Re: [PATCH v6 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group

[Joerg Roedel <joro@8bytes.org>]

On Wed, Sep 07, 2022 at 02:10:33PM -0300, Jason Gunthorpe wrote:
> Sure, rust has all sorts of nice things. But the kernel doesn't follow
> rust idioms, and I don't think this is a great place to start
> experimenting with them.

It is actually a great place to start experimenting. The IOMMU
interfaces are rather domain specific and if we get something wrong the
damage is limited to a few callers. There are APIs much more exposed in
the kernel which would be worse for that.

But anyway, I am not insisting on it.

> It has been 3 months since EMEDIUMTYPE was first proposed and 6
> iterations of the series, don't you think it is a bit late in the game
> to try to experiment with rust error handling idioms?

If I am not mistaken, I am the person who gets blamed when crappy IOMMU
code is sent upstream. So it is also up to me to decide in which state
and how close to merging a given patch series is an whether it is
already 'late in the game'.

> So, again, would you be happy with a simple 
> 
>  #define IOMMU_EINCOMPATIBLE_DEVICE xx
> 
> to make it less "re-using random error codes"?

I am wondering if this can be solved by better defining what the return
codes mean and adjust the call-back functions to match the definition.
Something like:

	-ENODEV : Device not mapped my an IOMMU
	-EBUSY  : Device attached and domain can not be changed
	-EINVAL : Device and domain are incompatible
	...

That would be much more intuitive than using something obscure like
EMEDIUMTYPE.

Regards,

	Joerg