Re: [PATCH v13 3/7] genirq: Add mechanism to multiplex a single HW IPI

[Marc Zyngier]

On Tue, 29 Nov 2022 14:24:45 +0000,
Anup Patel <apatel@xxxxxxxxxxxxxxxx> wrote:
> 
> All RISC-V platforms have a single HW IPI provided by the INTC local
> interrupt controller. The HW method to trigger INTC IPI can be through
> external irqchip (e.g. RISC-V AIA), through platform specific device
> (e.g. SiFive CLINT timer), or through firmware (e.g. SBI IPI call).
> 
> To support multiple IPIs on RISC-V, we add a generic IPI multiplexing
> mechanism which help us create multiple virtual IPIs using a single
> HW IPI. This generic IPI multiplexing is inspired from the Apple AIC
> irqchip driver and it is shared by various RISC-V irqchip drivers.
> 
> Signed-off-by: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
> ---
>  include/linux/irq.h  |   3 +
>  kernel/irq/Kconfig   |   5 ++
>  kernel/irq/Makefile  |   1 +
>  kernel/irq/ipi-mux.c | 193 +++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 202 insertions(+)
>  create mode 100644 kernel/irq/ipi-mux.c

[...]

This is finally starting to look acceptable. My only changes are:

diff --git a/kernel/irq/ipi-mux.c b/kernel/irq/ipi-mux.c
index 626c564233e9..1a6ba19b736c 100644
--- a/kernel/irq/ipi-mux.c
+++ b/kernel/irq/ipi-mux.c
@@ -69,6 +69,12 @@ static void ipi_mux_send_mask(struct irq_data *d, const struct cpumask *mask)
 		 */
 		smp_mb__after_atomic();
 
+		/*
+		 * The flag writes must complete before the physical IPI is
+		 * issued to another CPU. This is implied by the control
+		 * dependency on the result of atomic_read() below, which is
+		 * itself already ordered after the vIPI flag write.
+		 */
 		if (!(pending & ibit) && (atomic_read(&icpu->enable) & ibit))
 			ipi_mux_send(cpu);
 	}
@@ -160,7 +166,7 @@ int ipi_mux_create(unsigned int nr_ipi, void (*mux_send)(unsigned int cpu))
 		goto fail_free_cpu;
 	}
 
-	domain = irq_domain_create_simple(fwnode, nr_ipi, 0,
+	domain = irq_domain_create_linear(fwnode, nr_ipi,
 					  &ipi_mux_domain_ops, NULL);
 	if (!domain) {
 		pr_err("unable to add IPI Mux domain\n");

The first hunk preserve an important piece of information about how
delicate the ordering is, while the second only allocates the irqdesc
structures as needed, not upfront.

I'll shortly go over the rest of the irqchip code and can apply the
above myself if there is nothing more to fix.

I've also converted the AIC driver over to this[1], and nothing broke
so far...

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=irq/ipi-mux

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