PCI: Add pci_get_base_class() helper

There is no function to get all PCI devices in a system by matching
against the base class code only, ignoring the sub-class code and
the programming interface.  Add pci_get_base_class() to suit the
need.

For example, if a driver wants to process all PCI display devices in
a system, it can do so like this:

  pdev = NULL;
  while ((pdev = pci_get_base_class(PCI_BASE_CLASS_DISPLAY, pdev))) {
    do_something_for_pci_display_device(pdev);
  }

Link: https://lore.kernel.org/r/20230825062714.6325-2-sui.jingfeng@linux.devSigned-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
[bhelgaas: reword commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

drivers/pci/search.c

@@ -363,6 +363,37 @@ struct pci_dev *pci_get_class(unsigned int class, struct pci_dev *from)
 }
 EXPORT_SYMBOL(pci_get_class);
 
+/**
+ * pci_get_base_class - searching for a PCI device by matching against the base class code only
+ * @class: search for a PCI device with this base class code
+ * @from: Previous PCI device found in search, or %NULL for new search.
+ *
+ * Iterates through the list of known PCI devices. If a PCI device is found
+ * with a matching base class code, the reference count to the device is
+ * incremented. See pci_match_one_device() to figure out how does this works.
+ * A new search is initiated by passing %NULL as the @from argument.
+ * Otherwise if @from is not %NULL, searches continue from next device on the
+ * global list. The reference count for @from is always decremented if it is
+ * not %NULL.
+ *
+ * Returns:
+ * A pointer to a matched PCI device, %NULL Otherwise.
+ */
+struct pci_dev *pci_get_base_class(unsigned int class, struct pci_dev *from)
+{
+	struct pci_device_id id = {
+		.vendor = PCI_ANY_ID,
+		.device = PCI_ANY_ID,
+		.subvendor = PCI_ANY_ID,
+		.subdevice = PCI_ANY_ID,
+		.class_mask = 0xFF0000,
+		.class = class << 16,
+	};
+
+	return pci_get_dev_by_id(&id, from);
+}
+EXPORT_SYMBOL(pci_get_base_class);
+
 /**
  * pci_dev_present - Returns 1 if device matching the device list is present, 0 if not.
  * @ids: A pointer to a null terminated list of struct pci_device_id structures

include/linux/pci.h

@@ -1181,6 +1181,8 @@ struct pci_dev *pci_get_slot(struct pci_bus *bus, unsigned int devfn);
 struct pci_dev *pci_get_domain_bus_and_slot(int domain, unsigned int bus,
 					    unsigned int devfn);
 struct pci_dev *pci_get_class(unsigned int class, struct pci_dev *from);
+struct pci_dev *pci_get_base_class(unsigned int class, struct pci_dev *from);
+
 int pci_dev_present(const struct pci_device_id *ids);
 
 int pci_bus_read_config_byte(struct pci_bus *bus, unsigned int devfn,
@@ -1924,6 +1926,9 @@ static inline struct pci_dev *pci_get_class(unsigned int class,
 					    struct pci_dev *from)
 { return NULL; }
 
+static inline struct pci_dev *pci_get_base_class(unsigned int class,
+						 struct pci_dev *from)
+{ return NULL; }
 
 static inline int pci_dev_present(const struct pci_device_id *ids)
 { return 0; }