Re: [PATCH 3/6] irqchip: RISC-V Local Interrupt Controller Driver

From: Christoph Hellwig
Date: Thu Aug 02 2018 - 12:03:12 EST

On Thu, Aug 02, 2018 at 11:35:43AM +0200, Thomas Gleixner wrote:
> On Thu, 2 Aug 2018, Christoph Hellwig wrote:
> > The cpu local interrupt handling, which was irq-riscv-intc.c in this
> > series and has been moved to arch/riscv/kernel/irq.c in my new series
> > is split over a few control registers (CSRs in RISC-V speak):
> >
> > The exception handler, which includes the delivery of interrupts to
> > the CPU is set up using the stvec CSR (Section 4.1.4). The vector mode
> > mentioned there is not supported by Linux (and not by any hardware known
> > to me), so ignore it.
> And even if it would be available then it would just avoid the software
> decoding of the cause register. So no fundamental difference.
> > Once an exception has been triggered Linux reads the scause CSR
> > (Section 4.1.10) to check the exception cause. If the interrupt
> > bit is set we have three possible exception causes that matter for
> > the kernel: Supervisor software interrupt, Supervisor timer interrupt,
> > Supervisor external interrupt (ignore the user versions, I'm not even
> > sure they are implementable, and they certainly are not at the moment).
> Yeah. I would upfront declare the user stuff broken and not supported.
> > To enable / disable any of these logical interrupt sources the sie
> > CSR (Section 4.1.5) has a bit for each kind thast can be set/cleared.
> >
> > Also there is the sip CSR (also section 4.1.5) which tells if any of those
> > is pending at the moment.
> So that's the low level per cpu interrupt/exception distribution mechanism,
> i.e. a distinct per cpu 'vector' space with fixed functionality. It does
> not make sense to actually handle that as an irq chip. It has absolutely no
> relevance. The software interrupts are enabled when the CPU is started and
> the external ones as well as they are gated by the PLIC.
> The only thing which might need to access the enable register is the local
> timer interrupt. That really does not require an extra irq chip as the
> enable/disable is really just at cpu up/down time and the magic happens on
> the local CPU so no smp functional call hackery is required.
> The PLIC is the beast which wants a proper irqdomain/irqchip
> implementation.

And that is exactly what I've done in the repost.

Local interrupt handling:

PLIC driver:


I need to polish the DT binding a little more and will repost later today.

> Thanks,
> tglx
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