[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Rabin Vincent]

From: Rabin Vincent <rabinv@axis.com>

According to the errata document for the Cortex A9 MPCore, the correct
code sequence in the interrupt handler to workaround erratum 740657
"Global Timer can send two interrupts for the same event" is:

 (1) Read the ICCIAR (Interrupt Acknowledge) register
 (2) Modify the comparator value, to set it to a higher value
 (3) Clear the Global Timer flag
 (4) Clear the Pending Status information for Interrupt 27 (Global
     Timer interrupt) in the Distributor of the Interrupt Controller.
 (5) Write the ICCEOIR (End of Interrupt) register

(1) and (5) are done by the GIC driver and (2) and (3) are done by the
Global Timer driver.  However, nobody does (4) and thus the workaround
is inactive and the timer triggers many spurious interrupts:

 <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
 <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
 <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
 <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled

Make the Global Timer driver perform step (4) via the GIC driver with
the help of the irq_set_irqchip_state() function, to prevent the
spurious interrupts.

Signed-off-by: Rabin Vincent <rabinv@axis.com>
---
 drivers/clocksource/arm_global_timer.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
index 9df0d16..b9d0f86 100644
--- a/drivers/clocksource/arm_global_timer.c
+++ b/drivers/clocksource/arm_global_timer.c
@@ -140,26 +140,31 @@ static int gt_clockevent_set_next_event(unsigned long evt,
 static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
 {
 	struct clock_event_device *evt = dev_id;
+	bool workaround = clockevent_state_oneshot(evt);
 
 	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
 				GT_INT_STATUS_EVENT_FLAG))
 		return IRQ_NONE;
 
 	/**
-	 * ERRATA 740657( Global Timer can send 2 interrupts for
+	 * ERRATA 740657 (Global Timer can send 2 interrupts for
 	 * the same event in single-shot mode)
 	 * Workaround:
-	 *	Either disable single-shot mode.
-	 *	Or
-	 *	Modify the Interrupt Handler to avoid the
-	 *	offending sequence. This is achieved by clearing
-	 *	the Global Timer flag _after_ having incremented
-	 *	the Comparator register	value to a higher value.
+	 * - Read the ICCIAR (Interrupt Acknowledge) register
+	 * - Modify the comparator value, to set it to a higher value
+	 * - Clear the Global Timer flag
+	 * - Clear the Pending Status information for Interrupt 27 (Global
+	 *   Timer interrupt) in the Distributor of the Interrupt Controller.
+	 * - Write the ICCEOIR (End of Interrupt) register
 	 */
-	if (clockevent_state_oneshot(evt))
+	if (workaround)
 		gt_compare_set(ULONG_MAX, 0);
 
 	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
+
+	if (workaround)
+		irq_set_irqchip_state(irq, IRQCHIP_STATE_PENDING, false);
+
 	evt->event_handler(evt);
 
 	return IRQ_HANDLED;
-- 
2.7.0

[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Rabin Vincent]

From: Rabin Vincent <rabinv@axis.com>

According to the errata document for the Cortex A9 MPCore, the correct
code sequence in the interrupt handler to workaround erratum 740657
"Global Timer can send two interrupts for the same event" is:

 (1) Read the ICCIAR (Interrupt Acknowledge) register
 (2) Modify the comparator value, to set it to a higher value
 (3) Clear the Global Timer flag
 (4) Clear the Pending Status information for Interrupt 27 (Global
     Timer interrupt) in the Distributor of the Interrupt Controller.
 (5) Write the ICCEOIR (End of Interrupt) register

(1) and (5) are done by the GIC driver and (2) and (3) are done by the
Global Timer driver.  However, nobody does (4) and thus the workaround
is inactive and the timer triggers many spurious interrupts:

 <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
 <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
 <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
 <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled

Make the Global Timer driver perform step (4) via the GIC driver with
the help of the irq_set_irqchip_state() function, to prevent the
spurious interrupts.

