linux-kernel - Re: [PATCH 05/15] drm/panfrost: use spinlock instead of atomic

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Message-ID: <20155d49-fa2b-722e-9e9e-789838c07b61@arm.com>
Date:   Thu, 28 May 2020 14:22:44 +0100
From:   Steven Price <steven.price@....com>
To:     Clément Péron <peron.clem@...il.com>,
        Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/15] drm/panfrost: use spinlock instead of atomic

On 10/05/2020 17:55, Clément Péron wrote:
> Convert busy_count to a simple int protected by spinlock.
> 
> Signed-off-by: Clément Péron <peron.clem@...il.com>

Looks like a fairly mechanical cleanup.

Reviewed-by: Steven Price <steven.price@....com>

> ---
>   drivers/gpu/drm/panfrost/panfrost_devfreq.c | 43 +++++++++++++++------
>   drivers/gpu/drm/panfrost/panfrost_devfreq.h | 10 ++++-
>   2 files changed, 41 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_devfreq.c b/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> index 962550363391..78753cfb59fb 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> @@ -12,16 +12,12 @@
>   
>   static void panfrost_devfreq_update_utilization(struct panfrost_devfreq *pfdevfreq)
>   {
> -	ktime_t now;
> -	ktime_t last;
> -
> -	if (!pfdevfreq->devfreq)
> -		return;
> +	ktime_t now, last;
>   
>   	now = ktime_get();
>   	last = pfdevfreq->time_last_update;
>   
> -	if (atomic_read(&pfdevfreq->busy_count) > 0)
> +	if (pfdevfreq->busy_count > 0)
>   		pfdevfreq->busy_time += ktime_sub(now, last);
>   	else
>   		pfdevfreq->idle_time += ktime_sub(now, last);
> @@ -59,10 +55,14 @@ static int panfrost_devfreq_get_dev_status(struct device *dev,
>   {
>   	struct panfrost_device *pfdev = dev_get_drvdata(dev);
>   	struct panfrost_devfreq *pfdevfreq = &pfdev->pfdevfreq;
> +	unsigned long irqflags;
> +
> +	status->current_frequency = clk_get_rate(pfdev->clock);
> +
> +	spin_lock_irqsave(&pfdevfreq->lock, irqflags);
>   
>   	panfrost_devfreq_update_utilization(pfdevfreq);
>   
> -	status->current_frequency = clk_get_rate(pfdev->clock);
>   	status->total_time = ktime_to_ns(ktime_add(pfdevfreq->busy_time,
>   						   pfdevfreq->idle_time));
>   
> @@ -70,6 +70,8 @@ static int panfrost_devfreq_get_dev_status(struct device *dev,
>   
>   	panfrost_devfreq_reset(pfdevfreq);
>   
> +	spin_unlock_irqrestore(&pfdevfreq->lock, irqflags);
> +
>   	dev_dbg(pfdev->dev, "busy %lu total %lu %lu %% freq %lu MHz\n",
>   		status->busy_time, status->total_time,
>   		status->busy_time / (status->total_time / 100),
> @@ -100,6 +102,8 @@ int panfrost_devfreq_init(struct panfrost_device *pfdev)
>   	else if (ret)
>   		return ret;
>   
> +	spin_lock_init(&pfdevfreq->lock);
> +
>   	panfrost_devfreq_reset(pfdevfreq);
>   
>   	cur_freq = clk_get_rate(pfdev->clock);
> @@ -162,15 +166,32 @@ void panfrost_devfreq_suspend(struct panfrost_device *pfdev)
>   
>   void panfrost_devfreq_record_busy(struct panfrost_devfreq *pfdevfreq)
>   {
> +	unsigned long irqflags;
> +
> +	if (!pfdevfreq->devfreq)
> +		return;
> +
> +	spin_lock_irqsave(&pfdevfreq->lock, irqflags);
> +
>   	panfrost_devfreq_update_utilization(pfdevfreq);
> -	atomic_inc(&pfdevfreq->busy_count);
> +
> +	pfdevfreq->busy_count++;
> +
> +	spin_unlock_irqrestore(&pfdevfreq->lock, irqflags);
>   }
>   
>   void panfrost_devfreq_record_idle(struct panfrost_devfreq *pfdevfreq)
>   {
> -	int count;
> +	unsigned long irqflags;
> +
> +	if (!pfdevfreq->devfreq)
> +		return;
> +
> +	spin_lock_irqsave(&pfdevfreq->lock, irqflags);
>   
>   	panfrost_devfreq_update_utilization(pfdevfreq);
> -	count = atomic_dec_if_positive(&pfdevfreq->busy_count);
> -	WARN_ON(count < 0);
> +
> +	WARN_ON(--pfdevfreq->busy_count < 0);
> +
> +	spin_unlock_irqrestore(&pfdevfreq->lock, irqflags);
>   }
> diff --git a/drivers/gpu/drm/panfrost/panfrost_devfreq.h b/drivers/gpu/drm/panfrost/panfrost_devfreq.h
> index 0697f8d5aa34..e6629900a618 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_devfreq.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_devfreq.h
> @@ -4,6 +4,7 @@
>   #ifndef __PANFROST_DEVFREQ_H__
>   #define __PANFROST_DEVFREQ_H__
>   
> +#include <linux/spinlock.h>
>   #include <linux/ktime.h>
>   
>   struct devfreq;
> @@ -14,10 +15,17 @@ struct panfrost_device;
>   struct panfrost_devfreq {
>   	struct devfreq *devfreq;
>   	struct thermal_cooling_device *cooling;
> +
>   	ktime_t busy_time;
>   	ktime_t idle_time;
>   	ktime_t time_last_update;
> -	atomic_t busy_count;
> +	int busy_count;
> +	/*
> +	 * Protect busy_time, idle_time, time_last_update and busy_count
> +	 * because these can be updated concurrently, for example by the GP
> +	 * and PP interrupts.
> +	 */
> +	spinlock_t lock;
>   };
>   
>   int panfrost_devfreq_init(struct panfrost_device *pfdev);
>