linux-kernel - [PATCH v2] PCI: Speed up device init by parsing capabilities all at once

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Date:   Fri, 28 Jan 2022 04:51:43 +0000
From:   Vikash Bansal <bvikas@...are.com>
To:     "bhelgaas@...gle.com" <bhelgaas@...gle.com>
CC:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Srivatsa Bhat <srivatsab@...are.com>,
        "srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        Srinidhi Rao <srinidhir@...are.com>,
        Vikash Bansal <bvikas@...are.com>,
        Anish Swaminathan <anishs@...are.com>,
        Vasavi Sirnapalli <vsirnapalli@...are.com>,
        Ajay Kaher <akaher@...are.com>
Subject: [PATCH v2] PCI: Speed up device init by parsing capabilities all at
 once

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>
---

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   | 43 ++++++++++++++++++++++++++++++++++++-------
drivers/pci/probe.c |  1 +
include/linux/pci.h |  2 ++
3 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 9ecce435fb3f..b361788bcc27 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -468,6 +468,41 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
	return 0;
}

+
+/**
+ * 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
@@ -489,13 +524,7 @@ 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;
+	return dev->cap_off[cap];
}
EXPORT_SYMBOL(pci_find_capability);

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 17a969942d37..b2fa5b2c42f6 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1831,6 +1831,7 @@ 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 8253a5413d7c..abcf7fdc4c98 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -335,6 +335,7 @@ 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 */
@@ -1140,6 +1141,7 @@ void pci_sort_breadthfirst(void);

u8 pci_bus_find_capability(struct pci_bus *bus, unsigned int devfn, int cap);
u8 pci_find_capability(struct pci_dev *dev, int cap);
+void pci_find_all_capabilities(struct pci_dev *dev);
u8 pci_find_next_capability(struct pci_dev *dev, u8 pos, int cap);
u8 pci_find_ht_capability(struct pci_dev *dev, int ht_cap);
u8 pci_find_next_ht_capability(struct pci_dev *dev, u8 pos, int ht_cap);
--
2.30.0