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From:   Anup Patel <anup@brainfault.org>
Date:   Mon, 10 Sep 2018 19:59:15 +0530
Message-ID: <CAAhSdy1be_ibiowifBdgBZjbHuFU=CR4rZXQDwQghVYkex5bMg@mail.gmail.com>
Subject: Re: [PATCH v2 3/5] irqchip: RISC-V Local Interrupt Controller Driver
To:     Christoph Hellwig <hch@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Palmer Dabbelt <palmer@sifive.com>,
        Jason Cooper <jason@lakedaemon.net>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>,
        Atish Patra <atish.patra@wdc.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        linux-riscv@lists.infradead.org
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On Mon, Sep 10, 2018 at 7:19 PM, Christoph Hellwig <hch@infradead.org> wrote:
> On Mon, Sep 10, 2018 at 03:45:42PM +0200, Thomas Gleixner wrote:
>> > He has an irqchip that is called from the RISC-V exception handler
>> > when the interrupt flag is set in scause and then dispatches to one
>> > of:  IPI, timer, actual irqchip.
>>
>> So the per cpu timer is the only per cpu interrupt and that thing is used
>> unconditionally, right?
>
> Yes.  external is chained and IPI is still handled explicitly.

On riscv64, there are 64 local interrupts (i.e. per-CPU interrupts).

Three of these local interrupts have clearly defined use:
1. Software interrupt (inter-processor interrupt)
2. External interrupt (interrupt from PLIC)
3. Timer interrupt (interrupt from per-CPU timer)

Other local interrupts are available for future use.

Taking inspiration from ARM world, I had give quite a few
examples how these RISC-V local interrupts can be used
for other purposes:
1. per-CPU interrupt for per-HART performance monitoring unit
2. interrupt controller virtualizaton extension can use per-CPU
interrupts for managing resources (just like ARM GICv2/v3 virt
extensions)
3. bus errors can be reported as per-CPU interrupts

Considering above, it is better to have a distinct irqchip and
irq_domain for all local interrupts (just like this patch).

Regards,
Anup