PCI: Improvements for native PCIe hotplug

System BIOS sometimes allocates extra bus space for hotplug capable PCIe root/downstream ports. This space is needed if the device plugged to the port will have more hotplug capable downstream ports. A good example of this is Thunderbolt. Each Thunderbolt device contains a PCIe switch and one or more hotplug capable PCIe downstream ports where the daisy chain can be extended.

Currently Linux only allocates minimal bus space to make sure all the enumerated devices barely fit there. The BIOS reserved extra space is not taken into consideration at all. Because of this we run out of bus space pretty quickly when more PCIe devices are attached to hotplug downstream ports in order to extend the chain.

This modifies PCI core so that we distribute the available BIOS allocated bus space equally between hotplug capable PCIe downstream ports to make sure there is enough bus space for extending the hierarchy later on.

Signed-off-by Mika Westerberg <[email protected]>

---
 drivers/pci/hotplug-pci.c |  13 +++++-
 drivers/pci/probe.c       | 114 ++++++++++++++++++++++++++++++++++++++--------
 include/linux/pci.h       |  19 ++++++--
 3 files changed, 123 insertions(+), 23 deletions(-)

drivers/pci/hotplug-pci.c +12 -1

index c68366cee6b7..deb06fe22b26 100644
@@ -8,6 +8,7 @@ int pci_hp_add_bridge(struct pci_dev *dev)
 {
 	struct pci_bus *parent = dev->bus;
 	int pass, busnr, start = parent->busn_res.start;
+	unsigned int available_buses = 0;
 	int end = parent->busn_res.end;
 
 	for (busnr = start; busnr <= end; busnr++) {

@@ -19,8 +20,18 @@ int pci_hp_add_bridge(struct pci_dev *dev)
 				pci_name(dev));
 		return -1;
 	}
+
+	/*
+	 * In case of PCIe the hierarchy can be extended through hotplug
+	 * capable downstream ports. Distribute the available bus
+	 * numbers between them to make extending the chain possible.
+	 */
+	if (pci_is_pcie(dev))
+		available_buses = end - busnr;
+
 	for (pass = 0; pass < 2; pass++)
-		busnr = pci_scan_bridge(parent, dev, busnr, pass);
+		busnr = pci_scan_bridge_extend(parent, dev, busnr,
+					       available_buses, pass);
 	if (!dev->subordinate)
 		return -1;
 

drivers/pci/probe.c +18 -3

index f285cd74088e..ae0bf3c807f5 100644
@@ -960,6 +960,17 @@ static void pci_enable_crs(struct pci_dev *pdev)
 }
 
 /*
+ * pci_scan_bridge_extend() - Scan buses behind a bridge
+ * @bus: Parent bus the bridge is on
+ * @dev: Bridge itself
+ * @max: Starting subordinate number of buses behind this bridge
+ * @available_buses: Total number of buses available for this bridge and
+ *		     the devices below. After the minimal bus space has
+ *		     been allocated the remaining buses will be
+ *		     distributed equally between hotplug capable bridges.
+ * @pass: Either %0 (scan already configured bridges) or %1 (scan bridges
+ *        that need to be reconfigured.
+ *
  * If it's a bridge, configure it and scan the bus behind it.
  * For CardBus bridges, we don't scan behind as the devices will
  * be handled by the bridge driver itself.

@@ -969,7 +980,8 @@ static void pci_enable_crs(struct pci_dev *pdev)
  * them, we proceed to assigning numbers to the remaining buses in
  * order to avoid overlaps between old and new bus numbers.
  */
-int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
+int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev, int max,
+			   unsigned int available_buses, int pass)
 {
 	struct pci_bus *child;
 	int is_cardbus = (dev->hdr_type == PCI_HEADER_TYPE_CARDBUS);

@@ -1080,6 +1092,9 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
 						bus->busn_res.end);
 		}
 		max++;
+		if (available_buses)
+			available_buses--;
+
 		buses = (buses & 0xff000000)
 		      | ((unsigned int)(child->primary)     <<  0)
 		      | ((unsigned int)(child->busn_res.start)   <<  8)

@@ -1101,7 +1116,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
 
 		if (!is_cardbus) {
 			child->bridge_ctl = bctl;
-			max = pci_scan_child_bus(child);
+			max = pci_scan_child_bus_extend(child, available_buses);
 		} else {
 			/*
 			 * For CardBus bridges, we leave 4 bus numbers

