linux-kernel - Re: [PATCH v9 5/7] drm/panthor: Serialize GPU cache flush operations

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Message-ID: <f7ea81af-398f-4a58-8193-1681ebafa678@arm.com>
Date: Fri, 15 Aug 2025 10:50:28 +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 v9 5/7] drm/panthor: Serialize GPU cache flush operations

On 07/08/2025 17:26, 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.
> 
> Reviewed-by: Liviu Dudau <liviu.dudau@....com>
> Co-developed-by: Dennis Tsiang <dennis.tsiang@....com>
> Signed-off-by: Dennis Tsiang <dennis.tsiang@....com>
> Signed-off-by: Karunika Choo <karunika.choo@....com>

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

> ---
>  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 5e2c3173ae27..db69449a5be0 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;
>  };
>  
>  #define GPU_INTERRUPTS_MASK	\
> @@ -110,6 +113,7 @@ 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;
>  
>  	dma_set_max_seg_size(ptdev->base.dev, UINT_MAX);
> @@ -258,6 +262,9 @@ 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)) {