Re: [RFC PATCH 1/5] RISC-V: Make IPI triggering flexible

[Palmer Dabbelt]

On Tue, 04 Sep 2018 11:50:02 PDT (-0700), Christoph Hellwig wrote:
> On Tue, Sep 04, 2018 at 06:15:10PM +0530, Anup Patel wrote:
> > The mechanism to trigger IPI is generally part of interrupt-controller
> > driver for various architectures. On RISC-V, we have an option to trigger
> > IPI using SBI or SOC vendor can implement RISC-V CPU where IPI will be
> > triggered using SOC interrupt-controller (e.g. custom PLIC).
>
> Which is exactly what we want to avoid, and should not make it easy.
>
> The last thing we need is non-standard whacky IPI mechanisms, and
> that is why we habe SBI calls for it.  I think we should simply
> stat that if an RISC-V cpu design bypasse the SBI for no good reason
> we'll simply not support it.

I agree.  Hiding this sort of stuff is the whole point of the SBI.

Anup: do you have some concrete reason for trying to avoid the SBI?  If it's 
just to add non-standard interrupt controllers then I don't think that's a 
sufficient reason, as you can just add support for whatever the non-standard 
interrupt mechanism is in the SBI implementation -- that's what we're doing 
with BBL's CLINT driver, though there's not a whole lot of wackiness there so 
at least the SBI implementation is pretty small.

> So NAK for this patch.

Certainly without a compelling reason, and even then I'd only want to take some 
standard interrupt controller -- for example, the CLIC (or whatever the result 
of the fast interrupts task group is called) could be a viable option.  Even 
with a standard interrupt controller, we'd need a really compelling reason to 
do so.