ptp: Remove 'default y' for VMCLOCK PTP device

The VMCLOCK device gives support for accurate timekeeping even across
live migration, unlike the KVM PTP clock. To help ensure that users can
always use ptp_vmclock where it's available in preference to ptp_kvm,
set it to 'default PTP_1588_CLOCK_VMCLOCK' instead of 'default y'.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/89955b74d225129d6e3d79b53aa8d81d1b50560f.camel@infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: dwmw2

drivers/ptp/Kconfig

@@ -135,12 +135,16 @@ config PTP_1588_CLOCK_VMCLOCK
 	tristate "Virtual machine PTP clock"
 	depends on X86_TSC || ARM_ARCH_TIMER
 	depends on PTP_1588_CLOCK && ACPI && ARCH_SUPPORTS_INT128
-	default y
+	default PTP_1588_CLOCK_KVM
 	help
 	  This driver adds support for using a virtual precision clock
 	  advertised by the hypervisor. This clock is only useful in virtual
 	  machines where such a device is present.
 
+	  Unlike the KVM virtual PTP clock, the VMCLOCK device offers support
+	  for reliable timekeeping even across live migration. So this driver
+	  is enabled by default whenever the KVM PTP clock is.
+
 	  To compile this driver as a module, choose M here: the module
 	  will be called ptp_vmclock.