powerpc/powernv: replace the hard coded boundary with gate

At the moment 64bit-prefetchable window can be maximum 64GB, which is
currently got from device tree. This means that in shared mode the maximum
supported VF BAR size is 64GB/256=256MB. While this size could exhaust the
whole 64bit-prefetchable window. This is a design decision to set a
boundary to 64MB of the VF BAR size. Since VF BAR size with 64MB would
occupy a quarter of the 64bit-prefetchable window, this is affordable.

This patch replaces magic limit of 64MB with "gate", which is 1/4 of the
M64 Segment Size(m64_segsize >> 2) and adds comment to explain the reason
for it.
Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Reviewed-by: Gavin Shan <gwshan@linux.vent.ibm.com>
Acked-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/pci-ioda.c

@@ -2798,8 +2798,9 @@ static void pnv_pci_init_ioda_msis(struct pnv_phb *phb) { }
 #ifdef CONFIG_PCI_IOV
 static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
 {
-	struct pci_controller *hose;
-	struct pnv_phb *phb;
+	struct pci_controller *hose = pci_bus_to_host(pdev->bus);
+	struct pnv_phb *phb = hose->private_data;
+	const resource_size_t gate = phb->ioda.m64_segsize >> 2;
 	struct resource *res;
 	int i;
 	resource_size_t size;
@@ -2809,9 +2810,6 @@ static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
 	if (!pdev->is_physfn || pdev->is_added)
 		return;
 
-	hose = pci_bus_to_host(pdev->bus);
-	phb = hose->private_data;
-
 	pdn = pci_get_pdn(pdev);
 	pdn->vfs_expanded = 0;
 	pdn->m64_single_mode = false;
@@ -2832,10 +2830,22 @@ static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
 
 		size = pci_iov_resource_size(pdev, i + PCI_IOV_RESOURCES);
 
-		/* bigger than 64M */
-		if (size > (1 << 26)) {
-			dev_info(&pdev->dev, "PowerNV: VF BAR%d: %pR IOV size is bigger than 64M, roundup power2\n",
-				 i, res);
+		/*
+		 * If bigger than quarter of M64 segment size, just round up
+		 * power of two.
+		 *
+		 * Generally, one M64 BAR maps one IOV BAR. To avoid conflict
+		 * with other devices, IOV BAR size is expanded to be
+		 * (total_pe * VF_BAR_size).  When VF_BAR_size is half of M64
+		 * segment size , the expanded size would equal to half of the
+		 * whole M64 space size, which will exhaust the M64 Space and
+		 * limit the system flexibility.  This is a design decision to
+		 * set the boundary to quarter of the M64 segment size.
+		 */
+		if (size > gate) {
+			dev_info(&pdev->dev, "PowerNV: VF BAR%d: %pR IOV size "
+				"is bigger than %lld, roundup power2\n",
+				 i, res, gate);
 			mul = roundup_pow_of_two(total_vfs);
 			pdn->m64_single_mode = true;
 			break;