pciexp_device: Rewrite LTR configuration

I was bugged by spurious "Failed to enable LTR" messages for years.
Looking at the the current algorithm, it is flawed in multiple ways:

* It looks like the author didn't know they implemented a
  recursive algorithm (pciexp_enable_ltr()) inside another
  recursive algorithm (pciexp_scan_bridge()). Thus, at every
  tree level, everything is run again for the whole sub-
  tree.

* LTR is enabled no matter if `.set_ltr_max_latencies` is
  implemented or not. Leaving the endpoints' LTR settings
  at 0: They are told to always report zero tolerance.
  In theory, depending on the root-complex implementation,
  this may result in higher power consumption than without
  LTR messages.

* `.set_ltr_max_latencies` is only considered for the direct
  parent of a device. Thus, even with it implemented, an
  endpoint below a (non-root) bridge may suffer from the 0
  settings as described above.

* Due to the double-recursive nature, LTR is enabled starting
  with the endpoints, then moving up the tree, while the PCIe
  spec tells us to do it in the exact opposite order.

With the current implementation of pciexp_scan_bridge(), it is
hard to hook anything in that runs for each device from top to
bottom. So the proposed solution still adds some redundancy:

First, for every device that uses pciexp_scan_bus(), we enable
LTR if possible (see below). Then, when returning from the bus-
scanning recursion, we enable LTR for every device and configure
the maximum latencies (if supported). The latter runs again on
all bridges, because it's hard to know if pciexp_scan_bus() was
used for them.

When to enable LTR:

* For all devices that implement `.set_ltr_max_latencies`.
* For all devices below a bridge that has it enabled already.

Change-Id: I2c5b8658f1fc8cec15e8b0824464c6fc9bee7e0e
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51328
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

5 files changed, 82 insertions, 52 deletions

diff --git a/src/device/pciexp_device.c b/src/device/pciexp_device.c
index 70f65144b6..d8ed5d9e3d 100644
--- a/src/device/pciexp_device.c
+++ b/src/device/pciexp_device.c
@@ -127,66 +127,92 @@ static void pciexp_enable_clock_power_pm(struct device *endp, unsigned int endp_
 	pci_write_config16(endp, endp_cap + PCI_EXP_LNKCTL, lnkctl);
 }
 
-static void pciexp_config_max_latency(struct device *root, struct device *dev)
+static bool _pciexp_ltr_supported(struct device *dev, unsigned int cap)
 {
-	unsigned int cap;
-	cap = pciexp_find_extended_cap(dev, PCIE_EXT_CAP_LTR_ID);
-	if ((cap) && (root->ops->ops_pci != NULL) &&
-		(root->ops->ops_pci->set_ltr_max_latencies != NULL))
-			root->ops->ops_pci->set_ltr_max_latencies(dev, cap + 4);
+	return pci_read_config16(dev, cap + PCI_EXP_DEVCAP2) & PCI_EXP_DEVCAP2_LTR;
 }
 
-static bool pciexp_is_ltr_supported(struct device *dev, unsigned int cap)
+static bool _pciexp_ltr_enabled(struct device *dev, unsigned int cap)
 {
-	unsigned int val;
+	return pci_read_config16(dev, cap + PCI_EXP_DEVCTL2) & PCI_EXP_DEV2_LTR;
+}
 
-	val = pci_read_config16(dev, cap + PCI_EXP_DEVCAP2);
+static bool _pciexp_enable_ltr(struct device *parent, unsigned int parent_cap,
+			       struct device *dev, unsigned int cap)
+{
+	if (!_pciexp_ltr_supported(dev, cap)) {
+		printk(BIOS_DEBUG, "%s: No LTR support\n", dev_path(dev));
+		return false;
+	}
 
-	if (val & PCI_EXP_DEVCAP2_LTR)
+	if (_pciexp_ltr_enabled(dev, cap))
 		return true;
 
-	return false;
+	if (parent &&
+	    (parent->path.type != DEVICE_PATH_PCI ||
+	     !_pciexp_ltr_supported(parent, parent_cap) ||
+	     !_pciexp_ltr_enabled(parent, parent_cap)))
+		return false;
+
+	pci_or_config16(dev, cap + PCI_EXP_DEVCTL2, PCI_EXP_DEV2_LTR);
+	printk(BIOS_INFO, "%s: Enabled LTR\n", dev_path(dev));
+	return true;
 }
 
