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From: Mika Westerberg <mika.westerberg@linux.intel.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Len Brown <lenb@kernel.org>,
	Mario.Limonciello@dell.com,
	Michael Jamet <michael.jamet@intel.com>,
	Yehezkel Bernat <YehezkelShB@gmail.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Lukas Wunner <lukas@wunner.de>,
	linux-pci@vger.kernel.org, linux-acpi@vger.kernel.org
Subject: Re: [PATCH v5 4/9] ACPI / hotplug / PCI: Do not scan all bridges when native PCIe hotplug is used
Date: Mon, 7 May 2018 14:34:49 +0300	[thread overview]
Message-ID: <20180507113449.GR17277@lahna.fi.intel.com> (raw)
In-Reply-To: <20180505000420.GB9529@bhelgaas-glaptop.roam.corp.google.com>

On Fri, May 04, 2018 at 07:04:20PM -0500, Bjorn Helgaas wrote:
> On Thu, May 03, 2018 at 01:22:41PM +0300, Mika Westerberg wrote:
> > On Wed, May 02, 2018 at 03:49:32PM -0500, Bjorn Helgaas wrote:
> > > On Mon, Apr 16, 2018 at 01:34:48PM +0300, Mika Westerberg wrote:
> > > > When a system is using native PCIe hotplug for Thunderbolt and the
> > > > controller is not enabled for full RTD3 (runtime D3) it will be only
> > > > present in the system when there is a device connected. This pretty much
> > > > follows the BIOS assisted hotplug behaviour.
> > > 
> > > This is a tangent, but what exactly does "controller is not enabled
> > > for full RTD3 (runtime D3)" mean?  The way that's worded sounds like
> > > it *could* be a setting in a PCI config register, but I suspect it's
> > > really something in Linux, e.g., some bit in struct pci_dev or device?
> > 
> > It means that the controller can to into D3 runtime. With BIOS assisted
> > mode and native PCIe mode, the controller is hot-removed when there is
> > nothing connected. In RTD3 mode it is there always and the OS expected
> > to put it into D3 when idle using standard PCI PM registers.
> 
> How do I tell if a controller is enabled for runtime D3?
> 
> I'm still struggling to figure out exactly how this runtime D3 part is
> relevant to this patch.  It might be behavior that happens in this
> particular scenario, but I'm not sure yet that it is a required part
> of the picture.
> 
> I think you're trying to separate enumeration handled by pciehp from
> enumeration handled by acpiphp, and runtime D3 sounds like an
> orthogonal issue.

It is not an issue at all. I just thought it is good to mention where
this all comes from in the changelog. In can reword it to:

  When a system is using native PCIe hotplug for Thunderbolt it will be
  only present in the system when there is a device connected. This pretty
  much follows the BIOS assisted hotplug behaviour.

if that is more understandable.

> > > > Thunderbolt host router integrated PCIe switch has two additional PCIe
> > > > downstream bridges that lead to NHI (Thunderbolt host controller) and xHCI
> > > > (USB 3 host controller) respectively. These downstream bridges are not
> > > > marked being hotplug capable. Reason for that is to preserve resources.
> > > > Otherwise the OS would distribute remaining resources between all
> > > > downstream bridges making these two bridges consume precious resources
> > > > of the actual hotplug bridges.
> > > > 
> > > > Now, because these two bridges are not marked being hotplug capable the OS
> > > > will not enable hotplug interrupt for them either and will not receive
> > > > interrupt when devices behind them are hot-added. Solution to this is
> > > > that the BIOS sends ACPI Notify() to the root port let the OS know it
> > > > needs to rescan for added and/or removed devices.
> > > 
> > > We're building stuff based on "is_hotplug_bridge", but I'm not sure
> > > that bit is really useful.
> > > 
> > > set_pcie_hotplug_bridge() sets it for downstream ports with
> > > PCI_EXP_SLTCAP_HPC, which is easy enough to understand.
> > > 
> > > In acpiphp, check_hotplug_bridge() sets in some scenario that I can't
> > > quite figure out.  I assume it's based on something in the namespace.
> > > 
> > > But it seems like the whole point of this patch is that even if a
> > > bridge doesn't have "is_hotplug_bridge" set, ACPI hotplug can hot-add
> > > devices below it.
> > 
> > Correct.
> > 
> > > So I'm not sure what "is_hotplug_bridge" is really telling us.  Is it
> > > just a hint about how many resources we want to assign to the bridge,
> > > i.e., we assign only a few when it's not set and more if it is set?
> > 
> > Good question. I did not invent the flag so hard to say. I've been using
> > it because IIRC you prefer it.
> 
> It's not a question of whether I prefer it.  It's only useful if it
> means something specific and we agree on what that is.
> 
> So what do you think it means?

