PCI: Improve pci_msi_supported() comments

 * pci_msi_supported - check whether MSI may be enabled on device

 * @dev: pointer to the pci_dev data structure of MSI device function

 *

 * MSI must be globally enabled and supported by the device and its root

 * bus. But, the root bus is not easy to find since some architectures

 * have virtual busses on top of the PCI hierarchy (for instance the

 * hypertransport bus), while the actual bus where MSI must be supported

 * is below. So we test the MSI flag on all parent busses and assume

 * that no quirk will ever set the NO_MSI flag on a non-root bus.

 * Look at global flags, the device itself, and its parent busses

 * to return 0 if MSI are supported for the device.

 **/

static

int pci_msi_supported(struct pci_dev * dev)

{

	struct pci_bus *bus;

	/* MSI must be globally enabled and supported by the device */

	if (!pci_msi_enable || !dev || dev->no_msi)

		return -EINVAL;

	/* check MSI flags of all parent busses */

	/* Any bridge which does NOT route MSI transactions from it's

	 * secondary bus to it's primary bus must set NO_MSI flag on

	 * the secondary pci_bus.

	 * We expect only arch-specific PCI host bus controller driver

	 * or quirks for specific PCI bridges to be setting NO_MSI.

	 */

	for (bus = dev->bus; bus; bus = bus->parent)

		if (bus->bus_flags & PCI_BUS_FLAGS_NO_MSI)

			return -EINVAL;