-static void pciexp_configure_ltr(struct device *dev)
+static void pciexp_enable_ltr(struct device *dev)
 {
-	unsigned int cap;
-
-	cap = pci_find_capability(dev, PCI_CAP_ID_PCIE);
+	const unsigned int cap = pci_find_capability(dev, PCI_CAP_ID_PCIE);
+	if (!cap)
+		return;
 
 	/*
-	 * Check if capability pointer is valid and
-	 * device supports LTR mechanism.
+	 * If we have get_ltr_max_latencies(), treat `dev` as the root.
+	 * If not, let _pciexp_enable_ltr() query the parent's state.
 	 */
-	if (!cap || !pciexp_is_ltr_supported(dev, cap)) {
-		printk(BIOS_INFO, "Failed to enable LTR for dev = %s\n",
-				dev_path(dev));
-		return;
+	struct device *parent = NULL;
+	unsigned int parent_cap = 0;
+	if (!dev->ops->ops_pci || !dev->ops->ops_pci->get_ltr_max_latencies) {
+		parent = dev->bus->dev;
+		parent_cap = pci_find_capability(dev, PCI_CAP_ID_PCIE);
+		if (!parent_cap)
+			return;
 	}
 
-	cap += PCI_EXP_DEVCTL2;
+	(void)_pciexp_enable_ltr(parent, parent_cap, dev, cap);
+}
 
-	/* Enable LTR for device */
-	pci_update_config32(dev, cap, ~PCI_EXP_DEV2_LTR, PCI_EXP_DEV2_LTR);
+static bool pciexp_get_ltr_max_latencies(struct device *dev, u16 *max_snoop, u16 *max_nosnoop)
+{
+	/* Walk the hierarchy up to find get_ltr_max_latencies(). */
+	do {
+		if (dev->ops->ops_pci && dev->ops->ops_pci->get_ltr_max_latencies)
+			break;
+		if (dev->bus->dev == dev || dev->bus->dev->path.type != DEVICE_PATH_PCI)
+			return false;
+		dev = dev->bus->dev;
+	} while (true);
 
-	/* Configure Max Snoop Latency */
-	pciexp_config_max_latency(dev->bus->dev, dev);
+	dev->ops->ops_pci->get_ltr_max_latencies(max_snoop, max_nosnoop);
+	return true;
 }
 
-static void pciexp_enable_ltr(struct device *dev)
+static void pciexp_configure_ltr(struct device *parent, unsigned int parent_cap,
+				 struct device *dev, unsigned int cap)
 {
-	struct bus *bus;
-	struct device *child;
+	if (!_pciexp_enable_ltr(parent, parent_cap, dev, cap))
+		return;
 
-	for (bus = dev->link_list ; bus ; bus = bus->next) {
-		for (child = bus->children; child; child = child->sibling) {
-			if (child->path.type != DEVICE_PATH_PCI)
-				continue;
-			pciexp_configure_ltr(child);
-			if (child->ops && child->ops->scan_bus)
-				pciexp_enable_ltr(child);
-		}
-	}
+	const unsigned int ltr_cap = pciexp_find_extended_cap(dev, PCIE_EXT_CAP_LTR_ID);
+	if (!ltr_cap)
+		return;
+
+	u16 max_snoop, max_nosnoop;
+	if (!pciexp_get_ltr_max_latencies(dev, &max_snoop, &max_nosnoop))
+		return;
+
+	pci_write_config16(dev, ltr_cap + PCI_LTR_MAX_SNOOP, max_snoop);
+	pci_write_config16(dev, ltr_cap + PCI_LTR_MAX_NOSNOOP, max_nosnoop);
+	printk(BIOS_INFO, "%s: Programmed LTR max latencies\n", dev_path(dev));
 }
 
 static unsigned char pciexp_L1_substate_cal(struct device *dev, unsigned int endp_cap,
@@ -471,12 +497,17 @@ static void pciexp_tune_dev(struct device *dev)
 
 	/* Adjust Max_Payload_Size of link ends. */
 	pciexp_set_max_payload_size(root, root_cap, dev, cap);
+
+	pciexp_configure_ltr(root, root_cap, dev, cap);
 }
 
 void pciexp_scan_bus(struct bus *bus, unsigned int min_devfn,
 			     unsigned int max_devfn)
 {
 	struct device *child;
+
+	pciexp_enable_ltr(bus->dev);
+
 	pci_scan_bus(bus, min_devfn, max_devfn);
 
 	for (child = bus->children; child; child = child->sibling) {
@@ -493,7 +524,6 @@ void pciexp_scan_bus(struct bus *bus, unsigned int min_devfn,
 void pciexp_scan_bridge(struct device *dev)
 {
 	do_pci_scan_bridge(dev, pciexp_scan_bus);
-	pciexp_enable_ltr(dev);
 }
 