Signed-off-by: Rabin Vincent <rabinv@axis.com>
---
 drivers/clocksource/arm_global_timer.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
index 9df0d16..b9d0f86 100644
--- a/drivers/clocksource/arm_global_timer.c
+++ b/drivers/clocksource/arm_global_timer.c
@@ -140,26 +140,31 @@ static int gt_clockevent_set_next_event(unsigned long evt,
 static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
 {
 	struct clock_event_device *evt = dev_id;
+	bool workaround = clockevent_state_oneshot(evt);
 
 	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
 				GT_INT_STATUS_EVENT_FLAG))
 		return IRQ_NONE;
 
 	/**
-	 * ERRATA 740657( Global Timer can send 2 interrupts for
+	 * ERRATA 740657 (Global Timer can send 2 interrupts for
 	 * the same event in single-shot mode)
 	 * Workaround:
-	 *	Either disable single-shot mode.
-	 *	Or
-	 *	Modify the Interrupt Handler to avoid the
-	 *	offending sequence. This is achieved by clearing
-	 *	the Global Timer flag _after_ having incremented
-	 *	the Comparator register	value to a higher value.
+	 * - Read the ICCIAR (Interrupt Acknowledge) register
+	 * - Modify the comparator value, to set it to a higher value
+	 * - Clear the Global Timer flag
+	 * - Clear the Pending Status information for Interrupt 27 (Global
+	 *   Timer interrupt) in the Distributor of the Interrupt Controller.
+	 * - Write the ICCEOIR (End of Interrupt) register
 	 */
-	if (clockevent_state_oneshot(evt))
+	if (workaround)
 		gt_compare_set(ULONG_MAX, 0);
 
 	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
+
+	if (workaround)
+		irq_set_irqchip_state(irq, IRQCHIP_STATE_PENDING, false);
+
 	evt->event_handler(evt);
 
 	return IRQ_HANDLED;
-- 
2.7.0

Re: [PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Daniel Lezcano]

On 03/29/2016 10:08 AM, Rabin Vincent wrote:
> From: Rabin Vincent <rabinv@axis.com>
>
> According to the errata document for the Cortex A9 MPCore, the correct
> code sequence in the interrupt handler to workaround erratum 740657
> "Global Timer can send two interrupts for the same event" is:
>
>   (1) Read the ICCIAR (Interrupt Acknowledge) register
>   (2) Modify the comparator value, to set it to a higher value
>   (3) Clear the Global Timer flag
>   (4) Clear the Pending Status information for Interrupt 27 (Global
>       Timer interrupt) in the Distributor of the Interrupt Controller.
>   (5) Write the ICCEOIR (End of Interrupt) register
>
> (1) and (5) are done by the GIC driver and (2) and (3) are done by the
> Global Timer driver.  However, nobody does (4) and thus the workaround
> is inactive and the timer triggers many spurious interrupts:
>
>   <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
>   <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
>   <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
>   <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled

Hi Rabin,

what platform are you using to test that ?


> Make the Global Timer driver perform step (4) via the GIC driver with
> the help of the irq_set_irqchip_state() function, to prevent the
> spurious interrupts.
>
> Signed-off-by: Rabin Vincent <rabinv@axis.com>
> ---
>   drivers/clocksource/arm_global_timer.c | 21 +++++++++++++--------
>   1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
> index 9df0d16..b9d0f86 100644
> --- a/drivers/clocksource/arm_global_timer.c
> +++ b/drivers/clocksource/arm_global_timer.c
> @@ -140,26 +140,31 @@ static int gt_clockevent_set_next_event(unsigned long evt,
>   static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
>   {
>   	struct clock_event_device *evt = dev_id;
> +	bool workaround = clockevent_state_oneshot(evt);
>
>   	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
>   				GT_INT_STATUS_EVENT_FLAG))
>   		return IRQ_NONE;
>
>   	/**
> -	 * ERRATA 740657( Global Timer can send 2 interrupts for
> +	 * ERRATA 740657 (Global Timer can send 2 interrupts for
>   	 * the same event in single-shot mode)
>   	 * Workaround:
> -	 *	Either disable single-shot mode.
> -	 *	Or
> -	 *	Modify the Interrupt Handler to avoid the
> -	 *	offending sequence. This is achieved by clearing
> -	 *	the Global Timer flag _after_ having incremented
> -	 *	the Comparator register	value to a higher value.
> +	 * - Read the ICCIAR (Interrupt Acknowledge) register
> +	 * - Modify the comparator value, to set it to a higher value
> +	 * - Clear the Global Timer flag
> +	 * - Clear the Pending Status information for Interrupt 27 (Global
> +	 *   Timer interrupt) in the Distributor of the Interrupt Controller.
> +	 * - Write the ICCEOIR (End of Interrupt) register
>   	 */
> -	if (clockevent_state_oneshot(evt))
> +	if (workaround)
>   		gt_compare_set(ULONG_MAX, 0);
>
>   	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
> +
> +	if (workaround)
> +		irq_set_irqchip_state(irq, IRQCHIP_STATE_PENDING, false);
> +
>   	evt->event_handler(evt);
>
>   	return IRQ_HANDLED;
>


