PCI: Introduce pci_walk_bus_reverse(), for_each_pci_dev_reverse()

PCI/TSM, the PCI core functionality for the PCIe TEE Device Interface
Security Protocol (TDISP), has a need to walk all subordinate functions of
a Device Security Manager (DSM) to setup a device security context. A DSM
is physical function 0 of multi-function or SR-IOV device endpoint, or it
is an upstream switch port.

In error scenarios or when a TEE Security Manager (TSM) device is removed
it needs to unwind all established DSM contexts.

Introduce reverse versions of PCI device iteration helpers to mirror the
setup path and ensure that dependent children are handled before parents.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20251031212902.2256310-4-dan.j.williams@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/drivers/base/bus.c b/drivers/base/bus.c
index 5e75e1bce551..d19dae8f9d1b 100644
--- a/drivers/base/bus.c
+++ b/drivers/base/bus.c
@@ -334,6 +334,19 @@ static struct device *next_device(struct klist_iter *i)
 	return dev;
 }
 
+static struct device *prev_device(struct klist_iter *i)
+{
+	struct klist_node *n = klist_prev(i);
+	struct device *dev = NULL;
+	struct device_private *dev_prv;
+
+	if (n) {
+		dev_prv = to_device_private_bus(n);
+		dev = dev_prv->device;
+	}
+	return dev;
+}
+
 /**
  * bus_for_each_dev - device iterator.
  * @bus: bus type.
@@ -414,6 +427,31 @@ struct device *bus_find_device(const struct bus_type *bus,
 }
 EXPORT_SYMBOL_GPL(bus_find_device);
 
+struct device *bus_find_device_reverse(const struct bus_type *bus,
+				       struct device *start, const void *data,
+				       device_match_t match)
+{
+	struct subsys_private *sp = bus_to_subsys(bus);
+	struct klist_iter i;
+	struct device *dev;
+
+	if (!sp)
+		return NULL;
+
+	klist_iter_init_node(&sp->klist_devices, &i,
+			     (start ? &start->p->knode_bus : NULL));
+	while ((dev = prev_device(&i))) {
+		if (match(dev, data)) {
+			get_device(dev);
+			break;
+		}
+	}
+	klist_iter_exit(&i);
+	subsys_put(sp);
+	return dev;
+}
+EXPORT_SYMBOL_GPL(bus_find_device_reverse);
+
 static struct device_driver *next_driver(struct klist_iter *i)
 {
 	struct klist_node *n = klist_next(i);