RE: [patch V2 02/33] genirq/msi: Provide struct msi_parent_ops

[Thomas Gleixner]

On Wed, Nov 23 2022 at 07:57, Kevin Tian wrote:
>> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> + * One solution is to let the root domain handle the initialization that's
>> + * why there is the @domain and the @msi_parent_domain pointer.
>  
> This is the part which I don't quite understand (sorry with limited knowledge
> in this area).
>
> In concept a hierarchical model has restrictions added up when moving
> down to lower layers i.e. presumably the root domain decides the minimal
> supported capabilities. In this case there is no need of a real parent pointer
> as long as every domain in the stack incrementally adds its restrictions to
> info->flags.
>
> I can see why this is required for x86 given that MULTI_MSI is supported
> only with IR. and we cannot make vector domain inclusively claiming
> MULTI_MSI since it's completely broken when the vector domain becomes
> the parent itself, in absence of IR.
>
> Just be curious whether this intermediate-parent-deciding-restrictions
> is generic instead of x86 specific, e.g. is it possible to have a 4-layers
> hierarchy where the root parent wants to check both two intermediate
> parents?

Sure. Nothing prevents you from doing so:

dom4:
    .init... = dom4_init

dom4_init()
    do_stuff()
    invoke parent init

dom3:
    .init... = parent_init

dom2:
    .init... = dom2_init

dom2_init()
    do_stuff()
    invoke parent init

....

See?

Thanks,

        tglx