fast_get: Enable buffer sharing when using module driver heap

Allow fast_get sram buffer sharing across multiple userspace module
instances when CONFIG_SOF_USERSPACE_USE_DRIVER_HEAP is enabled. The module
driver heap is shared by all instances of a given module, so allocated
buffers can safely be reused between them.

Simplify checking whether a calling thread runs in userspace by verifying
if the K_USER flag is set.

Signed-off-by: Adrian Warecki <adrian.warecki@intel.com>

Author: softwarecki

zephyr/lib/fast-get.c

@@ -150,7 +150,11 @@ const void *fast_get(struct k_heap *heap, const void *dram_ptr, size_t size)
 		alloc_align = PLATFORM_DCACHE_ALIGN;
 	}
 
-	if (size > FAST_GET_MAX_COPY_SIZE || !IS_ENABLED(CONFIG_USERSPACE))
+	/* The module driver heap is shared by all instances of a given module.
+	 * Instances can share the allocated buffer.
+	 */
+	if (size > FAST_GET_MAX_COPY_SIZE || !IS_ENABLED(CONFIG_USERSPACE) ||
+	    IS_ENABLED(CONFIG_SOF_USERSPACE_USE_DRIVER_HEAP))
 		alloc_ptr = dram_ptr;
 	else
 		/* When userspace is enabled only share large buffers */
@@ -188,8 +192,12 @@ const void *fast_get(struct k_heap *heap, const void *dram_ptr, size_t size)
 		/*
 		 * We only get there for large buffers, since small buffers with
 		 * enabled userspace don't create fast-get entries
+		 * 
+		 * We also reach this point when using the module driver heap.
+		 * Since the heap is already shared across module instances,
+		 * we skip memory domain manipulation.
 		 */
-		if (mdom->num_partitions > 1) {
+		if (k_current_get()->base.user_options & K_USER && size > FAST_GET_MAX_COPY_SIZE) {
 			/* A userspace thread makes the request */
 			if (mdom != entry->mdom &&
 			    !fast_get_partition_exists(k_current_get(), ret,