patch-2.4.22 linux-2.4.22/Documentation/mips/time.README

• Lines: 60
• Date: 2003-08-25 04:44:39.000000000 -0700
• Orig file: linux-2.4.21/Documentation/mips/time.README
• Orig date: 2001-04-20 16:23:12.000000000 -0700

diff -urN linux-2.4.21/Documentation/mips/time.README linux-2.4.22/Documentation/mips/time.README
@@ -103,9 +103,9 @@
      Do you plan to use the CPU counter register as the timer interrupt
      or use an exnternal timer?
 
-     In order to CPU counter register as the timer interrupt source, you must
-     know the counter speed (mips_counter_frequency).  It is usually the
-     same as the CPU speed (Or it is ALWAYS the same?)
+     In order to use CPU counter register as the timer interrupt source, you 
+     must know the counter speed (mips_counter_frequency).  It is usually the
+     same as the CPU speed or an integral divisor of it.
 
   d) decide on whether you want to use high-level or low-level timer
      interrupt routines.  The low-level one is presumably faster, but should
@@ -154,8 +154,45 @@
 for some of the functions in time.c.  
 
 For example, you may define your own timer interrupt routine, which does
-its own processing and in turn calls timer_interrupt().
+some of its own processing and then calls timer_interrupt().
 
 You can also over-ride any of the built-in functions (gettimeoffset,
 RTC routines and/or timer interrupt routine).
 
+
+PORTING NOTES FOR SMP
+----------------------
+
+If you have a SMP box, things are slightly more complicated.  
+
+The time service running every jiffy is logically divided into two parts:
+
+  1) the one for the whole system  (defined in timer_interrupt())
+  2) the one that should run for each CPU (defined in local_timer_interrupt())
+
+You need to decide on your timer interrupt sources.
+
+  case 1) - whole system has only one timer interrupt delivered to one CPU
+
+	In this case, you set up timer interrupt as in UP systems.  In addtion,
+	you need to set emulate_local_timer_interrupt to 1 so that other
+	CPUs get to call local_timer_interrupt().
+
+	THIS IS CURRENTLY NOT IMPLEMNETED.  However, it is rather easy to write
+	one should such a need arise.  You simply make a IPI call.
+
+  case 2) - each CPU has a separate timer interrupt
+
+	In this case, you need to set up IRQ such that each of them will
+	call local_timer_interrupt().  In addition, you need to arrange
+	one and only one of them to call timer_interrupt().
+
+	You can also do the low-level version of those interrupt routines,
+	following similar dispatching routes described above.
+
+Note about do_gettimeoffset():
+  
+  It is very likely the CPU counter registers are not sync'ed up in a SMP box.
+  Therefore you cannot really use the many of the existing routines that
+  are based on CPU counter.  You should wirte your own gettimeoffset rouinte
+  if you want intra-jiffy resolution.