@@ -1169,7 +1184,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
 
 	return max;
 }
-EXPORT_SYMBOL(pci_scan_bridge);
+EXPORT_SYMBOL(pci_scan_bridge_extend);
 
 /*
  * Read interrupt line and base address registers.

drivers/pci/probe.c +13 -2

+ * Scans devices below @bus including subordinate buses. Returns new
+ * subordinate number including all the found devices. Passing
+ * @available_buses causes the remaining bus space to be distributed
+ * equally between hotplug capable bridges to allow future extension of
+ * the hierarchy.
+ */
+unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
+				       unsigned int available_buses)
 {
-	unsigned int devfn, pass, max = bus->busn_res.start;
+	unsigned int used_buses, hotplug_bridges = 0;
+	unsigned int start = bus->busn_res.start;
+	unsigned int devfn, cmax, max = start;
 	struct pci_dev *dev;
 
 	dev_dbg(&bus->dev, "scanning bus\n");

@@ -2409,7 +2439,8 @@ unsigned int pci_scan_child_bus(struct pci_bus *bus)
 		pci_scan_slot(bus, devfn);
 
 	/* Reserve buses for SR-IOV capability. */
-	max += pci_iov_bus_range(bus);
+	used_buses = pci_iov_bus_range(bus);
+	max += used_buses;
 
 	/*
 	 * After performing arch-dependent fixup of the bus, look behind

drivers/pci/probe.c +4 -0

+	/*
+	 * First pass. Bridges that are already configured. We don't touch
+	 * these unless they are misconfigured (which we will do in second
+	 * pass).

drivers/pci/probe.c +28 -8

+			} else if (dev->is_hotplug_bridge) {
+				/*
+				 * Distribute the extra buses between
+				 * hotplug bridges if any.
+				 */
+				buses = available_buses / hotplug_bridges;
+				buses = min(buses, available_buses - used_buses);
+			}
+
+			cmax = max;
+			max = pci_scan_bridge_extend(bus, dev, cmax, buses, 1);
+			used_buses += max - cmax;
 		}
+	}
 
 	/*
 	 * Make sure a hotplug bridge has at least the minimum requested
-	 * number of buses.
+	 * number of buses but allow it to grow up to the maximum available
+	 * bus number of there is room.
 	 */
-	if (bus->self && bus->self->is_hotplug_bridge && pci_hotplug_bus_size) {
-		if (max - bus->busn_res.start < pci_hotplug_bus_size - 1)
-			max = bus->busn_res.start + pci_hotplug_bus_size - 1;
-
-		/* Do not allocate more buses than we have room left */
-		if (max > bus->busn_res.end)
-			max = bus->busn_res.end;
+	if (bus->self && bus->self->is_hotplug_bridge) {
+		used_buses = max_t(unsigned int, available_buses,
+				   pci_hotplug_bus_size - 1);
+		if (max - start < used_buses) {
+			max = start + used_buses;
+
+			/* Do not allocate more buses than we have room left */
+			if (max > bus->busn_res.end)
+				max = bus->busn_res.end;
+
+			dev_dbg(&bus->dev, "%pR extended by %#02x\n",
+				&bus->busn_res, max - start);
+		}
 	}
 
 	/*

drivers/pci/probe.c +0 -0

 /**
  * pcibios_root_bridge_prepare - Platform-specific host bridge setup.

include/linux/pci.h +16 -3

index 4397692be538..c9b34c2de0fb 100644
@@ -909,7 +909,14 @@ static inline void pci_dev_assign_slot(struct pci_dev *dev) { }
 int pci_scan_slot(struct pci_bus *bus, int devfn);
 struct pci_dev *pci_scan_single_device(struct pci_bus *bus, int devfn);
 void pci_device_add(struct pci_dev *dev, struct pci_bus *bus);
-unsigned int pci_scan_child_bus(struct pci_bus *bus);
+unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
+				       unsigned int available_buses);
+
+static inline unsigned int pci_scan_child_bus(struct pci_bus *bus)
+{
+	return pci_scan_child_bus_extend(bus, 0);
+}
+
 void pci_bus_add_device(struct pci_dev *dev);
 void pci_read_bridge_bases(struct pci_bus *child);
 struct resource *pci_find_parent_resource(const struct pci_dev *dev,

