Why two irq chips for MSI

[valmiki]

Hi,

In most of the RP drivers, why two irq chips are being used for MSI ?

One at irq_domain_set_info (which uses irq_compose_msi_msg and 
irq_set_affinity methods) and another being registered with struct 
msi_domain_info (which uses irq_mask/irq_unmask methods).

When will each chip be used w.r.t to virq ?

Thanks,
Valmiki

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