Re: [PATCH v2 3/7] irqchip/apple-aic: Add Fast IPI support

[Marc Zyngier <maz@kernel.org>]

On Thu, 24 Feb 2022 13:07:37 +0000,
Hector Martin <marcan@marcan.st> wrote:
> 
> The newer AICv2 present in t600x SoCs does not have legacy IPI support
> at all. Since t8103 also supports Fast IPIs, implement support for this
> first. The legacy IPI code is left as a fallback, so it can be
> potentially used by older SoCs in the future.
> 
> The vIPI code is shared; only the IPI firing/acking bits change for Fast
> IPIs.
> 
> Signed-off-by: Hector Martin <marcan@marcan.st>
> ---
>  drivers/irqchip/irq-apple-aic.c | 122 ++++++++++++++++++++++++++++----
>  1 file changed, 109 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-apple-aic.c b/drivers/irqchip/irq-apple-aic.c
> index 38091ebb9403..613e0ebdabdc 100644
> --- a/drivers/irqchip/irq-apple-aic.c
> +++ b/drivers/irqchip/irq-apple-aic.c
> @@ -24,7 +24,7 @@
>   * - Default "this CPU" register view and explicit per-CPU views
>   *
>   * In addition, this driver also handles FIQs, as these are routed to the same
> - * IRQ vector. These are used for Fast IPIs (TODO), the ARMv8 timer IRQs, and
> + * IRQ vector. These are used for Fast IPIs, the ARMv8 timer IRQs, and
>   * performance counters (TODO).
>   *
>   * Implementation notes:
> @@ -52,9 +52,11 @@
>  #include <linux/irqchip.h>
>  #include <linux/irqchip/arm-vgic-info.h>
>  #include <linux/irqdomain.h>
> +#include <linux/jump_label.h>
>  #include <linux/limits.h>
>  #include <linux/of_address.h>
>  #include <linux/slab.h>
> +#include <asm/cputype.h>
>  #include <asm/exception.h>
>  #include <asm/sysreg.h>
>  #include <asm/virt.h>
> @@ -106,7 +108,6 @@
>  
>  /*
>   * IMP-DEF sysregs that control FIQ sources
> - * Note: sysreg-based IPIs are not supported yet.
>   */
>  
>  /* Core PMC control register */
> @@ -155,6 +156,10 @@
>  #define SYS_IMP_APL_UPMSR_EL1		sys_reg(3, 7, 15, 6, 4)
>  #define UPMSR_IACT			BIT(0)
>  
> +/* MPIDR fields */
> +#define MPIDR_CPU(x)			MPIDR_AFFINITY_LEVEL(x, 0)
> +#define MPIDR_CLUSTER(x)		MPIDR_AFFINITY_LEVEL(x, 1)
> +
>  #define AIC_NR_FIQ		4
>  #define AIC_NR_SWIPI		32
>  
> @@ -173,11 +178,44 @@
>  #define AIC_TMR_EL02_PHYS	AIC_TMR_GUEST_PHYS
>  #define AIC_TMR_EL02_VIRT	AIC_TMR_GUEST_VIRT
>  
> +DEFINE_STATIC_KEY_TRUE(use_fast_ipi);
> +
> +struct aic_info {
> +	int version;
> +
> +	/* Features */
> +	bool fast_ipi;
> +};
> +
> +static const struct aic_info aic1_info = {
> +	.version	= 1,
> +};
> +
> +static const struct aic_info aic1_fipi_info = {
> +	.version	= 1,
> +
> +	.fast_ipi	= true,
> +};
> +
> +static const struct of_device_id aic_info_match[] = {
> +	{
> +		.compatible = "apple,t8103-aic",
> +		.data = &aic1_fipi_info,
> +	},
> +	{
> +		.compatible = "apple,aic",
> +		.data = &aic1_info,
> +	},
> +	{}
> +};
> +
>  struct aic_irq_chip {
>  	void __iomem *base;
>  	struct irq_domain *hw_domain;
>  	struct irq_domain *ipi_domain;
>  	int nr_hw;
> +
> +	struct aic_info info;
>  };
>  
>  static DEFINE_PER_CPU(uint32_t, aic_fiq_unmasked);
> @@ -386,8 +424,12 @@ static void __exception_irq_entry aic_handle_fiq(struct pt_regs *regs)
>  	 */
>  
>  	if (read_sysreg_s(SYS_IMP_APL_IPI_SR_EL1) & IPI_SR_PENDING) {
> -		pr_err_ratelimited("Fast IPI fired. Acking.\n");
> -		write_sysreg_s(IPI_SR_PENDING, SYS_IMP_APL_IPI_SR_EL1);
> +		if (static_branch_likely(&use_fast_ipi)) {
> +			aic_handle_ipi(regs);
> +		} else {
> +			pr_err_ratelimited("Fast IPI fired. Acking.\n");
> +			write_sysreg_s(IPI_SR_PENDING, SYS_IMP_APL_IPI_SR_EL1);
> +		}
>  	}
>  
>  	if (TIMER_FIRING(read_sysreg(cntp_ctl_el0)))
> @@ -563,6 +605,22 @@ static const struct irq_domain_ops aic_irq_domain_ops = {
>   * IPI irqchip
>   */
>  
> +static void aic_ipi_send_fast(int cpu)
> +{
> +	u64 mpidr = cpu_logical_map(cpu);
> +	u64 my_mpidr = read_cpuid_mpidr();
> +	u64 cluster = MPIDR_CLUSTER(mpidr);
> +	u64 idx = MPIDR_CPU(mpidr);
> +
> +	if (MPIDR_CLUSTER(my_mpidr) == cluster)
> +		write_sysreg_s(FIELD_PREP(IPI_RR_CPU, idx),
> +			       SYS_IMP_APL_IPI_RR_LOCAL_EL1);
> +	else
> +		write_sysreg_s(FIELD_PREP(IPI_RR_CPU, idx) | FIELD_PREP(IPI_RR_CLUSTER, cluster),
> +			       SYS_IMP_APL_IPI_RR_GLOBAL_EL1);
> +	isb();

