[PATCH v2 1/2] PCI/ATS: Add a helper function to configure ATS STU of a PF

[Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>]

As per PCI specification (PCI Express Base Specification Revision
6.0, Section 10.5) both PF and VFs of a PCI EP are permitted to be enabled
independently for ATS capability, however the STU(Smallest Translation
Unit) is shared between PF and VFs. For VFs, it is hardwired to Zero and
the associated PF's value applies to VFs.

In the current code, the STU is being configured while enabling the PF ATS.
Hence, it is not able to enable ATS for VFs, if it is not enabled on the
associated PF already.

Adding a function pci_ats_stu_configure(), which can be called to
configure the STU during PF enumeration.
Latter enumerations of VFs can successfully enable ATS independently.

Signed-off-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
---
 drivers/pci/ats.c       | 33 +++++++++++++++++++++++++++++++--
 include/linux/pci-ats.h |  3 +++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
index f9cc2e10b676..1611bfa1d5da 100644
--- a/drivers/pci/ats.c
+++ b/drivers/pci/ats.c
@@ -46,6 +46,35 @@ bool pci_ats_supported(struct pci_dev *dev)
 }
 EXPORT_SYMBOL_GPL(pci_ats_supported);
 
+/**
+ * pci_ats_stu_configure - Configure STU of a PF.
+ * @dev: the PCI device
+ * @ps: the IOMMU page shift
+ *
+ * Returns 0 on success, or negative on failure.
+ */
+int pci_ats_stu_configure(struct pci_dev *dev, int ps)
+{
+	u16 ctrl;
+
+	if (dev->ats_enabled || dev->is_virtfn)
+		return 0;
+
+	if (!pci_ats_supported(dev))
+		return -EINVAL;
+
+	if (ps < PCI_ATS_MIN_STU)
+		return -EINVAL;
+
+	dev->ats_stu = ps;
+	pci_read_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, &ctrl);
+	ctrl |= PCI_ATS_CTRL_STU(dev->ats_stu - PCI_ATS_MIN_STU);
+	pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(pci_ats_stu_configure);
+
 /**
  * pci_enable_ats - enable the ATS capability
  * @dev: the PCI device
@@ -68,8 +97,8 @@ int pci_enable_ats(struct pci_dev *dev, int ps)
 		return -EINVAL;
 
 	/*
-	 * Note that enabling ATS on a VF fails unless it's already enabled
-	 * with the same STU on the PF.
+	 * Note that enabling ATS on a VF fails unless it's already
+	 * configured with the same STU on the PF.
 	 */
 	ctrl = PCI_ATS_CTRL_ENABLE;
 	if (dev->is_virtfn) {
diff --git a/include/linux/pci-ats.h b/include/linux/pci-ats.h
index df54cd5b15db..7d62a92aaf23 100644
--- a/include/linux/pci-ats.h
+++ b/include/linux/pci-ats.h
@@ -8,6 +8,7 @@
 /* Address Translation Service */
 bool pci_ats_supported(struct pci_dev *dev);
 int pci_enable_ats(struct pci_dev *dev, int ps);
+int pci_ats_stu_configure(struct pci_dev *dev, int ps);
 void pci_disable_ats(struct pci_dev *dev);
 int pci_ats_queue_depth(struct pci_dev *dev);
 int pci_ats_page_aligned(struct pci_dev *dev);
@@ -16,6 +17,8 @@ static inline bool pci_ats_supported(struct pci_dev *d)
 { return false; }
 static inline int pci_enable_ats(struct pci_dev *d, int ps)
 { return -ENODEV; }
+static inline int pci_ats_stu_configure(struct pci_dev *d, int ps)
+{ return -ENODEV; }
 static inline void pci_disable_ats(struct pci_dev *d) { }
 static inline int pci_ats_queue_depth(struct pci_dev *d)
 { return -ENODEV; }
-- 
2.38.1