linux-kernel - [PATCH v3 4/8] PCI: Restore memory decoding after reallocation

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Message-ID: <20240807151723.613742-5-stewart.hildebrand@amd.com>
Date: Wed, 7 Aug 2024 11:17:13 -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 4/8] PCI: Restore memory decoding after reallocation

Currently, the PCI subsystem unconditionally clears the memory decoding
bit of devices with resource alignment specified. While drivers should
call pci_enable_device() to enable memory decoding, some platforms have
devices that expect memory decoding to be enabled even when no driver is
bound to the device. It is assumed firmware enables memory decoding in
these cases. For example, the vgacon driver uses the 0xb8000 buffer to
display a console without any PCI involvement, yet, on some platforms,
memory decoding must be enabled on the PCI VGA device in order for the
console to be displayed properly.

Restore the memory decoding bit after the resource has been reallocated.

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

v1->v2:
* capitalize subject text
* reword commit message

Testing notes / how to check if your platform needs memory decoding
enabled in order to use vgacon:
1. Boot your system with a monitor attached, without any driver bound to
   your PCI VGA device. You should see a console on your display.
2. Find the SBDF of your VGA device with lspci -v (00:01.0 in this
   example).
3. Disable memory decoding (replace 00:01.0 with your SBDF):
  $ sudo setpci -v -s 00:01.0 0x04.W=0x05
4. Type something to see if it appears on the console display
5. Re-enable memory decoding:
  $ sudo setpci -v -s 00:01.0 0x04.W=0x07

Relevant prior discussion at [1]

[1] https://lore.kernel.org/linux-pci/20160906165652.GE1214@localhost/
---
 drivers/pci/pci.c       |  1 +
 drivers/pci/setup-bus.c | 25 +++++++++++++++++++++++++
 include/linux/pci.h     |  2 ++
 3 files changed, 28 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index acecdd6edd5a..4b97d8d5c2d8 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6676,6 +6676,7 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
 
 	pci_read_config_word(dev, PCI_COMMAND, &command);
 	if (command & PCI_COMMAND_MEMORY) {
+		dev->dev_flags |= PCI_DEV_FLAGS_MEMORY_ENABLE;
 		command &= ~PCI_COMMAND_MEMORY;
 		pci_write_config_word(dev, PCI_COMMAND, command);
 	}
diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
index ab7510ce6917..6847b251e19a 100644
--- a/drivers/pci/setup-bus.c
+++ b/drivers/pci/setup-bus.c
@@ -2131,6 +2131,29 @@ pci_root_bus_distribute_available_resources(struct pci_bus *bus,
 	}
 }
 
+static void restore_memory_decoding(struct pci_bus *bus)
+{
+	struct pci_dev *dev;
+
+	list_for_each_entry(dev, &bus->devices, bus_list) {
+		struct pci_bus *b;
+
+		if (dev->dev_flags & PCI_DEV_FLAGS_MEMORY_ENABLE) {
+			u16 command;
+
+			pci_read_config_word(dev, PCI_COMMAND, &command);
+			command |= PCI_COMMAND_MEMORY;
+			pci_write_config_word(dev, PCI_COMMAND, command);
+		}
+
+		b = dev->subordinate;
+		if (!b)
+			continue;
+
+		restore_memory_decoding(b);
+	}
+}
+
 /*
  * First try will not touch PCI bridge res.
  * Second and later try will clear small leaf bridge res.
@@ -2229,6 +2252,8 @@ void pci_assign_unassigned_root_bus_resources(struct pci_bus *bus)
 	goto again;
 
 dump:
+	restore_memory_decoding(bus);
+
 	/* Dump the resource on buses */
 	pci_bus_dump_resources(bus);
 }
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 4246cb790c7b..74636acf152f 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -245,6 +245,8 @@ enum pci_dev_flags {
 	PCI_DEV_FLAGS_NO_RELAXED_ORDERING = (__force pci_dev_flags_t) (1 << 11),
 	/* Device does honor MSI masking despite saying otherwise */
 	PCI_DEV_FLAGS_HAS_MSI_MASKING = (__force pci_dev_flags_t) (1 << 12),
+	/* Firmware enabled memory decoding, to be restored if BAR is updated */
+	PCI_DEV_FLAGS_MEMORY_ENABLE = (__force pci_dev_flags_t) (1 << 13),
 };
 
 enum pci_irq_reroute_variant {
-- 
2.46.0