remoteproc: Introducing function rproc_attach()

Introducing function rproc_attach() to enact the same actions as
rproc_start(), but without the steps related to the handling of
a firmware image.  That way we can properly deal with scenarios
where the remoteproc core needs to attach with a remote processsor
that has been booted by another entity.
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/20200714195035.1426873-4-mathieu.poirier@linaro.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/remoteproc_core.c

@@ -1372,6 +1372,48 @@ static int rproc_start(struct rproc *rproc, const struct firmware *fw)
 	return ret;
 }
 
+static int __maybe_unused rproc_attach(struct rproc *rproc)
+{
+	struct device *dev = &rproc->dev;
+	int ret;
+
+	ret = rproc_prepare_subdevices(rproc);
+	if (ret) {
+		dev_err(dev, "failed to prepare subdevices for %s: %d\n",
+			rproc->name, ret);
+		goto out;
+	}
+
+	/* Attach to the remote processor */
+	ret = rproc_attach_device(rproc);
+	if (ret) {
+		dev_err(dev, "can't attach to rproc %s: %d\n",
+			rproc->name, ret);
+		goto unprepare_subdevices;
+	}
+
+	/* Start any subdevices for the remote processor */
+	ret = rproc_start_subdevices(rproc);
+	if (ret) {
+		dev_err(dev, "failed to probe subdevices for %s: %d\n",
+			rproc->name, ret);
+		goto stop_rproc;
+	}
+
+	rproc->state = RPROC_RUNNING;
+
+	dev_info(dev, "remote processor %s is now attached\n", rproc->name);
+
+	return 0;
+
+stop_rproc:
+	rproc->ops->stop(rproc);
+unprepare_subdevices:
+	rproc_unprepare_subdevices(rproc);
+out:
+	return ret;
+}
+
 /*
  * take a firmware and boot a remote processor with it.
  */