Message-ID: <53209790.2030100@redhat.com>
Date:	Wed, 12 Mar 2014 18:21:20 +0100
From:	Hans de Goede <hdegoede@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>
CC:	linux-arm-kernel@...ts.infradead.org,
	devicetree <devicetree@...r.kernel.org>,
	linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com
Subject: Re: [PATCH v2 1/4] irq: Add handle_fasteoi_late_irq irq handler

Hi,

On 03/12/2014 06:17 PM, Hans de Goede wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Add an irq handler for transparent controllers
> 
> Only a single callback will be issued to the chip: an ->eoi() call when
> the interrupt has been serviced. Same as handle_fasteoi_irq, but
> we avoid the eoi when the interrupt is masked due to a
> threaded handler. The eoi will be issued right before the unmask.
> 
> Signed-off-by: Hans de Goede <hdegoede@...hat.com>

Oops everything below this was not intended to be part of the commit msg.

Regards,

Hans

>> In some cases we want to do an ack right before the unmask of an irq.
>>
>> A typical example of such a case is having an i2c device which uses a level
>> interrupt. Such devices usually have an interrupt status register with
>> write to clear status bits. This means that the interrupt handler needs to
>> sleep to do i2c transfers to read / write the interrupt status register.
>>
>> This means using a threaded interrupt handler and IRQF_ONESHOT so that the
>> interrupt does not re-trigger while the interrupt handler is waiting for the
>> i2c transfer.
>>
>> Without ack-before-unmask the sequence of a single interrupt in this setup
>> looks like this:
>>
>> i2c-device raises interrupt line
>> interrupt gets triggered
>> interrupt gets masked
>> interrupt gets acked
>> interrupt handling thread starts running
>> interrupt handling thread does its stuff, write-clears irq-status register
>>  in i2c-device
>> i2c-device lowers interrupt line (note this happens after the ack!)
>> interrupt handling thread is done
>> interrupt gets unmasked
>> interrupt immediately retriggers again, because the line has been high
>>  after the ack
>> interrupt gets masked
>> interrupt gets acked
>> interrupt handling thread starts running
>> interrupt handling thread reads irq-status register, has nothing todo
>> interrupt handling thread is done
>> interrupt gets unmasked
>>
>> So in essence we get and handle 2 interrupts for what is 1 interrupt from
>> the i2c-device pov. By doing an ack before the unmask, the second interrupt
>> is avoided since at the point of the unmask the irq-thread has done its
>> job and cleared the interrupt source.
> 
> And how is that different from the non threaded case?
> 
>     mask()
>     ack()	<-- irq line is still active
>     handle()	<-- irq line goes inactive. So this happens
>     		     after the ack as above
>     unmask()
> 
>> Note that things work fine without this patch, the purpose of this patch is
>> soley to avoid the second unneeded interrupt.
>>
>> This patch uses an interrupt flag for this and not an irqchip flag because
>> the need for ack before unmask is based on the device and not on the irqchip.
> 
> No, it's not a device property. Its an irq chip property. There are
> interrupt chips which handle that case fine and making this a device
> property might even break such interrupt chips. e.g. ioapic handles
> that nicely but it would use ack_edge() on a level interrupt when the
> device driver requests that. And the edge and level mode of the ioapic
> are substantially different. Go figure.
> 
> The general policy is that device drivers should be oblivious of the
> underlying interrupt controller implementation as much as possible.
> 
> If the interrupt chip has this behaviour then handle_level_irq as the
> flow handler is the wrong thing to start with because it always acks
> before calling the handler.
> 
> Due to the fact that we run the primary handler with interrupts
> disabled, we should avoid the mask/unmask dance for the non threaded
> case completely. handle_fasteoi_irq does that, except that it does not
> provide the eoi() before unmask in the threaded case and it has an
> extra eoi() even in the masked case which is pointless for interrupt
> chips like the one you are dealing with.
> 
> Untested patch below.
> 
> Thanks,
> 
> 	tglx
> 
> ---------------->
> ---
>  include/linux/irq.h    |  4 +++
>  kernel/irq/chip.c      | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/irq/internals.h |  1 +
>  kernel/irq/manage.c    |  2 +-
>  4 files changed, 79 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/irq.h b/include/linux/irq.h
> index 7dc1003..51c199e 100644
> --- a/include/linux/irq.h
> +++ b/include/linux/irq.h
> @@ -349,6 +349,8 @@ struct irq_chip {
