hw/ppc: Support for an IBM PPE42 CPU decrementer

The IBM PPE42 processors support a 32-bit decrementer
that can raise an external interrupt when DEC[0]
transitions from a 0 to a -1 (a non-negative value to a
negative value).  It also continues decrementing
even after this condition is met.

The BookE timer is slightly different in that it
raises an interrupt when the DEC value reaches 0
and stops decrementing at that point.

Support a PPE42 version of the BookE timer by
adding a new PPC_TIMER_PPE flag that has the timer
code look for the transition from a non-negative value
to a negative value and allows the value to
continue decrementing.

Signed-off-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Link: https://lore.kernel.org/r/20250925201758.652077-8-milesg@linux.ibm.com
Message-ID: <20250925201758.652077-8-milesg@linux.ibm.com>
2 files changed