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

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Date:   Thu, 06 Sep 2018 02:45:26 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     Christoph Hellwig <hch@...radead.org>, anup@...infault.org
CC:     aou@...s.berkeley.edu, daniel.lezcano@...aro.org,
        tglx@...utronix.de, jason@...edaemon.net, marc.zyngier@....com,
        atish.patra@....com, Christoph Hellwig <hch@...radead.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject:     Re: [RFC PATCH 1/5] RISC-V: Make IPI triggering flexible

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.