Re: [PATCH v11 3/7] genirq: Add mechanism to multiplex a single HW IPI

From: Anup Patel <apatel@ventanamicro.com>
To: Marc Zyngier <maz@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	 Thomas Gleixner <tglx@linutronix.de>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	 Atish Patra <atishp@atishpatra.org>,
	Alistair Francis <Alistair.Francis@wdc.com>,
	 Anup Patel <anup@brainfault.org>,
	linux-riscv@lists.infradead.org,  linux-kernel@vger.kernel.org
Subject: Re: [PATCH v11 3/7] genirq: Add mechanism to multiplex a single HW IPI
Date: Sat, 26 Nov 2022 21:30:31 +0530	[thread overview]
Message-ID: <CAK9=C2V351pAjjP4pkrOuUDzOv1xBKO7+iJvPJgQ2swoBFo=8w@mail.gmail.com> (raw)
In-Reply-To: <87sfi5u6wx.wl-maz@kernel.org>

On Sat, Nov 26, 2022 at 7:58 PM Marc Zyngier <maz@kernel.org> wrote:
>
> On Sat, 26 Nov 2022 13:31:46 +0000,
> Anup Patel <apatel@ventanamicro.com> wrote:
> >
> > On Sat, Nov 26, 2022 at 6:12 PM Marc Zyngier <maz@kernel.org> wrote:
> > >
> > > On Mon, 14 Nov 2022 09:39:00 +0000,
> > > Anup Patel <apatel@ventanamicro.com> wrote:
> > > >
> > > > +struct ipi_mux_control {
> > > > +     void                            *data;
> > > > +     unsigned int                    nr;
> > >
> > > Honestly, I think we can get rid of this. The number of IPIs Linux
> > > uses is pretty small, and assuming a huge value (like 32) would be
> > > enough. It would save looking up this value on each IPI handling.
> >
> > I had kept in-case some driver wanted to create fewer (< 32)
> > muxed IPIs.
>
> I'm fine with being able to specifying the max, but I'm not sure there
> is a need to keep track of it any further. Certainly, the overhead of
> loading this value on each IPI could be removed. On most architecture,
> for_each_set_bit() and co and better optimised with a fixed number of
> bits.

Okay, I will update like you suggested.

>
> > > > +static const struct irq_chip ipi_mux_chip = {
> > > > +     .name           = "IPI Mux",
> > > > +     .irq_mask       = ipi_mux_mask,
> > > > +     .irq_unmask     = ipi_mux_unmask,
> > > > +     .ipi_send_mask  = ipi_mux_send_mask,
> > > > +};
> > >
> > > I really think this could either be supplied by the irqchip, or
> > > somehow patched to avoid the pointless imux->ops->ipi_mux_send
> > > indirection. Pointer chasing hurts.
> >
> > Once we remove ipi_mux_pre/post_handle() callbacks, the
> > "ops" will be pointless and we will be able to remove one level
> > of indirection here.
> >
> > We certainly need a mux irqchip to implement the
> > mask/unmask semantics for muxed IPIs.
>
> I'm not disputing that last point.
>
> > > > +/**
> > > > + * ipi_mux_create - Create virtual IPIs multiplexed on top of a single
> > > > + * parent IPI.
> > > > + * @parent_virq:     virq of the parent per-CPU IRQ
> > > > + * @nr_ipi:          number of virtual IPIs to create. This should
> > > > + *                   be <= BITS_PER_TYPE(int)
> > > > + * @ops:             multiplexing operations for the parent IPI
> > > > + * @data:            opaque data used by the multiplexing operations
> > >
> > > What is the use for data? If anything, that data should be passed via
> > > the mux interrupt. But the whole point of this is to make the mux
> > > invisible. So this whole 'data' business is a mystery to me.
> >
> > This is added only to pass back driver data in ipi_mux_send().
>
> Again, what is the purpose of such data? If you need per-interrupt
> data, this should be provided by the requester of the interrupt.

Currently, the irqchip drivers that we care about don't need this
data pointer so I will remove it. If required we can add it in future.

>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.

Thanks,
Anup

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