linux-kernel - [PATCH v2 2/3] x86/quirks: Reintroduce scanning of secondary buses

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Message-Id: <f0daa70dac1a9b2483abdb31887173eb6ab77bdf.1465690253.git.lukas@wunner.de>
Date:	Sun, 12 Jun 2016 12:31:53 +0200
From:	Lukas Wunner <lukas@...ner.de>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Yinghai Lu <yinghai@...nel.org>
Subject: [PATCH v2 2/3] x86/quirks: Reintroduce scanning of secondary buses

We used to scan secondary buses until commit 8659c406ade3 ("x86: only
scan the root bus in early PCI quirks") constrained early quirks to the
root bus only. Its motivation was to prevent application of the
nvidia_bugs quirk on secondary buses.

We're about to add a quirk to reset the Broadcom 4331 wireless card on
2011/2012 Macs, which is located on a secondary bus behind a PCIe root
port. To facilitate that, reintroduce scanning of secondary buses.

The commit message of 8659c406ade3 notes that scanning only the root bus
"saves quite some unnecessary scanning work". The algorithm used prior
to 8659c406ade3 was particularly time consuming because it scanned
buses 0 to 31 brute force. To avoid lengthening boot time, employ a
recursive strategy which only scans buses that are actually reachable
from the root bus.

Yinghai Lu pointed out that the secondary bus number read from a
bridge's config space may be invalid, in particular a value of 0 would
cause an infinite loop. The PCI core goes beyond that and recurses to a
child bus only if its bus number is greater than the parent bus number
(see pci_scan_bridge()). Since the root bus is numbered 0, this implies
that secondary buses may not be 0. Do the same on early scanning.

If this algorithm is found to significantly impact boot time or cause
infinite loops on broken hardware, it would be possible to limit its
recursion depth: The Broadcom 4331 quirk applies at depth 1, all others
at depth 0, so the bus need not be scanned deeper than that for now. An
alternative approach would be to revert to scanning only the root bus,
and apply the Broadcom 4331 quirk to the root ports 8086:1c12, 8086:1e12
and 8086:1e16. Apple always positioned the card behind either of these
three ports. The quirk would then check presence of the card in slot 0
below the root port and do its deed.

Cc: Yinghai Lu <yinghai@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-pci@...r.kernel.org
Signed-off-by: Lukas Wunner <lukas@...ner.de>
---
v2:
  - Validate secondary bus number (Yinghai Lu)

  - Move scanning of secondary buses to separate patch to allow for
    easier reviewing, bisecting and reverting (Bjorn Helgaas)

 arch/x86/kernel/early-quirks.c | 34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/early-quirks.c b/arch/x86/kernel/early-quirks.c
index 256976f..ea60c05 100644
--- a/arch/x86/kernel/early-quirks.c
+++ b/arch/x86/kernel/early-quirks.c
@@ -610,12 +610,6 @@ struct chipset {
 	void (*f)(int num, int slot, int func);
 };
 
-/*
- * Only works for devices on the root bus. If you add any devices
- * not on bus 0 readd another loop level in early_quirks(). But
- * be careful because at least the Nvidia quirk here relies on
- * only matching on bus 0.
- */
 static struct chipset early_qrk[] __initdata = {
 	{ PCI_VENDOR_ID_NVIDIA, PCI_ANY_ID,
 	  PCI_CLASS_BRIDGE_PCI, PCI_ANY_ID, QFLAG_APPLY_ONCE, nvidia_bugs },
@@ -648,6 +642,8 @@ static struct chipset early_qrk[] __initdata = {
 	{}
 };
 
+static void __init early_pci_scan_bus(int bus);
+
 /**
  * check_dev_quirk - apply early quirks to a given PCI device
  * @num: bus number
@@ -656,7 +652,7 @@ static struct chipset early_qrk[] __initdata = {
  *
  * Check the vendor & device ID against the early quirks table.
  *
- * If the device is single function, let early_quirks() know so we don't
+ * If the device is single function, let early_pci_scan_bus() know so we don't
  * poke at this device again.
  */
 static int __init check_dev_quirk(int num, int slot, int func)
@@ -665,6 +661,7 @@ static int __init check_dev_quirk(int num, int slot, int func)
 	u16 vendor;
 	u16 device;
 	u8 type;
+	u8 sec;
 	int i;
 
 	class = read_pci_config_16(num, slot, func, PCI_CLASS_DEVICE);
@@ -692,25 +689,36 @@ static int __init check_dev_quirk(int num, int slot, int func)
 
 	type = read_pci_config_byte(num, slot, func,
 				    PCI_HEADER_TYPE);
+
+	if ((type & 0x7f) == PCI_HEADER_TYPE_BRIDGE) {
+		sec = read_pci_config_byte(num, slot, func, PCI_SECONDARY_BUS);
+		if (sec > num)
+			early_pci_scan_bus(sec);
+	}
+
 	if (!(type & 0x80))
 		return -1;
 
 	return 0;
 }
 
-void __init early_quirks(void)
+static void __init early_pci_scan_bus(int bus)
 {
 	int slot, func;
 
-	if (!early_pci_allowed())
-		return;
-
 	/* Poor man's PCI discovery */
-	/* Only scan the root bus */
 	for (slot = 0; slot < 32; slot++)
 		for (func = 0; func < 8; func++) {
 			/* Only probe function 0 on single fn devices */
-			if (check_dev_quirk(0, slot, func))
+			if (check_dev_quirk(bus, slot, func))
 				break;
 		}
 }
+
+void __init early_quirks(void)
+{
+	if (!early_pci_allowed())
+		return;
+
+	early_pci_scan_bus(0);
+}
-- 
2.8.1