[PATCH 2/2] powerpc: add support for MPIC message register API

[Kushwaha Prabhakar-B32579]

> 
> >
> >
> > > > Hi,
> > > >
> > > > I have no comments about coding and architecture. It looks fine.
> > > >
> > > > Only have a query about its use case..
> > > >    "Any application intended to use message interrupt requires to
> > > > know
> > > reg_num because of struct mpic_msgr* mpic_msgr_get(unsigned int
> > > reg_num) API"
> > > >
> > > > It will be good to search available unit internally and provide
> > > > its
> > > pointer. It will make application more flexible.
> > > >
> > >
> > > The problem is that you fundamentally cannot implement an allocator
> > > for MSG registers if you're going to communicate with another kernel
> > > (how would both kernels' allocators be synchronized?). So the
> > > message register allocation must be decided at design time, not run
> time.
> > >
> >
> > I agree with you..  It is true while communicating with another kernel.
> > But message interrupts can be used by different independent drivers
> within same kernel. For eg. PCIe and Ethernet driver.
> > As per current design both drivers needs to be in sync before
> requesting any message unit for avoiding any conflict. As these drivers
> are completely independent. It is very difficult.
> >
> > Can it be possible to provide new API to take care it.
> 
> Do you have a real use case in mind where these message registers (not
> MSIs) are used internally in this manner?

Yes, we use for PCIe host/agent test case scenario.
Host usage message register to interrupt Agent...
Agent uses message register to generate irq_out (automatically generate MSI) to interrupt master. Please see RM for more details about irq_out
 

Note: PCIe host/agent test scenario is used internally and we are working on pushing it out..
 
> Perhaps an allocator could be added in the same patchset that adds such a
> user.
> 

Yaa. It can be done. Otherwise module has to query each message unit for its availability. 

--Prabhakar