Re: [PATCH 3.18-rc4 v10 4/6] irqchip: gic: Introduce plumbing for IPI FIQ

[Jason Cooper]

Daniel,

I've been a bit swamped this cycle and haven't kept as close an eye on
this as I should have. :(  fwiw, it's looking really good.  I have one
question below:

On Wed, Nov 26, 2014 at 04:23:28PM +0000, Daniel Thompson wrote:
> Currently it is not possible to exploit FIQ for systems with a GIC, even if
> the systems are otherwise capable of it. This patch makes it possible
> for IPIs to be delivered using FIQ.
> 
> To do so it modifies the register state so that normal interrupts are
> placed in group 1 and specific IPIs are placed into group 0. It also
> configures the controller to raise group 0 interrupts using the FIQ
> signal. It provides a means for architecture code to define which IPIs
> shall use FIQ and to acknowledge any IPIs that are raised.
> 
> All GIC hardware except GICv1-without-TrustZone support provides a means
> to group exceptions into group 0 and group 1 but the hardware
> functionality is unavailable to the kernel when a secure monitor is
> present because access to the grouping registers are prohibited outside
> "secure world". However when grouping is not available (or in the case
> of early GICv1 implementations is very hard to configure) the code to
> change groups does not deploy and all IPIs will be raised via IRQ.
> 
> It has been tested and shown working on two systems capable of
> supporting grouping (Freescale i.MX6 and STiH416). It has also been
> tested for boot regressions on two systems that do not support grouping
> (vexpress-a9 and Qualcomm Snapdragon 600).
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Jason Cooper <jason@xxxxxxxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Tested-by: Jon Medhurst <tixy@xxxxxxxxxx>
> ---
>  arch/arm/kernel/traps.c         |   5 +-
>  drivers/irqchip/irq-gic.c       | 155 +++++++++++++++++++++++++++++++++++++---
>  include/linux/irqchip/arm-gic.h |   8 +++
>  3 files changed, 158 insertions(+), 10 deletions(-)
...
> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> index 5d72823bc5e9..978e5e48d5c1 100644
> --- a/drivers/irqchip/irq-gic.c
> +++ b/drivers/irqchip/irq-gic.c
...
> +/*
> + * Test which group an interrupt belongs to.
> + *
> + * Returns 0 if the controller does not support grouping.
> + */
> +static int gic_get_group_irq(void __iomem *base, unsigned int hwirq)
> +{
> +	unsigned int grp_reg = hwirq / 32 * 4;
> +	u32 grp_val;
> +
> +	grp_val = readl_relaxed(base + GIC_DIST_IGROUP + grp_reg);
> +
> +	return (grp_val >> (hwirq % 32)) & 1;
> +}
...
> @@ -669,7 +802,11 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>  	dmb(ishst);
>  
>  	/* this always happens on GIC0 */
> -	writel_relaxed(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
> +	softint = map << 16 | irq;
> +	if (gic_get_group_irq(gic_data_dist_base(&gic_data[0]), irq))
> +		softint |= 0x8000;
> +	writel_relaxed(softint,
> +		       gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
>  
>  	bl_migration_unlock();
>  }

Is it worth the code complication to optimize this if the controller
doesn't support grouping?  Maybe set group_enabled at init so the above
would become:

	softint = map << 16 | irq;
	if (group_enabled &&
	    gic_get_group_irq(gic_data_dist_base(&gic_data[0]), irq))
		softint |= 0x8000;
	writel_relaxed(...);

thx,

Jason.
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