I think it currently means a bridge that may get new devices behind it
as a result of hot-add.

In case of ACPI based hotplug, it is not always possible to determine
and set it upfront so it would make more sense to me if is_hotplug_bridge 
means that this bridge is used only as part of native PCIe hotplug.

> In this code:
> 
> > > > @@ -291,7 +294,7 @@ static acpi_status acpiphp_add_context(acpi_handle handle, u32 lvl, void *data,
> > > >          * expose slots to user space in those cases.
> > > >          */
> > > >         if ((acpi_pci_check_ejectable(pbus, handle) || is_dock_device(adev))
> > > > -           && !(pdev && pdev->is_hotplug_bridge && pciehp_is_native(pdev))) {
> > > > +           && !(slot->flags & SLOT_IS_NATIVE && pdev->is_hotplug_bridge)) {
> 
> I *think* this part:
> 
>   slot->flags & SLOT_IS_NATIVE && pdev->is_hotplug_bridge
> 
> means "this bridge supports hotplug but it is handled by pciehp, and
> acpiphp should not expose the slot to user space".

Yes, I specifically added SLOT_IS_NATIVE flag there so we can get rid of
one condition. Note that:

  pdev && pdev->is_hotplug_bridge && pciehp_is_native(pdev)

and

  slot->flags & SLOT_IS_NATIVE && pdev->is_hotplug_bridge

are equivalent.

There are no functional changes in that hunk.

> If that's the case, maybe we should rework pciehp_is_native(bridge) so
> instead of answering the question "does the OS have permission to
> control PCIe hotplug in the hierarchy containing <bridge>?", it could
> answer the specific question "does pciehp handle hotplug for
> <bridge>?", e.g., something along these lines:
> 
>   bool pciehp_is_native(struct pci_dev *bridge)
>   {
>   #ifdef CONFIG_HOTPLUG_PCI_PCIE
>     u32 slot_cap;
>     struct pci_host_bridge *host;
> 
>     if (!pci_is_pcie(bridge))
>       return false;
> 
>     pcie_capability_read_dword(bridge, PCI_EXP_SLTCAP, &slot_cap);
>     if (!(slot_cap & PCI_EXP_SLTCAP_HPC))
>       return false;
> 
>     if (pcie_ports_native)
>       return true;
> 
>     host = pci_find_host_bridge(bridge->bus);
>     return host->native_hotplug;
>   #else
>     return false;
>   #endif
>   }
> 
> Then your test for whether acpiphp should expose the slot to user
> space could be:

[That test was there already before this patch.]

>   -   && !(pdev && pdev->is_hotplug_bridge && pciehp_is_native(pdev))) {
>   +   && !pciehp_is_native(pdev)) {
>  
> We could also use pciehp_is_native() directly in
> get_port_device_capability(), which is essentially the condition that
> allows pciehp to claim the device.

Fine, I agree it makes sense.

However, I intended *this* patch to be *fix* and thus did not want to
start reworking things too much. Especially code that is not related to
the issue at all (pciehp_is_native()).

> > > > Here is how the mechanism is supposed to work when a Thunderbolt
> > > > endpoint is connected to one of the ports. In case of a standard USB-C
> > > > device only the xHCI is hot-added otherwise steps are the same.
> > > > 
> > > > 1. Initially there is only the PCIe root port that is controlled by
> > > >    the pciehp driver
> > > > 
> > > >   00:1b.0 (Hotplug+) --
> > > > 
> > > > 2. Then we get native PCIe hotplug interrupt and once it is handled the
> > > >    topology looks as following
> > > > 
> > > >   00:1b.0 (Hotplug+) -- 01:00.0 --+- 02:00.0 --
> > > >                                   +- 02:01.0 (HotPlug+)
> > > >                                   \- 02:02.0 --
> > > > 
> > > > 3. Bridges 02:00.0 and 02:02.0 are not marked as hotplug capable and
> > > 
> > > By "not marked as hotplug capable", I guess you mean
> > > PCI_EXP_SLTCAP_HPC is not set, right?
> > 
> > Right.
> > 
> > > >    they don't have anything behind them currently. Bridge 02:01.0 is
> > > >    hotplug capable and used for extending the topology. At this point
> > > >    the required PCIe devices are enabled and ACPI Notify() is sent to
> > > >    the root port. The resulting topology is expected to look like
> > > > 
> > > >   00:1b.0 (Hotplug+) -- 01:00.0 --+- 02:00.0 -- Thunderbolt host controller
> > > >                                   +- 02:01.0 (HotPlug+)
> > > >                                   \- 02:02.0 -- xHCI host controller
> > > > 
> > > > However, the current ACPI hotplug implementation scans the whole 00:1b.0
> > > > hotplug slot and everything behind it regardless whether native PCIe is
> > > > used or not, and it expects that the BIOS has configured bridge
> > > > resources upfront. If that's not the case it assigns resources using
> > > > minimal allocation (everything currently found just barely fit)
> > > > preventing future extension. In addition to that, if there is another
> > > > native PCIe hotplug going on we may find the new PCIe switch only
> > > > partially ready (all links are not fully trained yet) confusing pciehp
> > > > when it finally starts to enumerate for new devices.
> > > > 
> > > > To make this work better with the native PCIe hotplug driver (pciehp),
> > > > we let it handle all slot management and resource allocation for hotplug
> > > > bridges and restrict ACPI hotplug to non-hotplug bridges.
> 
> I *think* the point of this patch is that:
> 
>   If X is a bridge, and pciehp manages hotplug on X, i.e., X has
>   PCI_EXP_SLTCAP_HPC set and the OS owns PCIe hotplug in this
>   hierarchy per _OSC, acpiphp should not enumerate devices on X's
>   secondary bus.
> 
>   Furthermore, acpiphp should avoid this enumeration no matter where X
>   is in the hierarchy.  So if the Notify() goes to a bridge Y far
>   above X, acpiphp should look at every existing bridge in the
>   hierarchy below Y.  If the bridge is managed by pciehp, acpiphp
>   should ignore it.  Otherwise, acpiphp should scan for new devices
>   below it.
> 
> This enumeration by acpiphp is recursive, and it has to avoid the
> pciehp-managed bridges at every level.  Your changes to enable_slot()
> check for SLOT_IS_NATIVE at the top level, but I don't see how they
> avoid pciehp-managed bridges at lower levels that may be scanned by
> pci_scan_bridge().

