ahci: per-port msix support

Some AHCI controllers support per-port MSI-X vectors.  At the same time
the Linux AHCI driver needs to support one-off architectures that
implement a single MSI-X vector for all ports.  The heuristic for
enabling AHCI ports becomes, in order of preference:

1/ per-port multi-MSI-X

2/ per-port multi-MSI

3/ single MSI

4/ single MSI-X

5/ legacy INTX

This all depends on AHCI implementations with potentially broken MSI-X
requesting less vectors than the number of ports.  If this assumption is
violated we will need to start explicitly white-listing AHCI-MSIX
implementations.

Reported-by: Ricardo Neri <ricardo.neri@intel.com>
[ricardo: fix struct msix_entry handling]
Reported-by: kernel test robot <ying.huang@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/ahci.h

@@ -242,6 +242,7 @@ enum {
 	AHCI_HFLAG_NO_FBS		= (1 << 18), /* no FBS */
 	AHCI_HFLAG_EDGE_IRQ		= (1 << 19), /* HOST_IRQ_STAT behaves as
 							Edge Triggered */
+	AHCI_HFLAG_MULTI_MSIX		= (1 << 20), /* per-port MSI-X */
 
 	/* ap->flags bits */
 
@@ -343,6 +344,7 @@ struct ahci_host_priv {
 	 * the PHY position in this array.
 	 */
 	struct phy		**phys;
+	struct msix_entry	*msix;		/* Optional MSI-X support */
 	unsigned		nports;		/* Number of ports */
 	void			*plat_data;	/* Other platform data */
 	unsigned int		irq;		/* interrupt line */