[PATCH v3] platform/x86: p2sb: Allow p2sb_bar() calls during PCI device probe

Shin'ichiro Kawasaki
2023-12-25T09:26:59Z

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.

Link: https://lore.kernel.org/linux-pci/6xb24fjmptxxn5js2fjrrddjae6twex5bjaftwqsuawuqqqydx@7cl3uik5ef6j/
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
---
This patch reflects discussions held at the Link tag. I confirmed this patch
fixes the problem using a system with i2c_i801 device, building i2c_i801
module as both built-in and loadable. Reviews will be appreicated.

Changes from v2:
* Improved p2sb_scan_and_cache() and p2sb_scan_and_cache_devfn()
* Reflected other review comments by Andy

Changes from v1:
* Reflected review comments by Andy
* Removed RFC prefix

Changes from RFC v2:
* Reflected review comments on the list

Changes from RFC v1:
* Fixed a build warning poitned out in llvm list by kernel test robot

drivers/platform/x86/p2sb.c | 166 +++++++++++++++++++++++++++---------
1 file changed, 124 insertions(+), 42 deletions(-)

diff --git a/drivers/platform/x86/p2sb.c b/drivers/platform/x86/p2sb.c
index 1cf2471d54dd..2cbfe8b38e14 100644
--- a/drivers/platform/x86/p2sb.c
+++ b/drivers/platform/x86/p2sb.c
@@ -26,6 +26,19 @@ static const struct x86_cpu_id p2sb_cpu_ids[] = {
{}
};

+/*
+ * Cache BAR0 of P2SB device functions 0 to 7.
+ * TODO: Move this definition to pci.h together with same other definitions.
+ */
+#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;
@@ -39,10 +52,15 @@ static int p2sb_get_devfn(unsigned int *devfn)
return 0;
}

+static int p2sb_valid_resource(struct resource *res)
+{
+ return res->flags ? 0 : -ENOENT;
+}
+
/* 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];
+ struct resource *bar0 = pci_resource_n(pdev, 0);

/* Make sure we have no dangling pointers in the output */
memset(mem, 0, sizeof(*mem));
@@ -56,47 +74,61 @@ static int p2sb_read_bar0(struct pci_dev *pdev, struct resource *mem)
mem->end = bar0->end;
mem->flags = bar0->flags;
mem->desc = bar0->desc;
-
- return 0;
}

-static int p2sb_scan_and_read(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
+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;
- int ret;

pdev = pci_scan_single_device(bus, devfn);
if (!pdev)
- return -ENODEV;
+ return;

- ret = p2sb_read_bar0(pdev, mem);
+ p2sb_read_bar0(pdev, &cache->res);
+ cache->bus_dev_id = bus->dev.id;

pci_stop_and_remove_bus_device(pdev);
- return ret;
+ return;
}

-/**
- * p2sb_bar - Get Primary to Sideband (P2SB) bridge device BAR
- * @bus: PCI bus to communicate with
- * @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 @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)
+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);
+ }
+
+ return p2sb_valid_resource(&p2sb_resources[PCI_FUNC(devfn)].res);
+}
+
+static struct pci_bus *p2sb_get_bus(struct pci_bus *bus)
+{
+ 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)
{
- struct pci_dev *pdev_p2sb;
+ struct pci_bus *bus;
unsigned int devfn_p2sb;
u32 value = P2SBC_HIDE;
int ret;
@@ -106,8 +138,9 @@ int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
if (ret)
return ret;

- /* if @bus is NULL, use bus 0 in domain 0 */
- bus = bus ?: pci_find_bus(0, 0);
+ bus = p2sb_get_bus(NULL);
+ if (!bus)
+ return -ENODEV;

/*
* Prevent concurrent PCI bus scan from seeing the P2SB device and
@@ -115,17 +148,16 @@ int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
*/
pci_lock_rescan_remove();

- /* Unhide the P2SB device, if needed */
+ /*
+ * 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);

- 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);
+ ret = p2sb_scan_and_cache(bus, devfn_p2sb);

/* Hide the P2SB device, if it was hidden */
if (value & P2SBC_HIDE)
@@ -133,12 +165,62 @@ int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)

pci_unlock_rescan_remove();

- if (ret)
- return ret;
+ return ret;
+}

- if (mem->flags == 0)
+/**
+ * p2sb_bar - Get Primary to Sideband (P2SB) bridge device BAR
+ * @bus: PCI bus to communicate with
+ * @devfn: PCI slot and function to communicate with
+ * @mem: memory resource to be filled in
+ *
+ * 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.
+ *
+ * Return:
+ * 0 on success or appropriate errno value on error.
+ */
+int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
+{
+ struct p2sb_res_cache *cache;
+ int ret;
+
+ 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;
+
+ ret = p2sb_valid_resource(&cache->res);
+ if (ret)
+ return ret;
+
+ 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 avoids access to unhidden P2SB devices, but it causes
+ * deadlock with sysfs pci bus rescan. To avoid the deadlock, access to P2SB
+ * devices 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);
--
2.43.0