@@ -1292,8 +1299,14 @@ int pci_add_dynid(struct pci_driver *drv,
 		  unsigned long driver_data);
 const struct pci_device_id *pci_match_id(const struct pci_device_id *ids,
 					 struct pci_dev *dev);
-int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max,
-		    int pass);
+int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev, int max,
+			   unsigned int available_buses, int pass);
+
+static inline int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev,
+				  int max, int pass)
+{
+	return pci_scan_bridge_extend(bus, dev, max, 0, pass);
+}
 
 void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
 		  void *userdata);

Signature · 1 line

2.14.1

Sent: 12 October 2017 00:33
Why do you limit it to PCIe? Isn't it conceivable that one could hot-add a conventional PCI card that contained a bridge leading to another hotplug slot? E.g., a PCI card with PCMCIA slot or something on it?

Quoted 13 lines · Mika Westerberg

System BIOS sometimes allocates extra bus space for hotplug capable PCIe root/downstream ports. This space is needed if the device plugged to the port will have more hotplug capable downstream ports. A good example of this is Thunderbolt. Each Thunderbolt device contains a PCIe switch and one or more hotplug capable PCIe downstream ports where the daisy chain can be extended.

Currently Linux only allocates minimal bus space to make sure all the enumerated devices barely fit there. The BIOS reserved extra space is not taken into consideration at all. Because of this we run out of bus space pretty quickly when more PCIe devices are attached to hotplug downstream ports in order to extend the chain.

Quoted 4 lines

This modifies PCI core so that we distribute the available BIOS allocated bus space equally between hotplug capable PCIe downstream ports to make sure there is enough bus space for extending the hierarchy later on.

I think this makes sense in general. It's a fairly complicated patch, so my comments here are just a first pass.

Why do you limit it to PCIe? Isn't it conceivable that one could hot-add a conventional PCI card that contained a bridge leading to another hotplug slot? E.g., a PCI card with PCMCIA slot or something on it?

Quoted 103 lines

Signed-off-by Mika Westerberg <[email protected]>

---
 drivers/pci/hotplug-pci.c |  13 +++++-
 drivers/pci/probe.c       | 114 ++++++++++++++++++++++++++++++++++++++--------
 include/linux/pci.h       |  19 ++++++--
 3 files changed, 123 insertions(+), 23 deletions(-)

drivers/pci/hotplug-pci.c +12 -1

index c68366cee6b7..deb06fe22b26 100644
@@ -8,6 +8,7 @@ int pci_hp_add_bridge(struct pci_dev *dev)
 {
 	struct pci_bus *parent = dev->bus;
 	int pass, busnr, start = parent->busn_res.start;
+	unsigned int available_buses = 0;
 	int end = parent->busn_res.end;
 
 	for (busnr = start; busnr <= end; busnr++) {

@@ -19,8 +20,18 @@ int pci_hp_add_bridge(struct pci_dev *dev)
 				pci_name(dev));
 		return -1;
 	}
+
+	/*
+	 * In case of PCIe the hierarchy can be extended through hotplug
+	 * capable downstream ports. Distribute the available bus
+	 * numbers between them to make extending the chain possible.
+	 */
+	if (pci_is_pcie(dev))
+		available_buses = end - busnr;
+
 	for (pass = 0; pass < 2; pass++)
-		busnr = pci_scan_bridge(parent, dev, busnr, pass);
+		busnr = pci_scan_bridge_extend(parent, dev, busnr,
+					       available_buses, pass);
 	if (!dev->subordinate)
 		return -1;
 

drivers/pci/probe.c +18 -3

index f285cd74088e..ae0bf3c807f5 100644
@@ -960,6 +960,17 @@ static void pci_enable_crs(struct pci_dev *pdev)
 }
 
 /*
+ * pci_scan_bridge_extend() - Scan buses behind a bridge
+ * @bus: Parent bus the bridge is on
+ * @dev: Bridge itself
+ * @max: Starting subordinate number of buses behind this bridge
+ * @available_buses: Total number of buses available for this bridge and
+ *		     the devices below. After the minimal bus space has
+ *		     been allocated the remaining buses will be
+ *		     distributed equally between hotplug capable bridges.
+ * @pass: Either %0 (scan already configured bridges) or %1 (scan bridges
+ *        that need to be reconfigured.
+ *
  * If it's a bridge, configure it and scan the bus behind it.
  * For CardBus bridges, we don't scan behind as the devices will
  * be handled by the bridge driver itself.

