[PATCH v4 4/4] PCI/ATS: Mandate checking pci_ats_supported() before pci_prepare_ats()

[Pranjal Shrivastava <praan@google.com>]

Currently, pci_prepare_ats() internally calls pci_ats_supported() and
returns -EINVAL if the device does not support ATS. While this provides
a silent safety check, it conflates support detection with configuration.

Update pci_prepare_ats() to wrap the internal pci_ats_supported check in
a WARN_ON(). This mandates all callers to call pci_prepare_ats() only if
the function supports ATS.

Update the function documentation to mention that callers must verify
ATS support (via pci_ats_supported()) before calling pci_prepare_ats().

Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Suggested-by: Baolu Lu <baolu.lu@linux.intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Pranjal Shrivastava <praan@google.com>
---
 drivers/pci/ats.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
index 9cb23780093d..f1434f86ac40 100644
--- a/drivers/pci/ats.c
+++ b/drivers/pci/ats.c
@@ -56,7 +56,9 @@ EXPORT_SYMBOL_GPL(pci_ats_supported);
  * @ps: the IOMMU page shift
  *
  * This must be done by the IOMMU driver on the PF before any VFs are created to
- * ensure that the VF can have ATS enabled.
+ * ensure that the VF can have ATS enabled. Callers must verify that ATS is
+ * supported by the device (e.g. via pci_ats_supported()) before calling this
+ * function.
  *
  * Returns 0 on success, or negative on failure.
  */
@@ -64,7 +66,7 @@ int pci_prepare_ats(struct pci_dev *dev, int ps)
 {
 	u16 ctrl;
 
-	if (!pci_ats_supported(dev))
+	if (WARN_ON(!pci_ats_supported(dev)))
 		return -EINVAL;
 
 	if (WARN_ON(dev->ats_enabled))
-- 
2.55.0.1003.g10538fe699-goog