linux-kernel - [PATCH v3 2/8] PCI: Don't unnecessarily disable memory decoding

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Message-ID: <20240807151723.613742-3-stewart.hildebrand@amd.com>
Date: Wed, 7 Aug 2024 11:17:11 -0400
From: Stewart Hildebrand <stewart.hildebrand@....com>
To: Bjorn Helgaas <bhelgaas@...gle.com>
CC: Stewart Hildebrand <stewart.hildebrand@....com>,
	<linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v3 2/8] PCI: Don't unnecessarily disable memory decoding

If alignment is requested for a device and all the BARs are already
sufficiently aligned, there is no need to disable memory decoding for
the device. Add a check for this scenario to save a PCI config space
access.

Also, there is no need to disable memory decoding if already disabled,
since the bit in question (PCI_COMMAND_MEMORY) is a RW bit. Make the
write conditional to save a PCI config space access.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@....com>
---
v2->v3:
* no change

v1->v2:
* new subject (was: "PCI: don't clear already cleared bit")
* don't disable memory decoding if alignment is not needed
---
 drivers/pci/pci.c | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index e3a49f66982d..acecdd6edd5a 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6564,7 +6564,7 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
 	return align;
 }
 
-static void pci_request_resource_alignment(struct pci_dev *dev, int bar,
+static bool pci_request_resource_alignment(struct pci_dev *dev, int bar,
 					   resource_size_t align, bool resize)
 {
 	struct resource *r = &dev->resource[bar];
@@ -6572,17 +6572,17 @@ static void pci_request_resource_alignment(struct pci_dev *dev, int bar,
 	resource_size_t size;
 
 	if (!(r->flags & IORESOURCE_MEM))
-		return;
+		return false;
 
 	if (r->flags & IORESOURCE_PCI_FIXED) {
 		pci_info(dev, "%s %pR: ignoring requested alignment %#llx\n",
 			 r_name, r, (unsigned long long)align);
-		return;
+		return false;
 	}
 
 	size = resource_size(r);
 	if (size >= align)
-		return;
+		return false;
 
 	/*
 	 * Increase the alignment of the resource.  There are two ways we
@@ -6625,6 +6625,8 @@ static void pci_request_resource_alignment(struct pci_dev *dev, int bar,
 		r->end = r->start + size - 1;
 	}
 	r->flags |= IORESOURCE_UNSET;
+
+	return true;
 }
 
 /*
@@ -6640,7 +6642,7 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
 	struct resource *r;
 	resource_size_t align;
 	u16 command;
-	bool resize = false;
+	bool resize = false, align_needed = false;
 
 	/*
 	 * VF BARs are read-only zero according to SR-IOV spec r1.1, sec
@@ -6662,12 +6664,21 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
 		return;
 	}
 
-	pci_read_config_word(dev, PCI_COMMAND, &command);
-	command &= ~PCI_COMMAND_MEMORY;
-	pci_write_config_word(dev, PCI_COMMAND, command);
+	for (i = 0; i <= PCI_ROM_RESOURCE; i++) {
+		bool ret;
 
-	for (i = 0; i <= PCI_ROM_RESOURCE; i++)
-		pci_request_resource_alignment(dev, i, align, resize);
+		ret = pci_request_resource_alignment(dev, i, align, resize);
+		align_needed = align_needed || ret;
+	}
+
+	if (!align_needed)
+		return;
+
+	pci_read_config_word(dev, PCI_COMMAND, &command);
+	if (command & PCI_COMMAND_MEMORY) {
+		command &= ~PCI_COMMAND_MEMORY;
+		pci_write_config_word(dev, PCI_COMMAND, command);
+	}
 
 	/*
 	 * Need to disable bridge's resource window,
-- 
2.46.0