ALSA: serial-generic: remove shared static buffer

If multiple instances of this driver are instantiated and try to send
concurrently then the single static buffer snd_serial_generic_tx_work()
will cause corruption in the data output.

Move the buffer into the per-instance driver data to avoid this.

Signed-off-by: John Keeping <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

sound/drivers/serial-generic.c

···
 #define SERIAL_TX_STATE_ACTIVE	1
 #define SERIAL_TX_STATE_WAKEUP	2
 
+#define INTERNAL_BUF_SIZE 256
+
 struct snd_serial_generic {
 	struct serdev_device *serdev;
 
···
 	struct work_struct tx_work;
 	unsigned long tx_state;
 
+	char tx_buf[INTERNAL_BUF_SIZE];
 };
 
 static void snd_serial_generic_tx_wakeup(struct snd_serial_generic *drvdata)
···
 	schedule_work(&drvdata->tx_work);
 }
 
-#define INTERNAL_BUF_SIZE 256
-
 static void snd_serial_generic_tx_work(struct work_struct *work)
 {
-	static char buf[INTERNAL_BUF_SIZE];
 	int num_bytes;
 	struct snd_serial_generic *drvdata = container_of(work, struct snd_serial_generic,
 						   tx_work);
···
 		if (!test_bit(SERIAL_MODE_OUTPUT_OPEN, &drvdata->filemode))
 			break;
 
-		num_bytes = snd_rawmidi_transmit_peek(substream, buf, INTERNAL_BUF_SIZE);
-		num_bytes = serdev_device_write_buf(drvdata->serdev, buf, num_bytes);
+		num_bytes = snd_rawmidi_transmit_peek(substream, drvdata->tx_buf,
+						      INTERNAL_BUF_SIZE);
+		num_bytes = serdev_device_write_buf(drvdata->serdev, drvdata->tx_buf,
+						    num_bytes);
 
 		if (!num_bytes)
 			break;