linux-kernel - Re: [PATCH v1] drm/panthor: Serialize GPU cache flush operations

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Message-ID: <63c7d33d-c475-448e-a928-570c6efe2387@arm.com>
Date: Thu, 31 Jul 2025 11:57:18 +0100
From: Steven Price <steven.price@....com>
To: Karunika Choo <karunika.choo@....com>, dri-devel@...ts.freedesktop.org
Cc: nd@....com, Boris Brezillon <boris.brezillon@...labora.com>,
 Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 linux-kernel@...r.kernel.org, Dennis Tsiang <dennis.tsiang@....com>
Subject: Re: [PATCH v1] drm/panthor: Serialize GPU cache flush operations

On 30/07/2025 18:43, Karunika Choo wrote:
> In certain scenarios, it is possible for multiple cache flushes to be
> requested before the previous one completes. This patch introduces the
> cache_flush_lock mutex to serialize these operations and ensure that
> any requested cache flushes are completed instead of dropped.
> 
> Signed-off-by: Karunika Choo <karunika.choo@....com>
> Co-developed-by: Dennis Tsiang <dennis.tsiang@....com>

A Co-Developed-By needs to have a signed-off-by too[1]

[1]
https://www.kernel.org/doc/html/latest/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by

But I also don't understand how this is happening. The only caller to
panthor_gpu_flush_caches() is in panthor_sched_suspend() and that is
holding the sched->lock mutex.

Steve

> ---
>  drivers/gpu/drm/panthor/panthor_gpu.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_gpu.c b/drivers/gpu/drm/panthor/panthor_gpu.c
> index cb7a335e07d7..030409371037 100644
> --- a/drivers/gpu/drm/panthor/panthor_gpu.c
> +++ b/drivers/gpu/drm/panthor/panthor_gpu.c
> @@ -35,6 +35,9 @@ struct panthor_gpu {
>  
>  	/** @reqs_acked: GPU request wait queue. */
>  	wait_queue_head_t reqs_acked;
> +
> +	/** @cache_flush_lock: Lock to serialize cache flushes */
> +	struct mutex cache_flush_lock;
>  };
>  
>  /**
> @@ -204,6 +207,7 @@ int panthor_gpu_init(struct panthor_device *ptdev)
>  
>  	spin_lock_init(&gpu->reqs_lock);
>  	init_waitqueue_head(&gpu->reqs_acked);
> +	mutex_init(&gpu->cache_flush_lock);
>  	ptdev->gpu = gpu;
>  	panthor_gpu_init_info(ptdev);
>  
> @@ -353,6 +357,9 @@ int panthor_gpu_flush_caches(struct panthor_device *ptdev,
>  	bool timedout = false;
>  	unsigned long flags;
>  
> +	/* Serialize cache flush operations. */
> +	guard(mutex)(&ptdev->gpu->cache_flush_lock);
> +
>  	spin_lock_irqsave(&ptdev->gpu->reqs_lock, flags);
>  	if (!drm_WARN_ON(&ptdev->base,
>  			 ptdev->gpu->pending_reqs & GPU_IRQ_CLEAN_CACHES_COMPLETED)) {