libata: implement libata.force module parameter

This patch implements libata.force module parameter which can
selectively override ATA port, link and device configurations
including cable type, SATA PHY SPD limit, transfer mode and NCQ.

For example, you can say "use 1.5Gbps for all fan-out ports attached
to the second port but allow 3.0Gbps for the PMP device itself, oh,
the device attached to the third fan-out port chokes on NCQ and
shouldn't go over UDMA4" by the following.

 libata.force=2:1.5g,2.15:3.0g,2.03:noncq,udma4

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

Documentation/kernel-parameters.txt

@@ -950,6 +950,41 @@ and is between 256 and 4096 characters. It is defined in the file
 			when set.
 			Format: <int>
 
+	libata.force=	[LIBATA] Force configurations.  The format is comma
+			separated list of "[ID:]VAL" where ID is
+			PORT[:DEVICE].  PORT and DEVICE are decimal numbers
+			matching port, link or device.  Basically, it matches
+			the ATA ID string printed on console by libata.  If
+			the whole ID part is omitted, the last PORT and DEVICE
+			values are used.  If ID hasn't been specified yet, the
+			configuration applies to all ports, links and devices.
+
+			If only DEVICE is omitted, the parameter applies to
+			the port and all links and devices behind it.  DEVICE
+			number of 0 either selects the first device or the
+			first fan-out link behind PMP device.  It does not
+			select the host link.  DEVICE number of 15 selects the
+			host link and device attached to it.
+
+			The VAL specifies the configuration to force.  As long
+			as there's no ambiguity shortcut notation is allowed.
+			For example, both 1.5 and 1.5G would work for 1.5Gbps.
+			The following configurations can be forced.
+
+			* Cable type: 40c, 80c, short40c, unk, ign or sata.
+			  Any ID with matching PORT is used.
+
+			* SATA link speed limit: 1.5Gbps or 3.0Gbps.
+
+			* Transfer mode: pio[0-7], mwdma[0-4] and udma[0-7].
+			  udma[/][16,25,33,44,66,100,133] notation is also
+			  allowed.
+
+			* [no]ncq: Turn on or off NCQ.
+
+			If there are multiple matching configurations changing
+			the same attribute, the last one is used.
+
 	load_ramdisk=	[RAM] List of ramdisks to load from floppy
 			See Documentation/ramdisk.txt.