcie_get_speed_cap(pdev) <= PCIE_SPEED_5_0GT) {
> > +		dev_dbg(&pdev->dev, "waiting downstream link for %d ms\n",
> 
> pci_dbg()
> s/waiting downstream link for %d ms/waiting %d ms for downstream link/

OK.

> > +			delay);
> > +		msleep(delay);
> > +	} else {
> > +		dev_dbg(&pdev->dev,
> > +			"waiting downstream link for %d ms after activation\n",
> > +			delay);
> > +		if (!pcie_wait_for_link_delay(pdev, true, delay)) {
> > +			/*
> > +			 * If the link did not train, no need to wait
> > +			 * further the device is probably gone.
> > +			 */
> > +			return;
> > +		}
> > +	}
> > +
> > +	if (!pci_device_is_present(child)) {
> > +		dev_dbg(&child->dev,
> > +			"waiting for additional 100 ms for the device to become accessible\n");
> > +		msleep(100);
> 
> This seems like magic.  Can we relate this back to anything in the
> spec?  If we need this, maybe we're seeing CRRS? 

I assume CRRS == Configuration Request Retry Status? Unfortunately
that does not seem to be the case here because the root ports in
question on that bugzilla entry do not support CRS software visibility.

Only thing I can relate to the spec regarding this (again in 6.6.1) is
that it says "Software must wait minimum of 100ms". The only other limit
it gives is 1.0s that the software must allow before it can determine
the device is broken (well or missing, I suppose). So it pretty much
gives us a range of 100ms - 1000ms.

> I suppose this is related to the note above about evidence that 100ms
> may not be enough?

Yes indeed it is. I figured that we only need this for certain devices
so no need to do it for all.

We could also assume something like doubling the 100ms delay to be on
the safe side or so but that slows down resume paths even more.

> > +	}
> > +}
> > +
> >  void pci_reset_secondary_bus(struct pci_dev *dev)
> >  {
> >  	u16 ctrl;
> > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > index d22d1b807701..9a83fcf612ca 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -498,6 +498,7 @@ void pcie_do_recovery(struct pci_dev *dev, enum pci_channel_state state,
> >  		      u32 service);
> >  
> >  bool pcie_wait_for_link(struct pci_dev *pdev, bool active);
> > +void pcie_wait_downstream_accessible(struct pci_dev *pdev);
> >  #ifdef CONFIG_PCIEASPM
> >  void pcie_aspm_init_link_state(struct pci_dev *pdev);
> >  void pcie_aspm_exit_link_state(struct pci_dev *pdev);
> > -- 
> > 2.23.0.rc1
> >