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

[Daniel Thompson]

On 26/11/14 17:42, Jason Cooper wrote:
> 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'll treat that good news. Thanks.

> 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(...);

No objections.

However given this code always calls gic_get_group_irq() with irq < 16
we might be able to do better even than this. The lower 16-bits of
IGROUP[0] are constant after boot so if we keep a shadow copy around
instead of just a boolean then we can avoid the register read on all
code paths.


Daniel.
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