ipc: (cosmetic) add comments for buffer binding

It is unclear in which context buffer binding should be executed when
an LL and a DP modules are being bound with both running in userspace
context. Since DP modules have limited visibility into the system and
the userspace LL context on the other hand has access to most DP
assets, perform binding and unbinding in LL thread context. Add
comments to explain that.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Author: lyakh

src/ipc/ipc4/handler-user.c

@@ -1685,6 +1685,13 @@ __cold int ipc4_user_process_module_message(struct ipc4_message_request *ipc4,
 		break;
 	case SOF_IPC4_MOD_BIND:
 #ifdef CONFIG_SOF_USERSPACE_LL
+		/*
+		 * bind and unbind can connect LL with DP. In that case it isn't
+		 * immediately clear whether the handler should run in the DP
+		 * thread context or in the LL IPC thread context. The LL IPC
+		 * thread has access to DP modules, so we have to perform
+		 * binding in the LL IPC thread context
+		 */
 		ret = ipc_user_forward_cmd(ipc4->primary.dat, ipc4->extension.dat,
 					   ipc4_user_target_core_module(ipc4));
 #else
@@ -1693,6 +1700,7 @@ __cold int ipc4_user_process_module_message(struct ipc4_message_request *ipc4,
 		break;
 	case SOF_IPC4_MOD_UNBIND:
 #ifdef CONFIG_SOF_USERSPACE_LL
+		/* DP / LL: see comment above */
 		ret = ipc_user_forward_cmd(ipc4->primary.dat, ipc4->extension.dat,
 					   ipc4_user_target_core_module(ipc4));
 #else