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

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Message-ID: <216ebb8e-c973-4306-a748-a030eae01e2d@arm.com>
Date: Thu, 31 Jul 2025 13:48:38 +0100
From: Karunika Choo <karunika.choo@....com>
To: Steven Price <steven.price@....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 31/07/2025 11:57, Steven Price wrote:
> 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]

Oops. I can push a v2 to add those.

> 
> [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.

The fix is in relation to the enablement of GPU Flush caches by default
for all GPUs [1]. While calls from the MMU are serialized, other calls
i.e. from panthor_sched_suspend() are not. As such, this patch
explicitly serializes these operations.

[1]
https://lore.kernel.org/all/20250724124210.3675094-6-karunika.choo@arm.com/

Kind regards,
Karunika Choo

> 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)) {
>