powerpc/powernv/npu: Fix reference leak

Since 902bdc57, get_pci_dev() calls pci_get_domain_bus_and_slot(). This
has the effect of incrementing the reference count of the PCI device, as
explained in drivers/pci/search.c:

 * Given a PCI domain, bus, and slot/function number, the desired PCI
 * device is located in the list of PCI devices. If the device is
 * found, its reference count is increased and this function returns a
 * pointer to its data structure.  The caller must decrement the
 * reference count by calling pci_dev_put().  If no device is found,
 * %NULL is returned.

Nothing was done to call pci_dev_put() and the reference count of GPU and
NPU PCI devices rockets up.

A natural way to fix this would be to teach the callers about the change,
so that they call pci_dev_put() when done with the pointer. This turns
out to be quite intrusive, as it affects many paths in npu-dma.c,
pci-ioda.c and vfio_pci_nvlink2.c. Also, the issue appeared in 4.16 and
some affected code got moved around since then: it would be problematic
to backport the fix to stable releases.

All that code never cared for reference counting anyway. Call pci_dev_put()
from get_pci_dev() to revert to the previous behavior.

Fixes: 902bdc57 ("powerpc/powernv/idoa: Remove unnecessary pcidev from pci_dn")
Cc: stable@vger.kernel.org # v4.16
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/npu-dma.c

@@ -31,9 +31,22 @@ static DEFINE_SPINLOCK(npu_context_lock);
 static struct pci_dev *get_pci_dev(struct device_node *dn)
 {
 	struct pci_dn *pdn = PCI_DN(dn);
+	struct pci_dev *pdev;
 
-	return pci_get_domain_bus_and_slot(pci_domain_nr(pdn->phb->bus),
+	pdev = pci_get_domain_bus_and_slot(pci_domain_nr(pdn->phb->bus),
 					   pdn->busno, pdn->devfn);
+
+	/*
+	 * pci_get_domain_bus_and_slot() increased the reference count of
+	 * the PCI device, but callers don't need that actually as the PE
+	 * already holds a reference to the device. Since callers aren't
+	 * aware of the reference count change, call pci_dev_put() now to
+	 * avoid leaks.
+	 */
+	if (pdev)
+		pci_dev_put(pdev);
+
+	return pdev;
 }
 
 /* Given a NPU device get the associated PCI device. */