media: ddbridge: Kconfig option to control the MSI modparam default

It is known that MSI interrupts - while working quite well so far - can
still cause issues on some hardware platforms (causing I2C timeouts due
to unhandled interrupts). The msi variable/option is set to 1 by default.
So, add a Kconfig option prefixed with "EXPERIMENTAL" that will control
the default value of that modparam, defaulting to off for a better
user experience and (guaranteed) stable operation "per default".

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Tested-by: Jasmin Jessich <jasmin@anw.at>
Tested-by: Dietmar Spingler <d_spingler@freenet.de>
Tested-by: Manfred Knick <Manfred.Knick@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/pci/ddbridge/Kconfig

@@ -26,3 +26,18 @@ config DVB_DDBRIDGE
 	  - CineS2 V7/V7A and DuoFlex S2 V4 (ST STV0910-based)
 
 	  Say Y if you own such a card and want to use it.
+
+config DVB_DDBRIDGE_MSIENABLE
+	bool "Enable Message Signaled Interrupts (MSI) per default (EXPERIMENTAL)"
+	depends on DVB_DDBRIDGE
+	depends on PCI_MSI
+	default n
+	---help---
+	  Use PCI MSI (Message Signaled Interrupts) per default. Enabling this
+	  might lead to I2C errors originating from the bridge in conjunction
+	  with certain SATA controllers, requiring a reload of the ddbridge
+	  module. MSI can still be disabled by passing msi=0 as option, as
+	  this will just change the msi option default value.
+
+	  If you're unsure, concerned about stability and don't want to pass
+	  module options in case of troubles, say N.

drivers/media/pci/ddbridge/ddbridge-main.c

@@ -47,10 +47,17 @@ MODULE_PARM_DESC(adapter_alloc,
 		 "0-one adapter per io, 1-one per tab with io, 2-one per tab, 3-one for all");
 
 #ifdef CONFIG_PCI_MSI
+#ifdef CONFIG_DVB_DDBRIDGE_MSIENABLE
 int msi = 1;
+#else
+int msi;
+#endif
 module_param(msi, int, 0444);
-MODULE_PARM_DESC(msi,
-		 " Control MSI interrupts: 0-disable, 1-enable (default)");
+#ifdef CONFIG_DVB_DDBRIDGE_MSIENABLE
+MODULE_PARM_DESC(msi, "Control MSI interrupts: 0-disable, 1-enable (default)");
+#else
+MODULE_PARM_DESC(msi, "Control MSI interrupts: 0-disable (default), 1-enable");
+#endif
 #endif
 
 int ci_bitrate = 70000;