struct pci_device_id
===================

The ``struct pci_device_id`` is a fundamental structure in Linux kernel programming, particularly for PCI device drivers. Its primary purpose is to hold PCI device identifiers, which are used to match the driver with specific PCI devices. This structure is essential for the kernel to determine which devices a particular driver can handle.

Key Points
----------

- **Purpose**:

  The ``struct pci_device_id`` is used to define a table of PCI device IDs that the driver supports. This table is used by the kernel to match the driver with the appropriate PCI devices.

- **Fields**:

  - ``vendor``: The vendor ID of the PCI device.
  - ``device``: The device ID of the PCI device.
  - ``subvendor``, ``subdevice``: Subsystem vendor and device IDs (optional).
  - ``class``, ``class_mask``: Class code and mask for class-based matching (optional).
  - ``driver_data``: Private data for the driver (optional).

- **Usage**:

  - The ``pci_device_id`` structure is typically used in conjunction with the ``MODULE_DEVICE_TABLE`` macro, which exports the device ID table to user space, allowing tools like ``modprobe`` to load the appropriate driver for a given device.
  - The ``id_table`` field in the ``pci_driver`` structure points to this table, enabling the kernel to match the driver with the correct devices.

- **Example**:

  .. code-block:: c

     static struct pci_device_id pci_watcher_ids[] = {
         { PCI_DEVICE(VENDOR_ID, DEVICE_ID) },
         {0, }
     };

  - In this example, ``pci_watcher_ids`` is an array of ``pci_device_id`` structures. The ``PCI_DEVICE`` macro is used to define a specific vendor and device ID pair. The array is terminated with an all-zero entry, which is a common practice to mark the end of the table.

- **Macros**:

  - ``PCI_DEVICE(vendor, device)``: A macro that simplifies the creation of a ``pci_device_id`` entry by specifying the vendor and device IDs.
  - ``MODULE_DEVICE_TABLE(pci, pci_watcher_ids)``: This macro exports the device ID table to user space, allowing the kernel to associate the driver with the specified PCI devices.

Summary
-------

The ``struct pci_device_id`` is a crucial part of PCI device driver development. It allows the driver to declare which PCI devices it supports, enabling the kernel to match the driver with the appropriate hardware. The ``pci_watcher_ids`` array in the provided code is an example of how this structure is used to define a list of supported devices.