irqchip/riscv-intc: Add empty irq_eoi() for chained irq handlers

We add empty irq_eoi() in RISC-V INTC driver for child irqchip
drivers (such as PLIC, SBI IPI, CLINT, APLIC, IMSIC, etc) which
implement chained handlers for parent per-HART local interrupts.
This hels us avoid unnecessary mask/unmask of per-HART local
interrupts at the time of handling interrupts.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230328035223.1480939-8-apatel@ventanamicro.com

Author: avpatel

drivers/irqchip/irq-riscv-intc.c

@@ -46,10 +46,27 @@ static void riscv_intc_irq_unmask(struct irq_data *d)
 	csr_set(CSR_IE, BIT(d->hwirq));
 }
 
+static void riscv_intc_irq_eoi(struct irq_data *d)
+{
+	/*
+	 * The RISC-V INTC driver uses handle_percpu_devid_irq() flow
+	 * for the per-HART local interrupts and child irqchip drivers
+	 * (such as PLIC, SBI IPI, CLINT, APLIC, IMSIC, etc) implement
+	 * chained handlers for the per-HART local interrupts.
+	 *
+	 * In the absence of irq_eoi(), the chained_irq_enter() and
+	 * chained_irq_exit() functions (used by child irqchip drivers)
+	 * will do unnecessary mask/unmask of per-HART local interrupts
+	 * at the time of handling interrupts. To avoid this, we provide
+	 * an empty irq_eoi() callback for RISC-V INTC irqchip.
+	 */
+}
+
 static struct irq_chip riscv_intc_chip = {
 	.name = "RISC-V INTC",
 	.irq_mask = riscv_intc_irq_mask,
 	.irq_unmask = riscv_intc_irq_unmask,
+	.irq_eoi = riscv_intc_irq_eoi,
 };
 
 static int riscv_intc_domain_map(struct irq_domain *d, unsigned int irq,