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

[Jeff Johnson <quic_jjohnson@quicinc.com>]

On 9/13/2022 1:24 AM, Nicolin Chen wrote:
> 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

It is my understanding that kernel-doc won't preserve your formatting 
without extra directives. See the "Note" after 
<https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#return-values>


>    * @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