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[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Daniel Lezcano]

On 03/29/2016 10:08 AM, Rabin Vincent wrote:
> From: Rabin Vincent <rabinv@axis.com>
>
> According to the errata document for the Cortex A9 MPCore, the correct
> code sequence in the interrupt handler to workaround erratum 740657
> "Global Timer can send two interrupts for the same event" is:
>
>   (1) Read the ICCIAR (Interrupt Acknowledge) register
>   (2) Modify the comparator value, to set it to a higher value
>   (3) Clear the Global Timer flag
>   (4) Clear the Pending Status information for Interrupt 27 (Global
>       Timer interrupt) in the Distributor of the Interrupt Controller.
>   (5) Write the ICCEOIR (End of Interrupt) register
>
> (1) and (5) are done by the GIC driver and (2) and (3) are done by the
> Global Timer driver.  However, nobody does (4) and thus the workaround
> is inactive and the timer triggers many spurious interrupts:
>
>   <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
>   <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
>   <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
>   <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
>   <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled

Hi Rabin,

what platform are you using to test that ?


> Make the Global Timer driver perform step (4) via the GIC driver with
> the help of the irq_set_irqchip_state() function, to prevent the
> spurious interrupts.
>
> Signed-off-by: Rabin Vincent <rabinv@axis.com>
> ---
>   drivers/clocksource/arm_global_timer.c | 21 +++++++++++++--------
>   1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
> index 9df0d16..b9d0f86 100644
> --- a/drivers/clocksource/arm_global_timer.c
> +++ b/drivers/clocksource/arm_global_timer.c
> @@ -140,26 +140,31 @@ static int gt_clockevent_set_next_event(unsigned long evt,
>   static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
>   {
>   	struct clock_event_device *evt = dev_id;
> +	bool workaround = clockevent_state_oneshot(evt);
>
>   	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
>   				GT_INT_STATUS_EVENT_FLAG))
>   		return IRQ_NONE;
>
>   	/**
> -	 * ERRATA 740657( Global Timer can send 2 interrupts for
> +	 * ERRATA 740657 (Global Timer can send 2 interrupts for
>   	 * the same event in single-shot mode)
>   	 * Workaround:
> -	 *	Either disable single-shot mode.
> -	 *	Or
> -	 *	Modify the Interrupt Handler to avoid the
> -	 *	offending sequence. This is achieved by clearing
> -	 *	the Global Timer flag _after_ having incremented
> -	 *	the Comparator register	value to a higher value.
> +	 * - Read the ICCIAR (Interrupt Acknowledge) register
> +	 * - Modify the comparator value, to set it to a higher value
> +	 * - Clear the Global Timer flag
> +	 * - Clear the Pending Status information for Interrupt 27 (Global
> +	 *   Timer interrupt) in the Distributor of the Interrupt Controller.
> +	 * - Write the ICCEOIR (End of Interrupt) register
>   	 */
> -	if (clockevent_state_oneshot(evt))
> +	if (workaround)
>   		gt_compare_set(ULONG_MAX, 0);
>
>   	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
> +
> +	if (workaround)
> +		irq_set_irqchip_state(irq, IRQCHIP_STATE_PENDING, false);
> +
>   	evt->event_handler(evt);
>
>   	return IRQ_HANDLED;
>


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Re: [PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Rabin Vincent]

On Tue, Mar 29, 2016 at 04:21:03PM +0200, Daniel Lezcano wrote:
> On 03/29/2016 10:08 AM, Rabin Vincent wrote:
> >  <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
> >  <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
> >  <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
> >  <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled
> 
> what platform are you using to test that ?

ARTPEC-6.

[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Rabin Vincent]

On Tue, Mar 29, 2016 at 04:21:03PM +0200, Daniel Lezcano wrote:
> On 03/29/2016 10:08 AM, Rabin Vincent wrote:
> >  <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
> >  <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
> >  <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
> >  <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
> >  <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled
> 
> what platform are you using to test that ?

ARTPEC-6.