>   * IRQCHIP_ONOFFLINE_ENABLED:	Only call irq_on/off_line callbacks
>   *				when irq enabled
>   * IRQCHIP_SKIP_SET_WAKE:	Skip chip.irq_set_wake(), for this irq chip
> + * IRQCHIP_ONESHOT_SAFE:	One shot does not require mask/unmask
> + * IRQCHIP_EOI_THREADED:	Chip requires eoi() on unmask in threaded mode
>   */
>  enum {
>  	IRQCHIP_SET_TYPE_MASKED		= (1 <<  0),
> @@ -357,6 +359,7 @@ enum {
>  	IRQCHIP_ONOFFLINE_ENABLED	= (1 <<  3),
>  	IRQCHIP_SKIP_SET_WAKE		= (1 <<  4),
>  	IRQCHIP_ONESHOT_SAFE		= (1 <<  5),
> +	IRQCHIP_EOI_THREADED		= (1 <<  6),
>  };
>  
>  /* This include will go away once we isolated irq_desc usage to core code */
> @@ -412,6 +415,7 @@ static inline int irq_set_parent(int irq, int parent_irq)
>   */
>  extern void handle_level_irq(unsigned int irq, struct irq_desc *desc);
>  extern void handle_fasteoi_irq(unsigned int irq, struct irq_desc *desc);
> +extern void handle_fasteoi_late_irq(unsigned int irq, struct irq_desc *desc);
>  extern void handle_edge_irq(unsigned int irq, struct irq_desc *desc);
>  extern void handle_edge_eoi_irq(unsigned int irq, struct irq_desc *desc);
>  extern void handle_simple_irq(unsigned int irq, struct irq_desc *desc);
> diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
> index dc04c16..bc3c099 100644
> --- a/kernel/irq/chip.c
> +++ b/kernel/irq/chip.c
> @@ -281,6 +281,19 @@ void unmask_irq(struct irq_desc *desc)
>  	}
>  }
>  
> +void unmask_threaded_irq(struct irq_desc *desc)
> +{
> +	struct irq_chip *chip = desc->irq_data.chip;
> +
> +	if (chip->flags & IRQCHIP_EOI_THREADED)
> +		chip->irq_eoi(&desc->irq_data);
> +
> +	if (chip->irq_unmask) {
> +		chip->irq_unmask(&desc->irq_data);
> +		irq_state_clr_masked(desc);
> +	}
> +}
> +
>  /*
>   *	handle_nested_irq - Handle a nested irq from a irq thread
>   *	@irq:	the interrupt number
> @@ -487,6 +500,66 @@ out:
>  	goto out_unlock;
>  }
>  
> +static void cond_unmask_and_eoi_irq(struct irq_desc *desc)
> +{
> +	if (!(desc->istate & IRQS_ONESHOT)) {
> +		desc->irq_data.chip->irq_eoi(&desc->irq_data);
> +		return;
> +	}
> +	/*
> +	 * We need to unmask in the following cases:
> +	 * - Oneshot irq which did not wake the thread (caused by a
> +	 *   spurious interrupt or a primary handler handling it
> +	 *   completely).
> +	 */
> +	if (!irqd_irq_disabled(&desc->irq_data) &&
> +	    irqd_irq_masked(&desc->irq_data) && !desc->threads_oneshot) {
> +		desc->irq_data.chip->irq_eoi(&desc->irq_data);
> +		unmask_irq(desc);
> +	}
> +}
> +
> +/**
> + *	handle_fasteoi_late_irq - irq handler for transparent controllers
> + *	@irq:	the interrupt number
> + *	@desc:	the interrupt description structure for this irq
> + *
> + *	Only a single callback will be issued to the chip: an ->eoi()
> + *	call when the interrupt has been serviced. Same as above, but
> + *	we avoid the eoi when the interrupt is masked due to a
> + *	threaded handler. The eoi will be issued right before the unmask.
> + */
> +void handle_fasteoi_late_irq(unsigned int irq, struct irq_desc *desc)
> +{
> +	raw_spin_lock(&desc->lock);
> +
> +	if (unlikely(irqd_irq_inprogress(&desc->irq_data)))
> +		if (!irq_check_poll(desc))
> +			goto out;
> +
> +	desc->istate &= ~(IRQS_REPLAY | IRQS_WAITING);
> +	kstat_incr_irqs_this_cpu(irq, desc);
> +
> +	/*
> +	 * If its disabled or no action available
> +	 * then mask it and get out of here:
> +	 */
> +	if (unlikely(!desc->action || irqd_irq_disabled(&desc->irq_data))) {
> +		desc->istate |= IRQS_PENDING;
> +		mask_irq(desc);
> +		goto out;
> +	}
> +
> +	if (desc->istate & IRQS_ONESHOT)
> +		mask_irq(desc);
> +
> +	handle_irq_event(desc);
> +
> +	cond_unmask_and_eoi_irq(desc);
> +out:
> +	raw_spin_unlock(&desc->lock);
> +}
> +
>  /**
>   *	handle_edge_irq - edge type IRQ handler
>   *	@irq:	the interrupt number
> diff --git a/kernel/irq/internals.h b/kernel/irq/internals.h
> index 001fa5b..e98bb56 100644
> --- a/kernel/irq/internals.h
> +++ b/kernel/irq/internals.h
> @@ -73,6 +73,7 @@ extern void irq_percpu_enable(struct irq_desc *desc, unsigned int cpu);
>  extern void irq_percpu_disable(struct irq_desc *desc, unsigned int cpu);
>  extern void mask_irq(struct irq_desc *desc);
>  extern void unmask_irq(struct irq_desc *desc);
> +extern void unmask_threaded_irq(struct irq_desc *desc);
>  
>  extern void init_kstat_irqs(struct irq_desc *desc, int node, int nr);
>  
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index 481a13c..9147fef 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -718,7 +718,7 @@ again:
>  
>  	if (!desc->threads_oneshot && !irqd_irq_disabled(&desc->irq_data) &&
>  	    irqd_irq_masked(&desc->irq_data))
> -		unmask_irq(desc);
> +		unmask_threaded_irq(desc);
>  
>  out_unlock:
>  	raw_spin_unlock_irq(&desc->lock);
> 
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