@@ -969,7 +980,8 @@ static void pci_enable_crs(struct pci_dev *pdev)
  * them, we proceed to assigning numbers to the remaining buses in
  * order to avoid overlaps between old and new bus numbers.
  */
-int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
+int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev, int max,
+			   unsigned int available_buses, int pass)
 {
 	struct pci_bus *child;
 	int is_cardbus = (dev->hdr_type == PCI_HEADER_TYPE_CARDBUS);

@@ -1080,6 +1092,9 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
 						bus->busn_res.end);
 		}
 		max++;
+		if (available_buses)
+			available_buses--;
+
 		buses = (buses & 0xff000000)
 		      | ((unsigned int)(child->primary)     <<  0)
 		      | ((unsigned int)(child->busn_res.start)   <<  8)

@@ -1101,7 +1116,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
 
 		if (!is_cardbus) {
 			child->bridge_ctl = bctl;
-			max = pci_scan_child_bus(child);
+			max = pci_scan_child_bus_extend(child, available_buses);
 		} else {
 			/*
 			 * For CardBus bridges, we leave 4 bus numbers

@@ -1169,7 +1184,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
 
 	return max;
 }
-EXPORT_SYMBOL(pci_scan_bridge);
+EXPORT_SYMBOL(pci_scan_bridge_extend);
 
 /*
  * Read interrupt line and base address registers.

@@ -2397,9 +2412,24 @@ void __weak pcibios_fixup_bus(struct pci_bus *bus)
        /* nothing to do, expected to be removed in the future */
 }
 

drivers/pci/probe.c void __weak pcibios_fixup_bus(struct pci_bus *bus) · line 2419

Quoted 6 lines

drivers/pci/probe.c +5 -1

-unsigned int pci_scan_child_bus(struct pci_bus *bus)
+/**
+ * pci_scan_child_bus_extend() - Scan devices below a bus
+ * @bus: Bus to scan for devices
+ * @available_buses: Total number of buses available (%0 does not try to
+ *		     extend beyond the minimal)

What does "%0" mean? Is that kernel-doc for something? 0?

Quoted 30 lines

drivers/pci/probe.c +13 -2

+ * Scans devices below @bus including subordinate buses. Returns new
+ * subordinate number including all the found devices. Passing
+ * @available_buses causes the remaining bus space to be distributed
+ * equally between hotplug capable bridges to allow future extension of
+ * the hierarchy.
+ */
+unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
+				       unsigned int available_buses)
 {
-	unsigned int devfn, pass, max = bus->busn_res.start;
+	unsigned int used_buses, hotplug_bridges = 0;
+	unsigned int start = bus->busn_res.start;
+	unsigned int devfn, cmax, max = start;
 	struct pci_dev *dev;
 
 	dev_dbg(&bus->dev, "scanning bus\n");

@@ -2409,7 +2439,8 @@ unsigned int pci_scan_child_bus(struct pci_bus *bus)
 		pci_scan_slot(bus, devfn);
 
 	/* Reserve buses for SR-IOV capability. */
-	max += pci_iov_bus_range(bus);
+	used_buses = pci_iov_bus_range(bus);
+	max += used_buses;
 
 	/*
 	 * After performing arch-dependent fixup of the bus, look behind

@@ -2421,23 +2452,68 @@ unsigned int pci_scan_child_bus(struct pci_bus *bus)
 		bus->is_added = 1;
 	}
 

drivers/pci/probe.c unsigned int pci_scan_child_bus(struct pci_bus *bus) · line 2427

Quoted 4 lines

drivers/pci/probe.c +0 -4

-	for (pass = 0; pass < 2; pass++)
-		list_for_each_entry(dev, &bus->devices, bus_list) {
-			if (pci_is_bridge(dev))
-				max = pci_scan_bridge(bus, dev, max, pass);

Thanks for getting rid of this "for (pass = 0; ...)" loop. Much nicer to just have it spelled out. It might even be worth pulling this looping change into a separate patch to simplify *this* patch a little bit.

I'd be happy if you did the same for the similar loop in pci_hp_add_bridge().

