drm/panthor: Serialize GPU cache flush operations

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@arm.com>
Co-developed-by: Dennis Tsiang <dennis.tsiang@arm.com>
Signed-off-by: Dennis Tsiang <dennis.tsiang@arm.com>
Signed-off-by: Karunika Choo <karunika.choo@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250807162633.3666310-6-karunika.choo@arm.com

Author: Karunika Choo

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)) {