firmware: arm_scmi: Reset Rx buffer to max size during async commands

During an async commands execution the Rx buffer length is at first set
to max_msg_sz when the synchronous part of the command is first sent.
However once the synchronous part completes the transport layer waits
for the delayed response which will be processed using the same xfer
descriptor initially allocated. Since synchronous response received at
the end of the xfer will shrink the Rx buffer length to the effective
payload response length, it needs to be reset again.

Raise the Rx buffer length again to max_msg_sz before fetching the
delayed response to ensure full response is read correctly from the
shared memory.

Link: https://lore.kernel.org/r/20210601102421.26581-2-cristian.marussi@arm.com
Fixes: 58ecdf03 ("firmware: arm_scmi: Add support for asynchronous commands and delayed response")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
[sudeep.holla: moved reset to scmi_handle_response as it could race with
               do_xfer_with_response]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/driver.c

@@ -335,6 +335,10 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
 		return;
 	}
 
+	/* rx.len could be shrunk in the sync do_xfer, so reset to maxsz */
+	if (msg_type == MSG_TYPE_DELAYED_RESP)
+		xfer->rx.len = info->desc->max_msg_size;
+
 	scmi_dump_header_dbg(dev, &xfer->hdr);
 
 	info->desc->ops->fetch_response(cinfo, xfer);