Quoted 9 lines

drivers/pci/probe.c +9 -0

+	/*
+	 * First pass. Bridges that are already configured. We don't touch
+	 * these unless they are misconfigured (which we will do in second
+	 * pass).
+	 */
+	list_for_each_entry(dev, &bus->devices, bus_list) {
+		if (pci_is_bridge(dev)) {
+			/*
+			 * Calculate how many hotplug bridges there are on

drivers/pci/probe.c unsigned int pci_scan_child_bus(struct pci_bus *bus) · line 2468

Quoted 5 lines

drivers/pci/probe.c +5 -0

+			 * this bus. We will distribute the additional
+			 * available buses between them.
+			 */
+			if (dev->is_hotplug_bridge)
+				hotplug_bridges++;

Maybe pull this out into a third list traversal, since it's not related to the "scan bridge" functionality?

drivers/pci/probe.c unsigned int pci_scan_child_bus(struct pci_bus *bus) · line 2474

Quoted 6 lines

drivers/pci/probe.c +6 -0

+
+			cmax = max;
+			max = pci_scan_bridge_extend(bus, dev, max, 0, 0);
+			used_buses += cmax - max;
+		}
+	}

I'm trying to relate the "Bridges that are already configured" comment

to this code. I don't see any test here for whether the bridge is already configured. Oh, I see -- the last parameter (0) to pci_scan_bridge_extend() means this is the first pass, and it basically just checks for secondary and subordinate being set. I thought "first pass" was related to the list_for_each_entry() loop here, but it's actually related to the internals of pci_scan_bridge_extend(). I think maybe rewording the comment can address this.

Quoted 7 lines

drivers/pci/probe.c +7 -0

+	/* Second pass. Bridges that need to be configured. */
+	list_for_each_entry(dev, &bus->devices, bus_list) {
+		if (pci_is_bridge(dev)) {
+			unsigned int buses = 0;
+
+			if (pcie_upstream_port(dev)) {
+				/* Upstream port gets all available buses */

drivers/pci/probe.c unsigned int pci_scan_child_bus(struct pci_bus *bus) · line 2483

Quoted 1 line

drivers/pci/probe.c +1 -0

+				buses = available_buses;

I guess this relies on the assumption that there can only be one upstream port on a bus? Is that true? Typically a switch only has a function 0 upstream port, but something niggles at me like the spec admits the possibility of a switch with multiple functions of upstream ports? I don't know where that is right now (or if it exists), but I'll try to find it.

Quoted 45 lines

drivers/pci/probe.c +28 -8

+			} else if (dev->is_hotplug_bridge) {
+				/*
+				 * Distribute the extra buses between
+				 * hotplug bridges if any.
+				 */
+				buses = available_buses / hotplug_bridges;
+				buses = min(buses, available_buses - used_buses);
+			}
+
+			cmax = max;
+			max = pci_scan_bridge_extend(bus, dev, cmax, buses, 1);
+			used_buses += max - cmax;
 		}
+	}
 
 	/*
 	 * Make sure a hotplug bridge has at least the minimum requested
-	 * number of buses.
+	 * number of buses but allow it to grow up to the maximum available
+	 * bus number of there is room.
 	 */
-	if (bus->self && bus->self->is_hotplug_bridge && pci_hotplug_bus_size) {
-		if (max - bus->busn_res.start < pci_hotplug_bus_size - 1)
-			max = bus->busn_res.start + pci_hotplug_bus_size - 1;
-
-		/* Do not allocate more buses than we have room left */
-		if (max > bus->busn_res.end)
-			max = bus->busn_res.end;
+	if (bus->self && bus->self->is_hotplug_bridge) {
+		used_buses = max_t(unsigned int, available_buses,
+				   pci_hotplug_bus_size - 1);
+		if (max - start < used_buses) {
+			max = start + used_buses;
+
+			/* Do not allocate more buses than we have room left */
+			if (max > bus->busn_res.end)
+				max = bus->busn_res.end;
+
+			dev_dbg(&bus->dev, "%pR extended by %#02x\n",
+				&bus->busn_res, max - start);
+		}
 	}
 
 	/*

drivers/pci/probe.c unsigned int pci_scan_child_bus(struct pci_bus *bus) · line 2529

Quoted 5 lines

drivers/pci/probe.c +1 -1

@@ -2450,7 +2526,7 @@ unsigned int pci_scan_child_bus(struct pci_bus *bus)
 	dev_dbg(&bus->dev, "bus scan returning with max=%02x\n", max);
 	return max;
 }
-EXPORT_SYMBOL_GPL(pci_scan_child_bus);
+EXPORT_SYMBOL_GPL(pci_scan_child_bus_extend);

Does this need to be exported? The only callers I see are pci_scan_bridge_extend() (already in the same module) and pci_scan_child_bus() (now an inline in linux/pci.h).

