ALSA: cmipci: Allow disabling MPU port via module option

Patrick May reported that his sound card with CMI8378 chip causes a
crash / reboot when accessing the MIDI port that isn't actually
present on the board.  Moreover, despite of the documentation,
passing mpu_port=0 doesn't disable the MIDI port on this board.

It implies that the chip is a newer revision and the MPU401 port is
integrated and mapped on the PCI register.  For this chip model, the
driver enables the MPU port unconditionally, so far.

Although fixing the unexpected reboot would be the best solution, it's
not so trivial to identify the cause.  So, as a plan B, this patch
extends the existing mpu_port option usage to allow disabling the port
by specifying the value 0, just like we applied for fm_port option in
commit 2f24d159 ("[ALSA] cmipci - Allow to disable integrated FM
port").  As default, the MPU port is still enabled, but user can pass
mpu_port=0 to disable it.
Reported-and-tested-by: Patrick May <dusthillresident@gmail.com>
Link: https://lore.kernel.org/r/20191217081448.1144-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa-configuration.rst

@@ -495,7 +495,8 @@ Module for C-Media CMI8338/8738/8768/8770 PCI sound cards.
 mpu_port
     port address of MIDI interface (8338 only):
     0x300,0x310,0x320,0x330 = legacy port,
-    0 = disable (default)
+    1 = integrated PCI port (default on 8738),
+    0 = disable
 fm_port
     port address of OPL-3 FM synthesizer (8x38 only):
     0x388 = legacy port,

sound/pci/cmipci.c

@@ -42,7 +42,7 @@ MODULE_SUPPORTED_DEVICE("{{C-Media,CMI8738},"
 static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;	/* Index 0-MAX */
 static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;	/* ID for this card */
 static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;	/* Enable switches */
-static long mpu_port[SNDRV_CARDS];
+static long mpu_port[SNDRV_CARDS] = {[0 ... (SNDRV_CARDS-1)] = 1};
 static long fm_port[SNDRV_CARDS] = {[0 ... (SNDRV_CARDS-1)]=1};
 static bool soft_ac3[SNDRV_CARDS] = {[0 ... (SNDRV_CARDS-1)]=1};
 #ifdef SUPPORT_JOYSTICK
@@ -3132,7 +3132,8 @@ static int snd_cmipci_create(struct snd_card *card, struct pci_dev *pci,
 	if (cm->chip_version >= 39) {
 		val = snd_cmipci_read_b(cm, CM_REG_MPU_PCI + 1);
 		if (val != 0x00 && val != 0xff) {
-			iomidi = cm->iobase + CM_REG_MPU_PCI;
+			if (mpu_port[dev])
+				iomidi = cm->iobase + CM_REG_MPU_PCI;
 			integrated_midi = 1;
 		}
 	}