Re: [PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Marc Zyngier]

Hi Rabin,

On 29/03/16 09:08, Rabin Vincent wrote:
> From: Rabin Vincent <rabinv@axis.com>
> 
> According to the errata document for the Cortex A9 MPCore, the correct
> code sequence in the interrupt handler to workaround erratum 740657
> "Global Timer can send two interrupts for the same event" is:
> 
>  (1) Read the ICCIAR (Interrupt Acknowledge) register
>  (2) Modify the comparator value, to set it to a higher value
>  (3) Clear the Global Timer flag
>  (4) Clear the Pending Status information for Interrupt 27 (Global
>      Timer interrupt) in the Distributor of the Interrupt Controller.
>  (5) Write the ICCEOIR (End of Interrupt) register
> 
> (1) and (5) are done by the GIC driver and (2) and (3) are done by the
> Global Timer driver.  However, nobody does (4) and thus the workaround
> is inactive and the timer triggers many spurious interrupts:
> 
>  <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
>  <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
>  <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
>  <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled
> 
> Make the Global Timer driver perform step (4) via the GIC driver with
> the help of the irq_set_irqchip_state() function, to prevent the
> spurious interrupts.
> 
> Signed-off-by: Rabin Vincent <rabinv@axis.com>
> ---
>  drivers/clocksource/arm_global_timer.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
> index 9df0d16..b9d0f86 100644
> --- a/drivers/clocksource/arm_global_timer.c
> +++ b/drivers/clocksource/arm_global_timer.c
> @@ -140,26 +140,31 @@ static int gt_clockevent_set_next_event(unsigned long evt,
>  static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
>  {
>  	struct clock_event_device *evt = dev_id;
> +	bool workaround = clockevent_state_oneshot(evt);
>  
>  	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
>  				GT_INT_STATUS_EVENT_FLAG))
>  		return IRQ_NONE;
>  
>  	/**
> -	 * ERRATA 740657( Global Timer can send 2 interrupts for
> +	 * ERRATA 740657 (Global Timer can send 2 interrupts for
>  	 * the same event in single-shot mode)
>  	 * Workaround:
> -	 *	Either disable single-shot mode.
> -	 *	Or
> -	 *	Modify the Interrupt Handler to avoid the
> -	 *	offending sequence. This is achieved by clearing
> -	 *	the Global Timer flag _after_ having incremented
> -	 *	the Comparator register	value to a higher value.
> +	 * - Read the ICCIAR (Interrupt Acknowledge) register
> +	 * - Modify the comparator value, to set it to a higher value
> +	 * - Clear the Global Timer flag
> +	 * - Clear the Pending Status information for Interrupt 27 (Global
> +	 *   Timer interrupt) in the Distributor of the Interrupt Controller.
> +	 * - Write the ICCEOIR (End of Interrupt) register
>  	 */
> -	if (clockevent_state_oneshot(evt))
> +	if (workaround)
>  		gt_compare_set(ULONG_MAX, 0);
>  
>  	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
> +
> +	if (workaround)
> +		irq_set_irqchip_state(irq, IRQCHIP_STATE_PENDING, false);
> +

This raises a few questions:

- What if my timer is not connected to a controller that implements this
API? Something that is not a GIC, for example?

- How does it work when the GIC (with EOImode==1) performs a priority
drop (by writing to the EOI register) before calling into the timer
handler, and finishing the handling with a write to DIR?

- What are the comparative costs of taking a spurious (but nonetheless
harmless) interrupt vs poking the distributor (which is by no mean a
cheap operation)?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Marc Zyngier]

