PCI: Add pcie_bandwidth_available() to compute bandwidth available to device

Add pcie_bandwidth_available() to compute the bandwidth available to a
device.  This may be limited by the device itself or by a slower upstream
link leading to the device.

The available bandwidth at each link along the path is computed as:

  link_width * 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.

Also return the device with the slowest link and the speed and width of
that link.

Signed-off-by: Tal Gilboa <talgi@mellanox.com>
[bhelgaas: changelog, leave pcie_get_minimum_link() alone for now, return
bw directly, use pci_upstream_bridge(), check "next_bw <= bw" to find
uppermost limiting device, return speed/width of the limiting device]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci.c

@@ -5146,6 +5146,64 @@ int pcie_get_minimum_link(struct pci_dev *dev, enum pci_bus_speed *speed,
 }
 EXPORT_SYMBOL(pcie_get_minimum_link);
 
+/**
+ * pcie_bandwidth_available - determine minimum link settings of a PCIe
+ *			      device and its bandwidth limitation
+ * @dev: PCI device to query
+ * @limiting_dev: storage for device causing the bandwidth limitation
+ * @speed: storage for speed of limiting device
+ * @width: storage for width of limiting device
+ *
+ * Walk up the PCI device chain and find the point where the minimum
+ * bandwidth is available.  Return the bandwidth available there and (if
+ * limiting_dev, speed, and width pointers are supplied) information about
+ * that point.  The bandwidth returned is in Mb/s, i.e., megabits/second of
+ * raw bandwidth.
+ */
+u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
+			     enum pci_bus_speed *speed,
+			     enum pcie_link_width *width)
+{
+	u16 lnksta;
+	enum pci_bus_speed next_speed;
+	enum pcie_link_width next_width;
+	u32 bw, next_bw;
+
+	if (speed)
+		*speed = PCI_SPEED_UNKNOWN;
+	if (width)
+		*width = PCIE_LNK_WIDTH_UNKNOWN;
+
+	bw = 0;
+
+	while (dev) {
+		pcie_capability_read_word(dev, PCI_EXP_LNKSTA, &lnksta);
+
+		next_speed = pcie_link_speed[lnksta & PCI_EXP_LNKSTA_CLS];
+		next_width = (lnksta & PCI_EXP_LNKSTA_NLW) >>
+			PCI_EXP_LNKSTA_NLW_SHIFT;
+
+		next_bw = next_width * PCIE_SPEED2MBS_ENC(next_speed);
+
+		/* Check if current device limits the total bandwidth */
+		if (!bw || next_bw <= bw) {
+			bw = next_bw;
+
+			if (limiting_dev)
+				*limiting_dev = dev;
+			if (speed)
+				*speed = next_speed;
+			if (width)
+				*width = next_width;
+		}
+
+		dev = pci_upstream_bridge(dev);
+	}
+
+	return bw;
+}
+EXPORT_SYMBOL(pcie_bandwidth_available);
+
 /**
  * pcie_get_speed_cap - query for the PCI device's link speed capability
  * @dev: PCI device to query

include/linux/pci.h

@@ -1083,6 +1083,9 @@ int pcie_get_mps(struct pci_dev *dev);
 int pcie_set_mps(struct pci_dev *dev, int mps);
 int pcie_get_minimum_link(struct pci_dev *dev, enum pci_bus_speed *speed,
 			  enum pcie_link_width *width);
+u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
+			     enum pci_bus_speed *speed,
+			     enum pcie_link_width *width);
 void pcie_flr(struct pci_dev *dev);
 int __pci_reset_function_locked(struct pci_dev *dev);
 int pci_reset_function(struct pci_dev *dev);