I'd rather move pci_scan_child_bus() back to probe.c and make pci_scan_child_bus_extend() static.

Same with pci_scan_bridge_extend(), although that looks harder because it's called from hotplug-pci.c. Really, I'm not sure hotplug-pci.c deserves to exist. I think the whole file (one function) could be folded into pci/probe.c (as a separate patch all by itself).

Quoted 14 lines · Bjorn Helgaas

Quoted 13 lines · Mika Westerberg

System BIOS sometimes allocates extra bus space for hotplug capable PCIe root/downstream ports. This space is needed if the device plugged to the port will have more hotplug capable downstream ports. A good example of this is Thunderbolt. Each Thunderbolt device contains a PCIe switch and one or more hotplug capable PCIe downstream ports where the daisy chain can be extended.

Currently Linux only allocates minimal bus space to make sure all the enumerated devices barely fit there. The BIOS reserved extra space is not taken into consideration at all. Because of this we run out of bus space pretty quickly when more PCIe devices are attached to hotplug downstream ports in order to extend the chain.

Quoted 7 lines

Quoted 4 lines

This modifies PCI core so that we distribute the available BIOS allocated bus space equally between hotplug capable PCIe downstream ports to make sure there is enough bus space for extending the hierarchy later on.

I think this makes sense in general. It's a fairly complicated patch, so my comments here are just a first pass.

Thanks for the comments!

Quoted 4 lines

Why do you limit it to PCIe? Isn't it conceivable that one could hot-add a conventional PCI card that contained a bridge leading to another hotplug slot? E.g., a PCI card with PCMCIA slot or something on it?

I guess this could be generalized to such configurations but I wanted to restrict this with PCIe for a couple of reasons. Firstly, I'm able to actually test this ;-) Second, the rules in PCIe are quite simple, whenever you have hotplug bridge (downstream port with a hotplug capability set) you distribute the available bus space with it. With a conventional PCI it is not so clear (at least to me).

Quoted 7 lines

Quoted 7 lines

drivers/pci/probe.c +7 -0

+	/* Second pass. Bridges that need to be configured. */
+	list_for_each_entry(dev, &bus->devices, bus_list) {
+		if (pci_is_bridge(dev)) {
+			unsigned int buses = 0;
+
+			if (pcie_upstream_port(dev)) {
+				/* Upstream port gets all available buses */

Quoted 8 lines

drivers/pci/probe.c unsigned int pci_scan_child_bus(struct pci_bus *bus) · line 2483

Quoted 1 line

drivers/pci/probe.c +1 -0

+				buses = available_buses;

I guess this relies on the assumption that there can only be one upstream port on a bus? Is that true? Typically a switch only has a function 0 upstream port, but something niggles at me like the spec admits the possibility of a switch with multiple functions of upstream ports? I don't know where that is right now (or if it exists), but I'll try to find it.

My understanding is that there can be only one upstream port on a bus. That said I looked at the spec again and there is this in chapter 7.3.1 of PCIe 3.1 spec:

Switches, and components wishing to incorporate more than eight Functions at their Upstream Port, are permitted to implement one or more “virtual switches” represented by multiple Type 1 (PCI-PCI Bridge) Configuration Space headers as illustrated in Figure 7-2. These virtual switches serve to allow fan-out beyond eight Functions. Since Switch Downstream Ports are permitted to appear on any Device Number, in this case all address information fields (Bus, Device, and Function Numbers) must be completely decoded to access the correct register. Any Configuration Request targeting an unimplemented Bus, Device, or Function must return a Completion with Unsupported Request Completion Status.

Not sure what it actually means, though. A "virtual switch" to me says it is a switch with one upstream port and multiple downstream ports, just like normal switch. Is this what you meant? Do you understand it so that there can be multiple upstream ports connected to a bus?

Quoted 30 lines · Mika Westerberg

Quoted 14 lines · Bjorn Helgaas

Quoted 13 lines · Mika Westerberg

System BIOS sometimes allocates extra bus space for hotplug capable PCIe root/downstream ports. This space is needed if the device plugged to the port will have more hotplug capable downstream ports. A good example of this is Thunderbolt. Each Thunderbolt device contains a PCIe switch and one or more hotplug capable PCIe downstream ports where the daisy chain can be extended.

