firmware: turris-mox-rwtm: fail probing when firmware does not support hwrng

When Marvell's rWTM firmware, which does not support the GET_RANDOM
command, is used, kernel prints an error message
  hwrng: no data available
every 10 seconds.

Fail probing of this driver if the rWTM firmware does not support the
GET_RANDOM command.

Fixes: 389711b3 ("firmware: Add Turris Mox rWTM firmware driver")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>

drivers/firmware/turris-mox-rwtm.c

@@ -260,6 +260,27 @@ static int mox_get_board_info(struct mox_rwtm *rwtm)
 	return 0;
 }
 
+static int check_get_random_support(struct mox_rwtm *rwtm)
+{
+	struct armada_37xx_rwtm_tx_msg msg;
+	int ret;
+
+	msg.command = MBOX_CMD_GET_RANDOM;
+	msg.args[0] = 1;
+	msg.args[1] = rwtm->buf_phys;
+	msg.args[2] = 4;
+
+	ret = mbox_send_message(rwtm->mbox, &msg);
+	if (ret < 0)
+		return ret;
+
+	ret = wait_for_completion_timeout(&rwtm->cmd_done, HZ / 2);
+	if (ret < 0)
+		return ret;
+
+	return mox_get_status(MBOX_CMD_GET_RANDOM, rwtm->reply.retval);
+}
+
 static int mox_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 {
 	struct mox_rwtm *rwtm = (struct mox_rwtm *) rng->priv;
@@ -497,6 +518,13 @@ static int turris_mox_rwtm_probe(struct platform_device *pdev)
 	if (ret < 0)
 		dev_warn(dev, "Cannot read board information: %i\n", ret);
 
+	ret = check_get_random_support(rwtm);
+	if (ret < 0) {
+		dev_notice(dev,
+			   "Firmware does not support the GET_RANDOM command\n");
+		goto free_channel;
+	}
+
 	rwtm->hwrng.name = DRIVER_NAME "_hwrng";
 	rwtm->hwrng.read = mox_hwrng_read;
 	rwtm->hwrng.priv = (unsigned long) rwtm;