I think you have an ordering issue with this, see below.

> +}
> +
>  static void aic_ipi_mask(struct irq_data *d)
>  {
>  	u32 irq_bit = BIT(irqd_to_hwirq(d));
> @@ -588,8 +646,12 @@ static void aic_ipi_unmask(struct irq_data *d)
>  	 * If a pending vIPI was unmasked, raise a HW IPI to ourselves.
>  	 * No barriers needed here since this is a self-IPI.
>  	 */
> -	if (atomic_read(this_cpu_ptr(&aic_vipi_flag)) & irq_bit)
> -		aic_ic_write(ic, AIC_IPI_SEND, AIC_IPI_SEND_CPU(smp_processor_id()));
> +	if (atomic_read(this_cpu_ptr(&aic_vipi_flag)) & irq_bit) {
> +		if (static_branch_likely(&use_fast_ipi))
> +			aic_ipi_send_fast(smp_processor_id());
> +		else
> +			aic_ic_write(ic, AIC_IPI_SEND, AIC_IPI_SEND_CPU(smp_processor_id()));
> +	}
>  }
>  
>  static void aic_ipi_send_mask(struct irq_data *d, const struct cpumask *mask)
> @@ -617,8 +679,12 @@ static void aic_ipi_send_mask(struct irq_data *d, const struct cpumask *mask)
>  		smp_mb__after_atomic();
>  
>  		if (!(pending & irq_bit) &&
> -		    (atomic_read(per_cpu_ptr(&aic_vipi_enable, cpu)) & irq_bit))
> -			send |= AIC_IPI_SEND_CPU(cpu);
> +		    (atomic_read(per_cpu_ptr(&aic_vipi_enable, cpu)) & irq_bit)) {
> +			if (static_branch_likely(&use_fast_ipi))
> +				aic_ipi_send_fast(cpu);

OK, this is suffering from the same issue that GICv3 has, which is
that memory barriers don't provide order against sysregs. You need a
DSB for that, which is a pain. Something like this:

diff --git a/drivers/irqchip/irq-apple-aic.c b/drivers/irqchip/irq-apple-aic.c
index 602c8b274170..579f22b72339 100644
--- a/drivers/irqchip/irq-apple-aic.c
+++ b/drivers/irqchip/irq-apple-aic.c
@@ -736,6 +736,15 @@ static void aic_ipi_send_fast(int cpu)
 	u64 cluster = MPIDR_CLUSTER(mpidr);
 	u64 idx = MPIDR_CPU(mpidr);
 
+	/*
+	 * A DSB is required here in to order prior writes to memory
+	 * with system register accesses having a side effect
+	 * (matching the behaviour of the IPI registers). Make sure we
+	 * only order stores with in the IS domain, keeping as light
+	 * as possible.
+	 */
+	dsb(ishst);
+
 	if (MPIDR_CLUSTER(my_mpidr) == cluster)
 		write_sysreg_s(FIELD_PREP(IPI_RR_CPU, idx),
 			       SYS_IMP_APL_IPI_RR_LOCAL_EL1);

Thanks,

	M.

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

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