Hi Rabin,

On 29/03/16 09:08, Rabin Vincent wrote:
> From: Rabin Vincent <rabinv@axis.com>
> 
> According to the errata document for the Cortex A9 MPCore, the correct
> code sequence in the interrupt handler to workaround erratum 740657
> "Global Timer can send two interrupts for the same event" is:
> 
>  (1) Read the ICCIAR (Interrupt Acknowledge) register
>  (2) Modify the comparator value, to set it to a higher value
>  (3) Clear the Global Timer flag
>  (4) Clear the Pending Status information for Interrupt 27 (Global
>      Timer interrupt) in the Distributor of the Interrupt Controller.
>  (5) Write the ICCEOIR (End of Interrupt) register
> 
> (1) and (5) are done by the GIC driver and (2) and (3) are done by the
> Global Timer driver.  However, nobody does (4) and thus the workaround
> is inactive and the timer triggers many spurious interrupts:
> 
>  <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
>  <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
>  <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
>  <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
>  <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled
> 
> Make the Global Timer driver perform step (4) via the GIC driver with
> the help of the irq_set_irqchip_state() function, to prevent the
> spurious interrupts.
> 
> Signed-off-by: Rabin Vincent <rabinv@axis.com>
> ---
>  drivers/clocksource/arm_global_timer.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
> index 9df0d16..b9d0f86 100644
> --- a/drivers/clocksource/arm_global_timer.c
> +++ b/drivers/clocksource/arm_global_timer.c
> @@ -140,26 +140,31 @@ static int gt_clockevent_set_next_event(unsigned long evt,
>  static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
>  {
>  	struct clock_event_device *evt = dev_id;
> +	bool workaround = clockevent_state_oneshot(evt);
>  
>  	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
>  				GT_INT_STATUS_EVENT_FLAG))
>  		return IRQ_NONE;
>  
>  	/**
> -	 * ERRATA 740657( Global Timer can send 2 interrupts for
> +	 * ERRATA 740657 (Global Timer can send 2 interrupts for
>  	 * the same event in single-shot mode)
>  	 * Workaround:
> -	 *	Either disable single-shot mode.
> -	 *	Or
> -	 *	Modify the Interrupt Handler to avoid the
> -	 *	offending sequence. This is achieved by clearing
> -	 *	the Global Timer flag _after_ having incremented
> -	 *	the Comparator register	value to a higher value.
> +	 * - Read the ICCIAR (Interrupt Acknowledge) register
> +	 * - Modify the comparator value, to set it to a higher value
> +	 * - Clear the Global Timer flag
> +	 * - Clear the Pending Status information for Interrupt 27 (Global
> +	 *   Timer interrupt) in the Distributor of the Interrupt Controller.
> +	 * - Write the ICCEOIR (End of Interrupt) register
>  	 */
> -	if (clockevent_state_oneshot(evt))
> +	if (workaround)
>  		gt_compare_set(ULONG_MAX, 0);
>  
>  	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
> +
> +	if (workaround)
> +		irq_set_irqchip_state(irq, IRQCHIP_STATE_PENDING, false);
> +

This raises a few questions:

- What if my timer is not connected to a controller that implements this
API? Something that is not a GIC, for example?

- How does it work when the GIC (with EOImode==1) performs a priority
drop (by writing to the EOI register) before calling into the timer
handler, and finishing the handling with a write to DIR?

- What are the comparative costs of taking a spurious (but nonetheless
harmless) interrupt vs poking the distributor (which is by no mean a
cheap operation)?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

Re: [PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Rabin Vincent]

On Tue, Mar 29, 2016 at 05:13:05PM +0100, Marc Zyngier wrote:
> - What if my timer is not connected to a controller that implements this
> API? Something that is not a GIC, for example?

As you know irq_set_irqchip_state() does nothing in that case, and the erratum
is thus presumably not worked around in such a system.

By the way, while the original document from ARM is not public, this
erratum can be seen here (among other places):
http://www.nxp.com/files/32bit/doc/errata/IMX6DQCE.pdf#page=13
https://www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/es/es-01042.pdf#page=26

> - How does it work when the GIC (with EOImode==1) performs a priority
> drop (by writing to the EOI register) before calling into the timer
> handler, and finishing the handling with a write to DIR?

I guess it doesn't.

> - What are the comparative costs of taking a spurious (but nonetheless
> harmless) interrupt vs poking the distributor (which is by no mean a
> cheap operation)?

The time taken by irq_set_irqchip_state() appears to be about 15%
shorter than the time gic_handle_irq() and its called functions take to
handle the spurious interrrupt.  The overhead of going in and out of
interrupt context for the spurious interrupt is not included in the
latter time.

[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Rabin Vincent]

On Tue, Mar 29, 2016 at 05:13:05PM +0100, Marc Zyngier wrote:
> - What if my timer is not connected to a controller that implements this
> API? Something that is not a GIC, for example?

As you know irq_set_irqchip_state() does nothing in that case, and the erratum
is thus presumably not worked around in such a system.

By the way, while the original document from ARM is not public, this
erratum can be seen here (among other places):
http://www.nxp.com/files/32bit/doc/errata/IMX6DQCE.pdf#page=13
https://www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/es/es-01042.pdf#page=26

> - How does it work when the GIC (with EOImode==1) performs a priority
> drop (by writing to the EOI register) before calling into the timer
> handler, and finishing the handling with a write to DIR?

I guess it doesn't.

> - What are the comparative costs of taking a spurious (but nonetheless
> harmless) interrupt vs poking the distributor (which is by no mean a
> cheap operation)?

The time taken by irq_set_irqchip_state() appears to be about 15%
shorter than the time gic_handle_irq() and its called functions take to
handle the spurious interrrupt.  The overhead of going in and out of
interrupt context for the spurious interrupt is not included in the
latter time.