PCI: Add pcie_bandwidth_capable() to compute max supported link bandwidth

Add pcie_bandwidth_capable() to compute the max link bandwidth supported by
a device, based on the max link speed and width, adjusted by the encoding
overhead.

The maximum bandwidth of the link is computed as:

  max_link_width * max_link_speed * (1 - encoding_overhead)

2.5 and 5.0 GT/s links use 8b/10b encoding, which reduces the raw bandwidth
available by 20%; 8.0 GT/s and faster links use 128b/130b encoding, which
reduces it by about 1.5%.

The result is in Mb/s, i.e., megabits/second, of raw bandwidth.

Signed-off-by: Tal Gilboa <talgi@mellanox.com>
[bhelgaas: add 16 GT/s, adjust for pcie_get_speed_cap() and
pcie_get_width_cap() signatures, don't export outside drivers/pci]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci.c

@@ -5208,6 +5208,28 @@ enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev)
 	return PCIE_LNK_WIDTH_UNKNOWN;
 }
 
+/**
+ * pcie_bandwidth_capable - calculate a PCI device's link bandwidth capability
+ * @dev: PCI device
+ * @speed: storage for link speed
+ * @width: storage for link width
+ *
+ * Calculate a PCI device's link bandwidth by querying for its link speed
+ * and width, multiplying them, and applying encoding overhead.  The result
+ * is in Mb/s, i.e., megabits/second of raw bandwidth.
+ */
+u32 pcie_bandwidth_capable(struct pci_dev *dev, enum pci_bus_speed *speed,
+			   enum pcie_link_width *width)
+{
+	*speed = pcie_get_speed_cap(dev);
+	*width = pcie_get_width_cap(dev);
+
+	if (*speed == PCI_SPEED_UNKNOWN || *width == PCIE_LNK_WIDTH_UNKNOWN)
+		return 0;
+
+	return *width * PCIE_SPEED2MBS_ENC(*speed);
+}
+
 /**
  * pci_select_bars - Make BAR mask from the type of resource
  * @dev: the PCI device for which BAR mask is made

drivers/pci/pci.h

@@ -261,8 +261,18 @@ void pci_disable_bridge_window(struct pci_dev *dev);
 	 (speed) == PCIE_SPEED_2_5GT ? "2.5 GT/s" : \
 	 "Unknown speed")
 
+/* PCIe speed to Mb/s reduced by encoding overhead */
+#define PCIE_SPEED2MBS_ENC(speed) \
+	((speed) == PCIE_SPEED_16_0GT ? 16000*128/130 : \
+	 (speed) == PCIE_SPEED_8_0GT  ?  8000*128/130 : \
+	 (speed) == PCIE_SPEED_5_0GT  ?  5000*8/10 : \
+	 (speed) == PCIE_SPEED_2_5GT  ?  2500*8/10 : \
+	 0)
+
 enum pci_bus_speed pcie_get_speed_cap(struct pci_dev *dev);
 enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev);
+u32 pcie_bandwidth_capable(struct pci_dev *dev, enum pci_bus_speed *speed,
+			   enum pcie_link_width *width);
 
 /* Single Root I/O Virtualization */
 struct pci_sriov {