[PATCH v2 13/13] PCI/CXL: Refuse an SBR of a CXL DPort unless authorized

CXL r4.0 sec 8.1.5.2 Table 8-32 describes a bit that makes the SBR bit
in Bridge Control take effect at all, so a Port left at the firmware
default is one the reset must not silently unmask.

Add cxl_sbr_allowed() to answer if SBR is allowed: check if a Port whose
Unmask SBR is already set needs no consent, as it happens with cxl_bus
reset.

Signed-off-by: Fabio M. De Francesco <[email protected]>
---
 drivers/pci/pci.c | 26 ++++++++++++++++++++++++++
 drivers/pci/pci.h |  1 +
 2 files changed, 27 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index eedd516f8484..08873ea3957b 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4881,6 +4881,11 @@ static int cxl_sbr_prepare(struct pci_dev *bridge, u16 dvsec,
 {
 	int rc;
 
+	if (action == CXL_SBR_OFFLINE_AND_UNBIND && !cxl_sbr_allowed(bridge)) {
+		pci_info(bridge, "SBR masked, write 1 to cxl_unmask_sbr to allow a bus reset\n");
+		return -ENOTTY;
+	}
+
 	/*
 	 * CXL_SBR_UNBIND: the link is already down, so offlining the regions'
 	 * memory would take the reads that page migration performs as a machine
@@ -5138,6 +5143,27 @@ static bool cxl_sbr_masked(struct pci_dev *dev)
 	return true;
 }
 
+/*
+ * cxl_sbr_allowed - whether an SBR of a CXL Downstream Port may go ahead
+ * @dev: Downstream Port to test
+ *
+ * Per CXL r4.0 sec 8.1.5.2 Table 8-32 the SBR bit in a CXL Port's Bridge
+ * Control register has no effect while the Port's Unmask SBR bit is clear, and
+ * sec 9.12.3 says System Firmware may leave it clear "to prevent CXL-unaware
+ * PCIe software from resetting the device and the link". A Port that already
+ * has it set needs no further permission; otherwise unmasking it takes the
+ * administrator's consent, given by writing 1 to the Port's cxl_unmask_sbr.
+ *
+ * Return: true if the reset paths may unmask and generate an SBR of @dev.
+ */
+bool cxl_sbr_allowed(struct pci_dev *dev)
+{
+	if (!cxl_port_dvsec(dev))
+		return false;
+
+	return dev->cxl_unmask_sbr || !cxl_sbr_masked(dev);
+}
+
 static int pci_reset_bus_function(struct pci_dev *dev, bool probe)
 {
 	struct pci_dev *bridge = pci_upstream_bridge(dev);
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index b6d873b077ed..bea05acc58f0 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -248,6 +248,7 @@ int __pci_bridge_secondary_bus_reset(struct pci_dev *dev,
 				     enum cxl_sbr_region_action action);
 bool is_cxl_dport(struct pci_dev *dev);
 u16 cxl_port_dvsec(struct pci_dev *dev);
+bool cxl_sbr_allowed(struct pci_dev *dev);
 int pci_bus_error_reset(struct pci_dev *dev);
 int pci_try_reset_bridge(struct pci_dev *bridge);
 
-- 
2.55.0