Re: [PATCH v8 7/7] iommu/riscv: Paging domain support

[Robin Murphy]

On 08/08/2024 5:40 pm, Easwar Hariharan wrote:
> On 8/8/2024 6:14 AM, Jason Gunthorpe wrote:
> > On Tue, Aug 06, 2024 at 12:24:40PM -0700, Easwar Hariharan wrote:
> > > On 6/14/2024 10:27 PM, Tomasz Jeznach wrote:
> > >
> > > > @@ -856,7 +1473,7 @@ static struct iommu_domain riscv_iommu_identity_domain = {
> > > >   
> > > >   static int riscv_iommu_device_domain_type(struct device *dev)
> > > >   {
> > > > -	return IOMMU_DOMAIN_IDENTITY;
> > > > +	return 0;
> > > >   }
> > >
> > > <snip>
> > > Sorry for the drive by comment, I just happen to be in the nearby code
> > > context.
> > >
> > > Nit: It may be better to use IOMMU_DOMAIN_BLOCKED here for readability
> > > rather than the bare value.
> >
> > It is weird and confusing, but 0 means "I have nothing to add" not
> > BLOCKED.
> >
> > You can't return BLOCKED from this op right now..
> >
> > Jason
>
> Ok, that's weird and confusing as you say. I went back and looked at the
> kerneldoc for iommu_ops and it IS called out, but it's confusing that a
> function named get_default_domain_type() can return a value that's a
> valid domain type but isn't treated as one.
>
> This was useful to fill in my mental model, thanks!

Yeah, it's perhaps a bit subtle, but the context is that not all domain 
types are valid *default domain* types, those are strictly only DMA or 
identity, because the other types would be nonsensical. If a driver 
really wanted to completely block a device for some specific reason, it 
should do that by refusing to attach it to the default domain at all.

Thanks,
Robin.