[14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy

Driver code has no business with the internals of the irq descriptor.

Aside of that the count is per interrupt line and therefore takes
interrupts from other devices into account which share the interrupt line
and are not handled by the graphics driver.

Replace it with a pmu private count which only counts interrupts which
originate from the graphics card.

To avoid atomics or heuristics of some sort make the counter field
'unsigned long'. That limits the count to 4e9 on 32bit which is a lot and
postprocessing can easily deal with the occasional wraparound.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
---
 drivers/gpu/drm/i915/i915_irq.c |   34 ++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/i915/i915_pmu.c |   18 +-----------------
 drivers/gpu/drm/i915/i915_pmu.h |    8 ++++++++
 3 files changed, 43 insertions(+), 17 deletions(-)

On Thu, 10 Dec 2020, Thomas Gleixner <tglx@linutronix.de> wrote:
> Driver code has no business with the internals of the irq descriptor.
>
> Aside of that the count is per interrupt line and therefore takes
> interrupts from other devices into account which share the interrupt line
> and are not handled by the graphics driver.
>
> Replace it with a pmu private count which only counts interrupts which
> originate from the graphics card.
>
> To avoid atomics or heuristics of some sort make the counter field
> 'unsigned long'. That limits the count to 4e9 on 32bit which is a lot and
> postprocessing can easily deal with the occasional wraparound.

I'll let Tvrtko and Chris review the substance here, but assuming they
don't object,

Acked-by: Jani Nikula <jani.nikula@intel.com>

for merging via whichever tree makes most sense.

>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: intel-gfx@lists.freedesktop.org
> Cc: dri-devel@lists.freedesktop.org
> ---
>  drivers/gpu/drm/i915/i915_irq.c |   34 ++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/i915/i915_pmu.c |   18 +-----------------
>  drivers/gpu/drm/i915/i915_pmu.h |    8 ++++++++
>  3 files changed, 43 insertions(+), 17 deletions(-)
>
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -60,6 +60,24 @@
>   * and related files, but that will be described in separate chapters.
>   */
>  
> +/*
> + * Interrupt statistic for PMU. Increments the counter only if the
> + * interrupt originated from the the GPU so interrupts from a device which
> + * shares the interrupt line are not accounted.
> + */
> +static inline void pmu_irq_stats(struct drm_i915_private *priv,
> +				 irqreturn_t res)
> +{
> +	if (unlikely(res != IRQ_HANDLED))
> +		return;
> +
> +	/*
> +	 * A clever compiler translates that into INC. A not so clever one
> +	 * should at least prevent store tearing.
> +	 */
> +	WRITE_ONCE(priv->pmu.irq_count, priv->pmu.irq_count + 1);
> +}
> +
>  typedef bool (*long_pulse_detect_func)(enum hpd_pin pin, u32 val);
>  
>  static const u32 hpd_ilk[HPD_NUM_PINS] = {
> @@ -1599,6 +1617,8 @@ static irqreturn_t valleyview_irq_handle
>  		valleyview_pipestat_irq_handler(dev_priv, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -1676,6 +1696,8 @@ static irqreturn_t cherryview_irq_handle
>  		valleyview_pipestat_irq_handler(dev_priv, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -2103,6 +2125,8 @@ static irqreturn_t ilk_irq_handler(int i
>  	if (sde_ier)
>  		raw_reg_write(regs, SDEIER, sde_ier);
>  
> +	pmu_irq_stats(i915, ret);
> +
>  	/* IRQs are synced during runtime_suspend, we don't require a wakeref */
>  	enable_rpm_wakeref_asserts(&i915->runtime_pm);
>  
> @@ -2419,6 +2443,8 @@ static irqreturn_t gen8_irq_handler(int
>  
>  	gen8_master_intr_enable(regs);
>  
> +	pmu_irq_stats(dev_priv, IRQ_HANDLED);
> +
>  	return IRQ_HANDLED;
>  }
>  
> @@ -2514,6 +2540,8 @@ static __always_inline irqreturn_t
>  
>  	gen11_gu_misc_irq_handler(gt, gu_misc_iir);
>  
> +	pmu_irq_stats(i915, IRQ_HANDLED);
> +
>  	return IRQ_HANDLED;
>  }
>  
> @@ -3688,6 +3716,8 @@ static irqreturn_t i8xx_irq_handler(int
>  		i8xx_pipestat_irq_handler(dev_priv, iir, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -3796,6 +3826,8 @@ static irqreturn_t i915_irq_handler(int
>  		i915_pipestat_irq_handler(dev_priv, iir, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -3941,6 +3973,8 @@ static irqreturn_t i965_irq_handler(int
>  		i965_pipestat_irq_handler(dev_priv, iir, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, IRQ_HANDLED);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -423,22 +423,6 @@ static enum hrtimer_restart i915_sample(
>  	return HRTIMER_RESTART;
>  }
>  
> -static u64 count_interrupts(struct drm_i915_private *i915)
> -{
> -	/* open-coded kstat_irqs() */
> -	struct irq_desc *desc = irq_to_desc(i915->drm.pdev->irq);
> -	u64 sum = 0;
> -	int cpu;
> -
> -	if (!desc || !desc->kstat_irqs)
> -		return 0;
> -
> -	for_each_possible_cpu(cpu)
> -		sum += *per_cpu_ptr(desc->kstat_irqs, cpu);
> -
> -	return sum;
> -}
> -
>  static void i915_pmu_event_destroy(struct perf_event *event)
>  {
>  	struct drm_i915_private *i915 =
> @@ -581,7 +565,7 @@ static u64 __i915_pmu_event_read(struct
>  				   USEC_PER_SEC /* to MHz */);
>  			break;
>  		case I915_PMU_INTERRUPTS:
> -			val = count_interrupts(i915);
> +			val = READ_ONCE(pmu->irq_count);
>  			break;
>  		case I915_PMU_RC6_RESIDENCY:
>  			val = get_rc6(&i915->gt);
> --- a/drivers/gpu/drm/i915/i915_pmu.h
> +++ b/drivers/gpu/drm/i915/i915_pmu.h
> @@ -108,6 +108,14 @@ struct i915_pmu {
>  	 */
>  	ktime_t sleep_last;
>  	/**
> +	 * @irq_count: Number of interrupts
> +	 *
> +	 * Intentionally unsigned long to avoid atomics or heuristics on 32bit.
> +	 * 4e9 interrupts are a lot and postprocessing can really deal with an
> +	 * occasional wraparound easily. It's 32bit after all.
> +	 */
> +	unsigned long irq_count;
> +	/**
>  	 * @events_attr_group: Device events attribute group.
>  	 */
>  	struct attribute_group events_attr_group;
>

