[PATCH v4 1/7] xen/arm: Improve readability of check for registered devices

[Stewart Hildebrand <stewart.hildebrand@amd.com>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Improve readability of check for devices already registered with the SMMU with
legacy mmu-masters DT bindings by using is_protected.

There are 2 device tree bindings for registering a device with the SMMU:
* mmu-masters (legacy, SMMUv1/2 only)
* iommus

A device tree may include both mmu-masters and iommus properties (although it is
unnecessary to do so). When a device appears in the mmu-masters list,
np->is_protected and dev->iommu_fwspec both get set by the SMMUv1/2 driver. The
function iommu_add_dt_device() is subsequently invoked for devices that have an
iommus specification.

The check as it was before this patch:

  if ( dev_iommu_fwspec_get(dev) )
      return 0;

and the new check:

  if ( dt_device_is_protected(np) )
      return 0;

are guarding against the same corner case: when a device has both mmu-masters
and iommus specifications in the device tree. The is_protected naming is more
descriptive.

If np->is_protected is not set (i.e. false), but dev->iommu_fwspec is set, it is
an error condition, so return an error in this case.

Expand the comment to further clarify the corner case.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
---
v3->v4:
* new patch: this change was split from ("xen/arm: Move is_protected flag to struct device")
---
 xen/drivers/passthrough/device_tree.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/xen/drivers/passthrough/device_tree.c b/xen/drivers/passthrough/device_tree.c
index 1c32d7b50cce..d9b63da7260a 100644
--- a/xen/drivers/passthrough/device_tree.c
+++ b/xen/drivers/passthrough/device_tree.c
@@ -141,12 +141,17 @@ int iommu_add_dt_device(struct dt_device_node *np)
         return -EINVAL;
 
     /*
-     * The device may already have been registered. As there is no harm in
-     * it just return success early.
+     * Devices that appear in the legacy mmu-masters list may have already been
+     * registered with the SMMU. In case a device has both a mmu-masters entry
+     * and iommus property, there is no need to register it again. In this case
+     * simply return success early.
      */
-    if ( dev_iommu_fwspec_get(dev) )
+    if ( dt_device_is_protected(np) )
         return 0;
 
+    if ( dev_iommu_fwspec_get(dev) )
+        return -EEXIST;
+
     /*
      * According to the Documentation/devicetree/bindings/iommu/iommu.txt
      * from Linux.
-- 
2.40.1