platform/x86: p2sb: Allow p2sb_bar() calls during PCI device probe

p2sb_bar() unhides P2SB device to get resources from the device. It
guards the operation by locking pci_rescan_remove_lock so that parallel
rescans do not find the P2SB device. However, this lock causes deadlock
when PCI bus rescan is triggered by /sys/bus/pci/rescan. The rescan
locks pci_rescan_remove_lock and probes PCI devices. When PCI devices
call p2sb_bar() during probe, it locks pci_rescan_remove_lock again.
Hence the deadlock.

To avoid the deadlock, do not lock pci_rescan_remove_lock in p2sb_bar().
Instead, do the lock at fs_initcall. Introduce p2sb_cache_resources()
for fs_initcall which gets and caches the P2SB resources. At p2sb_bar(),
refer the cache and return to the caller.

Before operating the device at P2SB DEVFN for resource cache, check
that its device class is PCI_CLASS_MEMORY_OTHER 0x0580 that PCH
specifications define. This avoids unexpected operation to other devices
at the same DEVFN.

Link: https://lore.kernel.org/linux-pci/6xb24fjmptxxn5js2fjrrddjae6twex5bjaftwqsuawuqqqydx@7cl3uik5ef6j/
Fixes: 9745fb07474f ("platform/x86/intel: Add Primary to Sideband (P2SB) bridge support")
Cc: [e-mail]
Suggested-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Shin'ichiro Kawasaki <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Ilpo Järvinen <[e-mail]>
Tested-by Klara Modin <[e-mail]>
Reviewed-by: Hans de Goede <[e-mail]>
Signed-off-by: Hans de Goede <[e-mail]>

drivers/platform/x86/p2sb.c

···
 	{}
 };
 
+/*
+ * Cache BAR0 of P2SB device functions 0 to 7.
+ * TODO: The constant 8 is the number of functions that PCI specification
+ *       defines. Same definitions exist tree-wide. Unify this definition and
+ *       the other definitions then move to include/uapi/linux/pci.h.
+ */
+#define NR_P2SB_RES_CACHE 8
+
+struct p2sb_res_cache {
+	u32 bus_dev_id;
+	struct resource res;
+};
+
+static struct p2sb_res_cache p2sb_resources[NR_P2SB_RES_CACHE];
+
 static int p2sb_get_devfn(unsigned int *devfn)
 {
 	unsigned int fn = P2SB_DEVFN_DEFAULT;
···
 	return 0;
 }
 
+static bool p2sb_valid_resource(struct resource *res)
+{
+	if (res->flags)
+		return true;
+
+	return false;
+}
+
 /* Copy resource from the first BAR of the device in question */
-static int p2sb_read_bar0(struct pci_dev *pdev, struct resource *mem)
+static void p2sb_read_bar0(struct pci_dev *pdev, struct resource *mem)
 {
 	struct resource *bar0 = &pdev->resource[0];
 
···
 	mem->end = bar0->end;
 	mem->flags = bar0->flags;
 	mem->desc = bar0->desc;
+}
+
+static void p2sb_scan_and_cache_devfn(struct pci_bus *bus, unsigned int devfn)
+{
+	struct p2sb_res_cache *cache = &p2sb_resources[PCI_FUNC(devfn)];
+	struct pci_dev *pdev;
+
+	pdev = pci_scan_single_device(bus, devfn);
+	if (!pdev)
+		return;
+
+	p2sb_read_bar0(pdev, &cache->res);
+	cache->bus_dev_id = bus->dev.id;
+
+	pci_stop_and_remove_bus_device(pdev);
+}
+
+static int p2sb_scan_and_cache(struct pci_bus *bus, unsigned int devfn)
+{
+	unsigned int slot, fn;
+
+	if (PCI_FUNC(devfn) == 0) {
+		/*
+		 * When function number of the P2SB device is zero, scan it and
+		 * other function numbers, and if devices are available, cache
+		 * their BAR0s.
+		 */
+		slot = PCI_SLOT(devfn);
+		for (fn = 0; fn < NR_P2SB_RES_CACHE; fn++)
+			p2sb_scan_and_cache_devfn(bus, PCI_DEVFN(slot, fn));
+	} else {
+		/* Scan the P2SB device and cache its BAR0 */
+		p2sb_scan_and_cache_devfn(bus, devfn);
+	}
+
+	if (!p2sb_valid_resource(&p2sb_resources[PCI_FUNC(devfn)].res))
+		return -ENOENT;
 
 	return 0;
 }
 
-static int p2sb_scan_and_read(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
+static struct pci_bus *p2sb_get_bus(struct pci_bus *bus)
 {
-	struct pci_dev *pdev;
+	static struct pci_bus *p2sb_bus;
+
+	bus = bus ?: p2sb_bus;
+	if (bus)
+		return bus;
+
+	/* Assume P2SB is on the bus 0 in domain 0 */
+	p2sb_bus = pci_find_bus(0, 0);
+	return p2sb_bus;
+}
+
+static int p2sb_cache_resources(void)
+{
+	unsigned int devfn_p2sb;
+	u32 value = P2SBC_HIDE;
+	struct pci_bus *bus;
+	u16 class;
 	int ret;
 
-	pdev = pci_scan_single_device(bus, devfn);
-	if (!pdev)
+	/* Get devfn for P2SB device itself */
+	ret = p2sb_get_devfn(&devfn_p2sb);
+	if (ret)
+		return ret;
+
+	bus = p2sb_get_bus(NULL);
+	if (!bus)
 		return -ENODEV;
 
