ALSA: dice: restrict the driver to playback-only devices

At the moment, this driver supports only playback, while FFADO supports
(only) full-duplex devices.  So, prevent conflicts by not claiming
devices that would be better handled by FFADO.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

sound/firewire/Kconfig

@@ -12,14 +12,16 @@ config SND_FIREWIRE_LIB
 	depends on SND_PCM
 
 config SND_DICE
-	tristate "DICE devices (EXPERIMENTAL)"
+	tristate "DICE-based DACs (EXPERIMENTAL)"
 	select SND_HWDEP
 	select SND_PCM
 	select SND_FIREWIRE_LIB
 	help
-	  Say Y here to include support for many FireWire audio interfaces
-	  based on the DICE chip family (DICE-II/Jr/Mini) from TC Applied
-	  Technologies.
+	  Say Y here to include support for many DACs based on the DICE
+	  chip family (DICE-II/Jr/Mini) from TC Applied Technologies.
+
+	  At the moment, this driver supports playback only.  If you
+	  want to use devices that support capturing, use FFADO instead.
 
 	  To compile this driver as a module, choose M here: the module
 	  will be called snd-dice.

sound/firewire/dice.c

@@ -1128,6 +1128,7 @@ static int dice_interface_check(struct fw_unit *unit)
 	int key, value, vendor = -1, model = -1, err;
 	unsigned int category, i;
 	__be32 pointers[ARRAY_SIZE(min_values)];
+	__be32 tx_data[4];
 	__be32 version;
 
 	/*
@@ -1171,6 +1172,14 @@ static int dice_interface_check(struct fw_unit *unit)
 			return -ENODEV;
 	}
 
+	/* We support playback only. Let capture devices be handled by FFADO. */
+	err = snd_fw_transaction(unit, TCODE_READ_BLOCK_REQUEST,
+				 DICE_PRIVATE_SPACE +
+				 be32_to_cpu(pointers[2]) * 4,
+				 tx_data, sizeof(tx_data), 0);
+	if (err < 0 || (tx_data[0] && tx_data[3]))
+		return -ENODEV;
+
 	/*
 	 * Check that the implemented DICE driver specification major version
 	 * number matches.