 /** Default device operations for PCI Express bridges */
diff --git a/src/include/device/pci.h b/src/include/device/pci.h
index e80cb22907..db7cc69c8d 100644
--- a/src/include/device/pci.h
+++ b/src/include/device/pci.h
@@ -31,7 +31,7 @@ struct pci_operations {
 	/* set the Subsystem IDs for the PCI device */
 	void (*set_subsystem)(struct device *dev, unsigned int vendor,
 		unsigned int device);
-	void (*set_ltr_max_latencies)(struct device *dev, unsigned int off);
+	void (*get_ltr_max_latencies)(u16 *max_snoop, u16 *max_nosnoop);
 };
 
 struct pci_driver {
diff --git a/src/include/device/pci_def.h b/src/include/device/pci_def.h
index b0558fa67e..d18e750520 100644
--- a/src/include/device/pci_def.h
+++ b/src/include/device/pci_def.h
@@ -518,6 +518,10 @@
 #define PCI_PWR_CAP		12	/* Capability */
 #define  PCI_PWR_CAP_BUDGET(x)	((x) & 1)	/* Included in system budget */
 
+/* Latency Tolerance Reporting */
+#define PCI_LTR_MAX_SNOOP	4
+#define PCI_LTR_MAX_NOSNOOP	6
+
 /*
  * The PCI interface treats multi-function devices as independent
  * devices.  The slot/function address of each device is encoded
diff --git a/src/soc/intel/broadwell/pch/pcie.c b/src/soc/intel/broadwell/pch/pcie.c
index 05619f8023..26fde0bfb2 100644
--- a/src/soc/intel/broadwell/pch/pcie.c
+++ b/src/soc/intel/broadwell/pch/pcie.c
@@ -609,17 +609,15 @@ static void pch_pcie_enable(struct device *dev)
 		root_port_commit_config();
 }
 
-static void pcie_set_ltr_max_latencies(struct device *dev, unsigned int off)
+static void pcie_get_ltr_max_latencies(u16 *max_snoop, u16 *max_nosnoop)
 {
-	/* Set max snoop and non-snoop latency for Broadwell */
-	pci_write_config32(dev, off,
-		PCIE_LTR_MAX_NO_SNOOP_LATENCY_3146US << 16 |
-		PCIE_LTR_MAX_SNOOP_LATENCY_3146US);
+	*max_snoop = PCIE_LTR_MAX_SNOOP_LATENCY_3146US;
+	*max_nosnoop = PCIE_LTR_MAX_NO_SNOOP_LATENCY_3146US;
 }
 
 static struct pci_operations pcie_ops = {
 	.set_subsystem = pci_dev_set_subsystem,
-	.set_ltr_max_latencies = pcie_set_ltr_max_latencies,
+	.get_ltr_max_latencies = pcie_get_ltr_max_latencies,
 };
 
 static struct device_operations device_ops = {
diff --git a/src/soc/intel/common/block/pcie/pcie.c b/src/soc/intel/common/block/pcie/pcie.c
index 346c0ff225..f7f0902ab7 100644
--- a/src/soc/intel/common/block/pcie/pcie.c
+++ b/src/soc/intel/common/block/pcie/pcie.c
@@ -45,16 +45,14 @@ static void pch_pcie_init(struct device *dev)
 	pci_write_config16(dev, PCI_SEC_STATUS, reg16);
 }
 
-static void pcie_set_ltr_max_latencies(struct device *dev, unsigned int offset)
+static void pcie_get_ltr_max_latencies(u16 *max_snoop, u16 *max_nosnoop)
 {
-	/* Set max snoop and non-snoop latency for the SOC */
-	pci_write_config32(dev, offset,
-		PCIE_LTR_MAX_NO_SNOOP_LATENCY_3146US << 16 |
-		PCIE_LTR_MAX_SNOOP_LATENCY_3146US);
+	*max_snoop = PCIE_LTR_MAX_SNOOP_LATENCY_3146US;
+	*max_nosnoop = PCIE_LTR_MAX_NO_SNOOP_LATENCY_3146US;
 }
 
 static struct pci_operations pcie_ops = {
-	.set_ltr_max_latencies = pcie_set_ltr_max_latencies,
+	.get_ltr_max_latencies = pcie_get_ltr_max_latencies,
 	.set_subsystem = pci_dev_set_subsystem,
 };