[PATCH 3/5] iommu: Add return errno rules to ->attach_dev ops

[Nicolin Chen <nicolinc@nvidia.com>]

Cases like VFIO wish to attach a device to an existing domain that was
not allocated specifically from the device. This raises a condition
where the IOMMU driver can fail the domain attach because the domain and
device are incompatible with each other.

This is a soft failure that can be resolved by using a different domain.

Provide a dedicated errno EINVAL from the IOMMU driver during attach that
the reason attached failed is because of domain incompatability.

VFIO can use this to know attach is a soft failure and it should continue
searching. Otherwise the attach will be a hard failure and VFIO will
return the code to userspace.

Update kdocs first to add rules of return errno to ->attach_dev ops.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
 include/linux/iommu.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index ea30f00dc145..c5d7ec0187c7 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -266,6 +266,17 @@ struct iommu_ops {
 /**
  * struct iommu_domain_ops - domain specific operations
  * @attach_dev: attach an iommu domain to a device
+ *              Rules of its return errno:
+ *               EINVAL  - Exclusively, device and domain are incompatible. Must
+ *                         avoid kernel prints along with this errno. Any EINVAL
+ *                         returned from kAPIs must be converted to ENODEV if it
+ *                         is device-specific, or to some other reasonable errno
+ *                         being listed below
+ *               ENOMEM  - Out of memory
+ *               ENOSPC  - No space left on device
+ *               EBUSY   - Device is attached to a domain and cannot be changed
+ *               ENODEV  - Device specific errors, not able to be attached
+ *              <others> - Treated as ENODEV by the caller. Use is discouraged
  * @detach_dev: detach an iommu domain from a device
  * @map: map a physically contiguous memory region to an iommu domain
  * @map_pages: map a physically contiguous set of pages of the same size to
-- 
2.17.1