[PATCH v4 2/3] PCI: Cache CRS Software Visibiliy in struct pci_dev

[Stanislav Spassov <stanspas@amazon.com>]

From: Stanislav Spassov <stanspas@amazon.de>

Arguably, since CRS SV is a capability of Root Ports and determines
how Root Ports will handle incoming CRS Completions, it makes more
sense to store this flag in the struct pci_bus that represents the
port's secondary bus, and to cache it in any buses further down the
hierarchy.

However, storing the flag in struct pci_dev allows individual devices
to be marked as not supporting CRS properly even when CRS SV is enabled
on their parent Root Port. This way, future code that relies on the new
flag will not be misled that it is safe to probe a device by relying on
CRS SV to not cause platform timeouts (See the note on CRS SV on p. 553
of PCI Express Base Specification r5.0 from May 22, 2019).

Note: Endpoints integrated into the Root Complex (RCiEP) may also be
capable of using CRS. In that case, the software visibility is
controlled using a Root Complex Register Block (RCRB). This is currently
not supported by the kernel. The code introduced here would simply not
set the newly introduced flag for RCiEP as for those bus->self is NULL.

Signed-off-by: Stanislav Spassov <stanspas@amazon.de>
---
 drivers/pci/probe.c | 8 +++++++-
 include/linux/pci.h | 3 +++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 512cb4312ddd..eeff8a07f330 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1080,9 +1080,11 @@ static void pci_enable_crs(struct pci_dev *pdev)
 
 	/* Enable CRS Software Visibility if supported */
 	pcie_capability_read_word(pdev, PCI_EXP_RTCAP, &root_cap);
-	if (root_cap & PCI_EXP_RTCAP_CRSVIS)
+	if (root_cap & PCI_EXP_RTCAP_CRSVIS) {
 		pcie_capability_set_word(pdev, PCI_EXP_RTCTL,
 					 PCI_EXP_RTCTL_CRSSVE);
+		pdev->crssv_enabled = true;
+	}
 }
 
 static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
@@ -2414,6 +2416,10 @@ void pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
 	list_add_tail(&dev->bus_list, &bus->devices);
 	up_write(&pci_bus_sem);
 
+	/* Propagate CRS Software Visibility bit from the parent bridge */
+	if (bus->self)
+		dev->crssv_enabled = bus->self->crssv_enabled;
+
 	ret = pcibios_add_device(dev);
 	WARN_ON(ret < 0);
 
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 3840a541a9de..1c222c7c2572 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -354,6 +354,9 @@ struct pci_dev {
 						      user sysfs */
 	unsigned int	clear_retrain_link:1;	/* Need to clear Retrain Link
 						   bit manually */
+	unsigned int	crssv_enabled:1;	/* Configuration Request Retry
+						   Status Software Visibility
+						   enabled on (parent) bridge */
 	unsigned int	d3_delay;	/* D3->D0 transition time in ms */
 	unsigned int	d3cold_delay;	/* D3cold->D0 transition time in ms */
 
-- 
2.25.1




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