linux-kernel - [PATCH v3] PCI: Speed up device init by parsing capabilities all at once

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Message-Id: <1645616049-19712-1-git-send-email-akaher@vmware.com>
Date:   Wed, 23 Feb 2022 17:04:09 +0530
From:   Ajay Kaher <akaher@...are.com>
To:     bhelgaas@...gle.com, helgaas@...nel.org, gregkh@...uxfoundation.org
Cc:     ymcavikas@...il.com, akaher@...are.com, amakhalov@...are.com,
        anishs@...are.com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, srinidhir@...are.com,
        srivatsa@...il.mit.edu, srivatsab@...are.com,
        vsirnapalli@...are.com, Vikash Bansal <bvikas@...are.com>
Subject: [PATCH v3] PCI: Speed up device init by parsing capabilities all at once

From: Vikash Bansal <bvikas@...are.com>

In the current implementation, the PCI capability list is parsed from
the beginning to find each capability, which results in a large number
of redundant PCI reads.

Instead, we can parse the complete list just once, store it in the
pci_dev structure, and get the offset of each capability directly from
the pci_dev structure.

This implementation improves pci devices initialization time  by ~2-3%
(from 270ms to 261ms) in case of bare metal and 7-8% (From 201ms to 184ms)
in case of VM running on ESXi.

It also adds a memory overhead of 20bytes (value of PCI_CAP_ID_MAX) per
PCI device.

Ran pahole for pci_dev structure. This patch is not adding any padding
bytes.

Signed-off-by: Vikash Bansal <bvikas@...are.com>
Signed-off-by: Ajay Kaher <akaher@...are.com>

---

Changes in v3:
  - Added check before access cap_off from pci_find_capability().
  - Moved declaration of pci_find_all_capabilities() to drivers/pci/pci.h.

Changes in v2:
  - Ran pahole tool.
  - Modified comments to add "clock time".
  - Removed comments before call to pci_find_all_capabilities.

---
 drivers/pci/pci.c   | 44 +++++++++++++++++++++++++++++++++++++-------
 drivers/pci/pci.h   |  1 +
 drivers/pci/probe.c |  1 +
 include/linux/pci.h |  1 +
 4 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 3d2fb39..cf54811 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -469,6 +469,40 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
 }
 
 /**
+ * pci_find_all_capabilities - Read all capabilities
+ * @dev: the PCI device
+ *
+ * Read all capabilities and store offsets in cap_off
+ * array in pci_dev structure.
+ */
+void pci_find_all_capabilities(struct pci_dev *dev)
+{
+	int ttl = PCI_FIND_CAP_TTL;
+	u16 ent;
+	u8 pos;
+	u8 id;
+
+	pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
+	if (!pos)
+		return;
+	pci_bus_read_config_byte(dev->bus, dev->devfn, pos, &pos);
+	while (ttl--) {
+		if (pos < 0x40)
+			break;
+		pos &= ~3;
+		pci_bus_read_config_word(dev->bus, dev->devfn, pos, &ent);
+		id = ent & 0xff;
+		if (id == 0xff)
+			break;
+
+		/* Read first instance of capability */
+		if (!(dev->cap_off[id]))
+			dev->cap_off[id] = pos;
+		pos = (ent >> 8);
+	}
+}
+
+/**
  * pci_find_capability - query for devices' capabilities
  * @dev: PCI device to query
  * @cap: capability code
@@ -489,13 +523,9 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
  */
 u8 pci_find_capability(struct pci_dev *dev, int cap)
 {
-	u8 pos;
-
-	pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
-	if (pos)
-		pos = __pci_find_next_cap(dev->bus, dev->devfn, pos, cap);
-
-	return pos;
+	if(cap >= PCI_CAP_ID_MAX)
+		return 0;
+	return dev->cap_off[cap];
 }
 EXPORT_SYMBOL(pci_find_capability);
 
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 3d60cab..3cb70d5 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -87,6 +87,7 @@ bool pci_bridge_d3_possible(struct pci_dev *dev);
 void pci_bridge_d3_update(struct pci_dev *dev);
 void pci_bridge_wait_for_secondary_bus(struct pci_dev *dev);
 void pci_bridge_reconfigure_ltr(struct pci_dev *dev);
+void pci_find_all_capabilities(struct pci_dev *dev);
 
 static inline void pci_wakeup_event(struct pci_dev *dev)
 {
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 087d365..d75e1fa 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1839,6 +1839,7 @@ int pci_setup_device(struct pci_dev *dev)
 	dev->hdr_type = hdr_type & 0x7f;
 	dev->multifunction = !!(hdr_type & 0x80);
 	dev->error_state = pci_channel_io_normal;
+	pci_find_all_capabilities(dev);
 	set_pcie_port_type(dev);
 
 	pci_set_of_node(dev);
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 18a75c8..bd61cee 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -326,6 +326,7 @@ struct pci_dev {
 	unsigned int	class;		/* 3 bytes: (base,sub,prog-if) */
 	u8		revision;	/* PCI revision, low byte of class word */
 	u8		hdr_type;	/* PCI header type (`multi' flag masked out) */
+	u8		cap_off[PCI_CAP_ID_MAX]; /* Offsets of all pci capabilities */
 #ifdef CONFIG_PCIEAER
 	u16		aer_cap;	/* AER capability offset */
 	struct aer_stats *aer_stats;	/* AER stats for this device */
-- 
2.7.4