Currently Linux only allocates minimal bus space to make sure all the enumerated devices barely fit there. The BIOS reserved extra space is not taken into consideration at all. Because of this we run out of bus space pretty quickly when more PCIe devices are attached to hotplug downstream ports in order to extend the chain.

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This modifies PCI core so that we distribute the available BIOS allocated bus space equally between hotplug capable PCIe downstream ports to make sure there is enough bus space for extending the hierarchy later on.

I think this makes sense in general. It's a fairly complicated patch, so my comments here are just a first pass.

Thanks for the comments!

Quoted 4 lines

Why do you limit it to PCIe? Isn't it conceivable that one could hot-add a conventional PCI card that contained a bridge leading to another hotplug slot? E.g., a PCI card with PCMCIA slot or something on it?

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I guess this could be generalized to such configurations but I wanted to restrict this with PCIe for a couple of reasons. Firstly, I'm able to actually test this ;-) Second, the rules in PCIe are quite simple, whenever you have hotplug bridge (downstream port with a hotplug capability set) you distribute the available bus space with it. With a conventional PCI it is not so clear (at least to me).

You're testing dev->is_hotplug_bridge, which I think is the right approach. It happens that we currently only set that for PCIe bridges with PCI_EXP_SLTCAP_HPC and for some ACPI cases (and a quirk for one device). But in principle we could and probably should set it if we can identify a conventional PCI hotplug bridge. So my suggestion is to just remove the explicit PCIe tests.

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drivers/pci/probe.c +7 -0

+	/* Second pass. Bridges that need to be configured. */
+	list_for_each_entry(dev, &bus->devices, bus_list) {
+		if (pci_is_bridge(dev)) {
+			unsigned int buses = 0;
+
+			if (pcie_upstream_port(dev)) {
+				/* Upstream port gets all available buses */

Quoted 8 lines

drivers/pci/probe.c unsigned int pci_scan_child_bus(struct pci_bus *bus) · line 2483

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drivers/pci/probe.c +1 -0

+				buses = available_buses;

I guess this relies on the assumption that there can only be one upstream port on a bus? Is that true? Typically a switch only has a function 0 upstream port, but something niggles at me like the spec admits the possibility of a switch with multiple functions of upstream ports? I don't know where that is right now (or if it exists), but I'll try to find it.

My understanding is that there can be only one upstream port on a bus. That said I looked at the spec again and there is this in chapter 7.3.1 of PCIe 3.1 spec:

Switches, and components wishing to incorporate more than eight Functions at their Upstream Port, are permitted to implement one or more “virtual switches” represented by multiple Type 1 (PCI-PCI Bridge) Configuration Space headers as illustrated in Figure 7-2. These virtual switches serve to allow fan-out beyond eight Functions. Since Switch Downstream Ports are permitted to appear on any Device Number, in this case all address information fields (Bus, Device, and Function Numbers) must be completely decoded to access the correct register. Any Configuration Request targeting an unimplemented Bus, Device, or Function must return a Completion with Unsupported Request Completion Status.

Quoted 4 lines

Not sure what it actually means, though. A "virtual switch" to me says it is a switch with one upstream port and multiple downstream ports, just like normal switch. Is this what you meant? Do you understand it so that there can be multiple upstream ports connected to a bus?

I agree with you; I think that section is just saying that if a component needs more then eight functions, it can incorporate a switch, so it could have one upstream port, one internal logical bus, up to 32 * 8 = 256 downstream ports on that logical bus, and 8 endpoints below each downstream port. Of course, there wouldn't be enough bus number space for all that. But I don't think this is talking about a multifunction switch upstream port.

Anyway, I think you're right that there can only be one upstream port on a bus, because an upstream port contains link management stuff, and it wouldn't make sense to have two upstream ports trying to manage the same end of a single link.

But I would really like to remove the PCIe-specific nature of this test somehow so it could work on a conventional PCI topology.

Hi,

Currently when plugging PCIe device using native PCIe hotplug Linux PCI core tries to allocate bus space and resources so that the newly enumerated topology barely fits there. Now, if the PCIe topology that was just plugged in has more PCIe hotplug ports we will run out of bus space and resources pretty quickly. There is a workaround for this by passing pci=hpbussize=N in the kernel command line but it runs to the same situation after next hotplug.