[RTEMS Project] #3175: Merge FreeBSD timecounter changes from 2015-01-20 to now

[RTEMS trac]

#3175: Merge FreeBSD timecounter changes from 2015-01-20 to now
-----------------------------+------------------------------
 Reporter:  Sebastian Huber  |       Owner:  Sebastian Huber
     Type:  enhancement      |      Status:  assigned
 Priority:  normal           |   Milestone:  4.12.0
Component:  score            |     Version:  4.12
 Severity:  normal           |  Resolution:
 Keywords:                   |
-----------------------------+------------------------------

Comment (by Ian Lepore <ian@…>):

 In [changeset:"51304ddeb797ce32c8c94c528caaf517edaa777f/rtems"
 51304dde/rtems]:
 {{{
 #!CommitTicketReference repository="rtems"
 revision="51304ddeb797ce32c8c94c528caaf517edaa777f"
 timecounter: Merge FreeBSD change r282424

 Implement a mechanism for making changes in the kernel<->driver PPS
 interface without breaking ABI or API compatibility with existing drivers.

 The existing data structures used to communicate between the kernel and
 driver portions of PPS processing contain no spare/padding fields and no
 flags field or other straightforward mechanism for communicating changes
 in the structures or behaviors of the code.  This makes it difficult to
 MFC new features added to the PPS facility.  ABI compatibility is
 important; out-of-tree drivers in module form are known to exist.  (Note
 that the existing api_version field in the pps_params structure must
 contain the value mandated by RFC 2783 and any RFCs that come along
 after.)

 These changes introduce a pair of abi-version fields which are filled in
 by the driver and the kernel respectively to indicate the interface
 version.  The driver sets its version field before calling the new
 pps_init_abi() function.  That lets the kernel know how much of the
 pps_state structure is understood by the driver and it can avoid using
 newer fields at the end of the structure that it knows about if the driver
 is a lower version.  The kernel fills in its version field during the init
 call, letting the driver know what features and data the kernel supports.

 To implement the new version information in a way that is backwards
 compatible with code from before these changes, the high bit of the
 lightly-used 'kcmode' field is repurposed as a flag bit that indicates the
 driver is aware of the abi versioning scheme.  Basically if this bit is
 clear that indicates a "version 0" driver and if it is set the driver_abi
 field indicates the version.

 These changes also move the recently-added 'mtx' field of pps_state from
 the middle to the end of the structure, and make the kernel code that uses
 this field conditional on the driver being abi version 1 or higher.  It
 changes the only driver currently supplying the mtx field, usb_serial, to
 use pps_init_abi().

 Reviewed by:    hselasky@

 Update #3175.
 }}}

--
Ticket URL: <http://devel.rtems.org/ticket/3175#comment:3>
RTEMS Project <http://www.rtems.org/>
RTEMS Project