-	ret = p2sb_read_bar0(pdev, mem);
+	/*
+	 * When a device with same devfn exists and its device class is not
+	 * PCI_CLASS_MEMORY_OTHER for P2SB, do not touch it.
+	 */
+	pci_bus_read_config_word(bus, devfn_p2sb, PCI_CLASS_DEVICE, &class);
+	if (!PCI_POSSIBLE_ERROR(class) && class != PCI_CLASS_MEMORY_OTHER)
+		return -ENODEV;
 
-	pci_stop_and_remove_bus_device(pdev);
+	/*
+	 * Prevent concurrent PCI bus scan from seeing the P2SB device and
+	 * removing via sysfs while it is temporarily exposed.
+	 */
+	pci_lock_rescan_remove();
+
+	/*
+	 * The BIOS prevents the P2SB device from being enumerated by the PCI
+	 * subsystem, so we need to unhide and hide it back to lookup the BAR.
+	 * Unhide the P2SB device here, if needed.
+	 */
+	pci_bus_read_config_dword(bus, devfn_p2sb, P2SBC, &value);
+	if (value & P2SBC_HIDE)
+		pci_bus_write_config_dword(bus, devfn_p2sb, P2SBC, 0);
+
+	ret = p2sb_scan_and_cache(bus, devfn_p2sb);
+
+	/* Hide the P2SB device, if it was hidden */
+	if (value & P2SBC_HIDE)
+		pci_bus_write_config_dword(bus, devfn_p2sb, P2SBC, P2SBC_HIDE);
+
+	pci_unlock_rescan_remove();
+
 	return ret;
 }
 
···
  * @devfn: PCI slot and function to communicate with
  * @mem: memory resource to be filled in
  *
- * The BIOS prevents the P2SB device from being enumerated by the PCI
- * subsystem, so we need to unhide and hide it back to lookup the BAR.
- *
- * if @bus is NULL, the bus 0 in domain 0 will be used.
+ * If @bus is NULL, the bus 0 in domain 0 will be used.
  * If @devfn is 0, it will be replaced by devfn of the P2SB device.
  *
  * Caller must provide a valid pointer to @mem.
- *
- * Locking is handled by pci_rescan_remove_lock mutex.
  *
  * Return:
  * 0 on success or appropriate errno value on error.
  */
 int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
 {
-	struct pci_dev *pdev_p2sb;
-	unsigned int devfn_p2sb;
-	u32 value = P2SBC_HIDE;
+	struct p2sb_res_cache *cache;
 	int ret;
 
-	/* Get devfn for P2SB device itself */
-	ret = p2sb_get_devfn(&devfn_p2sb);
-	if (ret)
-		return ret;
-
-	/* if @bus is NULL, use bus 0 in domain 0 */
-	bus = bus ?: pci_find_bus(0, 0);
-
-	/*
-	 * Prevent concurrent PCI bus scan from seeing the P2SB device and
-	 * removing via sysfs while it is temporarily exposed.
-	 */
-	pci_lock_rescan_remove();
-
-	/* Unhide the P2SB device, if needed */
-	pci_bus_read_config_dword(bus, devfn_p2sb, P2SBC, &value);
-	if (value & P2SBC_HIDE)
-		pci_bus_write_config_dword(bus, devfn_p2sb, P2SBC, 0);
-
-	pdev_p2sb = pci_scan_single_device(bus, devfn_p2sb);
-	if (devfn)
-		ret = p2sb_scan_and_read(bus, devfn, mem);
-	else
-		ret = p2sb_read_bar0(pdev_p2sb, mem);
-	pci_stop_and_remove_bus_device(pdev_p2sb);
-
-	/* Hide the P2SB device, if it was hidden */
-	if (value & P2SBC_HIDE)
-		pci_bus_write_config_dword(bus, devfn_p2sb, P2SBC, P2SBC_HIDE);
-
-	pci_unlock_rescan_remove();
-
-	if (ret)
-		return ret;
-
-	if (mem->flags == 0)
+	bus = p2sb_get_bus(bus);
+	if (!bus)
 		return -ENODEV;
 
+	if (!devfn) {
+		ret = p2sb_get_devfn(&devfn);
+		if (ret)
+			return ret;
+	}
+
+	cache = &p2sb_resources[PCI_FUNC(devfn)];
+	if (cache->bus_dev_id != bus->dev.id)
+		return -ENODEV;
+
+	if (!p2sb_valid_resource(&cache->res))
+		return -ENOENT;
+
+	memcpy(mem, &cache->res, sizeof(*mem));
 	return 0;
 }
 EXPORT_SYMBOL_GPL(p2sb_bar);
+
+static int __init p2sb_fs_init(void)
+{
+	p2sb_cache_resources();
+	return 0;
+}
+
+/*
+ * pci_rescan_remove_lock to avoid access to unhidden P2SB devices can
+ * not be locked in sysfs pci bus rescan path because of deadlock. To
+ * avoid the deadlock, access to P2SB devices with the lock at an early
+ * step in kernel initialization and cache required resources. This
+ * should happen after subsys_initcall which initializes PCI subsystem
+ * and before device_initcall which requires P2SB resources.
+ */
+fs_initcall(p2sb_fs_init);