On Fri, Dec 11 2020 at 14:19, David Laight wrote:
> From: Thomas Gleixner
>> You can't catch that. If this really becomes an issue you need a
>> sequence counter around it.
>
> Or just two copies of the high word.
> Provided the accesses are sequenced:
> writer:
> 	load high:low
> 	add small_value,high:low
> 	store high
> 	store low
> 	store high_copy
> reader:
> 	load high_copy
> 	load low
> 	load high
> 	if (high != high_copy)
> 		low = 0;

And low = 0 is solving what? You need to loop back and retry until it's
consistent and then it's nothing else than an open coded sequence count.

Thanks,

        tglx

From: Thomas Gleixner
> Sent: 11 December 2020 21:11
> 
> On Fri, Dec 11 2020 at 14:19, David Laight wrote:
> > From: Thomas Gleixner
> >> You can't catch that. If this really becomes an issue you need a
> >> sequence counter around it.
> >
> > Or just two copies of the high word.
> > Provided the accesses are sequenced:
> > writer:
> > 	load high:low
> > 	add small_value,high:low
> > 	store high
> > 	store low
> > 	store high_copy
> > reader:
> > 	load high_copy
> > 	load low
> > 	load high
> > 	if (high != high_copy)
> > 		low = 0;
> 
> And low = 0 is solving what? You need to loop back and retry until it's
> consistent and then it's nothing else than an open coded sequence count.

If it is a counter or timestamp then the high:0 value happened
some time between when you started trying to read the value and
when you finished trying to read it.

As such it is a perfectly reasonable return value.

	David

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