remoteproc: Refactor function rproc_free_vring()

When function rproc_free_vring() clears the virtio device section
it does so on the cached resource table rather than the one
installed in the remote processor memory.  When a remote processor
has been booted by another entity there is no need to use a cached
table and as such, no need to clear the virtio device section in
it.
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20200714195035.1426873-9-mathieu.poirier@linaro.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/remoteproc_core.c

@@ -411,10 +411,22 @@ void rproc_free_vring(struct rproc_vring *rvring)
 
 	idr_remove(&rproc->notifyids, rvring->notifyid);
 
-	/* reset resource entry info */
-	rsc = (void *)rproc->table_ptr + rvring->rvdev->rsc_offset;
-	rsc->vring[idx].da = 0;
-	rsc->vring[idx].notifyid = -1;
+	/*
+	 * At this point rproc_stop() has been called and the installed resource
+	 * table in the remote processor memory may no longer be accessible. As
+	 * such and as per rproc_stop(), rproc->table_ptr points to the cached
+	 * resource table (rproc->cached_table).  The cached resource table is
+	 * only available when a remote processor has been booted by the
+	 * remoteproc core, otherwise it is NULL.
+	 *
+	 * Based on the above, reset the virtio device section in the cached
+	 * resource table only if there is one to work with.
+	 */
+	if (rproc->table_ptr) {
+		rsc = (void *)rproc->table_ptr + rvring->rvdev->rsc_offset;
+		rsc->vring[idx].da = 0;
+		rsc->vring[idx].notifyid = -1;
+	}
 }
 
 static int rproc_vdev_do_start(struct rproc_subdev *subdev)