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

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Date:   Fri, 29 May 2020 13:20:24 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Clément Péron <peron.clem@...il.com>,
        Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Steven Price <steven.price@....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:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 05/15] drm/panfrost: use spinlock instead of atomic

On 2020-05-10 17:55, Clément Péron wrote:
> Convert busy_count to a simple int protected by spinlock.

A little more reasoning might be nice.

> Signed-off-by: Clément Péron <peron.clem@...il.com>
> ---
[...]
> 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.
> +	 */

Nit: this comment is clearly wrong, since we only have Job, GPU and MMU 
interrupts here. I guess if there is a race it would be between 
submission/completion/timeout on different job slots.

Given that, should this actually be considered a fix for 9e62b885f715 
("drm/panfrost: Simplify devfreq utilisation tracking")?

Robin.