This is pretty special case. For example below the code scans only the
two bridges (02:00.0 and 02:02.0) and they only have one bus reserved.
There won't be any further bridges downstream. The hotplug bridge
(02:01.0) is left for pciehp:

  00:1b.0 (Hotplug+) -- 01:00.0 --+- 02:00.0 --
                                  +- 02:01.0 (HotPlug+)
                                  \- 02:02.0 --

In theory of course it is possible that there will be another hotplug
bridge added behind those two bridges but in practice this is only used
for this special case to bring in the two devices (NHI, xHCI). I don't
think it makes much sense to complicate pci_scan_bridge() and friends
just to make more "generic" support for a case that in practice never
happens.

  reply	other threads:[~2018-05-07 11:34 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-16 10:34 [PATCH v5 0/9] PCI: Fixes and cleanups for native PCIe and ACPI hotplug Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 1/9] PCI: Take all bridges into account when calculating bus numbers for extension Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 2/9] PCI: Take bridge window alignment into account when distributing resources Mika Westerberg
2018-04-25 22:38   ` Bjorn Helgaas
2018-04-26 12:23     ` Mika Westerberg
2018-05-01 20:32       ` Bjorn Helgaas
2018-05-03 12:39         ` Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 3/9] PCI: pciehp: Clear Presence Detect and Data Link Layer Status Changed on resume Mika Westerberg
2018-05-01 21:52   ` Bjorn Helgaas
2018-05-02 11:55     ` Mika Westerberg
2018-05-02 13:41       ` Bjorn Helgaas
2018-05-03 10:42         ` Mika Westerberg
2018-05-03 23:01           ` Bjorn Helgaas
2018-05-04  7:20             ` Mika Westerberg
2018-05-30 10:40             ` Lukas Wunner
2018-05-30 13:27               ` Mika Westerberg
2018-05-04  7:18     ` Lukas Wunner
2018-05-04  8:02       ` Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 4/9] ACPI / hotplug / PCI: Do not scan all bridges when native PCIe hotplug is used Mika Westerberg
     [not found]   ` <20180502204932.GG11698@bhelgaas-glaptop.roam.corp.google.com>
2018-05-03 10:22     ` Mika Westerberg
2018-05-05  0:04       ` Bjorn Helgaas
2018-05-07 11:34         ` Mika Westerberg [this message]
2018-05-07 20:37           ` Bjorn Helgaas
2018-04-16 10:34 ` [PATCH v5 5/9] ACPI / hotplug / PCI: Mark stale PCI devices disconnected Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 6/9] PCI: Move resource distribution for a single bridge outside of the loop Mika Westerberg
2018-04-24 23:05   ` Bjorn Helgaas
2018-04-25  7:29     ` Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 7/9] PCI: Document return value of pci_scan_bridge() and pci_scan_bridge_extend() Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 8/9] PCI: Improve "partially hidden behind bridge" log message Mika Westerberg
2018-04-16 10:34 ` [PATCH v5 9/9] ACPI / hotplug / PCI: Drop unnecessary parentheses Mika Westerberg

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