dmaengine: dmatest: Prevent to run on misconfigured channel

Andy reported that commit 6b41030fdc79 ("dmaengine: dmatest:
Restore default for channel") broke his scripts for the case
where "busy" channel is used for configuration with expectation
that run command would do nothing. Instead, behavior was
(unintentionally) changed to treat such case as under-configuration
and progress with defaults, i.e. run command would start a test
with default setting for channel (which would use all channels).

Restore original behavior with tracking status of channel setter
so we can distinguish between misconfigured and under-configured
cases in run command and act accordingly.

Fixes: 6b41030fdc79 ("dmaengine: dmatest: Restore default for channel")
Reported-by: Andy Shevchenko <[e-mail]>
Tested-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Vladimir Murzin <[e-mail]>
Tested-by: Peter Ujfalusi <[e-mail]>
Signed-off-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/dmatest.c

···
  * @nr_channels:	number of channels under test
  * @lock:		access protection to the fields of this structure
  * @did_init:		module has been initialized completely
+ * @last_error:		test has faced configuration issues
  */
 static struct dmatest_info {
 	/* Test parameters */
···
 	/* Internal state */
 	struct list_head	channels;
 	unsigned int		nr_channels;
+	int			last_error;
 	struct mutex		lock;
 	bool			did_init;
 } test_info = {
···
 		return ret;
 	} else if (dmatest_run) {
 		if (!is_threaded_test_pending(info)) {
-			pr_info("No channels configured, continue with any\n");
-			if (!is_threaded_test_run(info))
-				stop_threaded_test(info);
-			add_threaded_test(info);
+			/*
+			 * We have nothing to run. This can be due to:
+			 */
+			ret = info->last_error;
+			if (ret) {
+				/* 1) Misconfiguration */
+				pr_err("Channel misconfigured, can't continue\n");
+				mutex_unlock(&info->lock);
+				return ret;
+			} else {
+				/* 2) We rely on defaults */
+				pr_info("No channels configured, continue with any\n");
+				if (!is_threaded_test_run(info))
+					stop_threaded_test(info);
+				add_threaded_test(info);
+			}
 		}
 		start_threaded_tests(info);
 	} else {
···
 	struct dmatest_info *info = &test_info;
 	struct dmatest_chan *dtc;
 	char chan_reset_val[20];
-	int ret = 0;
+	int ret;
 
 	mutex_lock(&info->lock);
 	ret = param_set_copystring(val, kp);
···
 		goto add_chan_err;
 	}
 
+	info->last_error = ret;
 	mutex_unlock(&info->lock);
 
 	return ret;
 
 add_chan_err:
 	param_set_copystring(chan_reset_val, kp);
+	info->last_error = ret;
 	mutex_unlock(&info->lock);
 
 	return ret;