> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Sent: Monday, November 21, 2022 10:38 PM > > +/** > + * msi_parent_init_dev_msi_info - Delegate initialization of device MSI info > down > + * in the domain hierarchy > + * @dev: The device for which the domain should be created > + * @domain: The domain in the hierarchy this op is being called on > + * @msi_parent_domain: The IRQ_DOMAIN_FLAG_MSI_PARENT > domain for the child to > + * be created > + * @msi_child_info: The MSI domain info of the > IRQ_DOMAIN_FLAG_MSI_DEVICE > + * domain to be created > + * > + * Return: true on success, false otherwise > + * > + * This is the most complex problem of per device MSI domains and the > + * underlying interrupt domain hierarchy: > + * > + * The device domain to be initialized requests the broadest feature set > + * possible and the underlying domain hierarchy puts restrictions on it. > + * > + * That's trivial for a simple parent->child relationship, but it gets > + * interesting with an intermediate domain: root->parent->child. The > + * intermediate 'parent' can expand the capabilities which the 'root' > + * domain is providing. So that creates a classic hen and egg problem: > + * Which entity is doing the restrictions/expansions? > + * > + * 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? Thanks Kevin
