busy_wait based on decrementer values inside interrupts.

Change-Id: Iceba2e502420cd03e173edfa7740c6539e120264
Reviewed-on: http://gfw160.aus.stglabs.ibm.com:8080/gerrit/21562
Reviewed-by: Wael Elessawy <welessa@us.ibm.com>
Tested-by: Wael Elessawy <welessa@us.ibm.com>

2 files changed, 36 insertions, 6 deletions

diff --git a/src/occ_gpe0/apss_util.c b/src/occ_gpe0/apss_util.c
index 2a78561..04e9879 100644
--- a/src/occ_gpe0/apss_util.c
+++ b/src/occ_gpe0/apss_util.c
@@ -115,17 +115,39 @@ int wait_spi_completion(GpeErrorStruct *error, uint32_t reg, uint8_t timeout)
  * End Function Specification
  */
 
-// result based on busy_wait(1) calibration against pk_sleep(1)
-#define BUSY_LOOP_CONSTANT 71
 
 void busy_wait(uint32_t t_microseconds)
 {
     int j;
     volatile uint32_t i;
-//    ppe42_mullw macro works fine, but compiler parameter settings seems broken, use nested loop instead
-//    int loop = t * 100;  // Assuming loop iteration takes 6 ppe cycles = 0.01 microseconds
 
-    for(j = 0; j < BUSY_LOOP_CONSTANT; j++)
-        for(i = 0; i < t_microseconds; i++);
+    uint32_t start_decrementer_value;      // The decrementer register value at the beginning
+    uint32_t end_decrementer_value;        // The decrementer register value at the end
+    uint32_t current_decrementer_value;    // The current decrementer register value
+    uint32_t duration;
+    MFDEC(start_decrementer_value);        // get the decrementer register value at the beginning
+    current_decrementer_value = start_decrementer_value;
+
+    // multiply the delay time by the 37.5 MHz external clock frequency.
+    // I believe that the ppe42_mullw macro works fine, but the
+    // compiler parameter settings seems broken.
+    // @todo: This is a temporary fix, use external frequency directive.
+    duration =  (t_microseconds << 5)
+        + (t_microseconds << 2)
+        + (t_microseconds)
+        + (t_microseconds >> 1);
+
+    // Calculate the decrementer register value at the end of the busy wait period
+    end_decrementer_value = start_decrementer_value - duration;
+
+    if(start_decrementer_value < end_decrementer_value);          // decrementer overflows during the busy wait?
+    {
+        MFDEC(current_decrementer_value);
+        while(current_decrementer_value < end_decrementer_value)  // Wait until Decrementer overflows
+            MFDEC(current_decrementer_value);
+    }
+
+    while (current_decrementer_value > end_decrementer_value)     // Wait until end_decrementer_value is reached
+        MFDEC(current_decrementer_value);
 
 }
diff --git a/src/occ_gpe0/apss_util.h b/src/occ_gpe0/apss_util.h
index 1f7a76c..1c53cde 100644
--- a/src/occ_gpe0/apss_util.h
+++ b/src/occ_gpe0/apss_util.h
@@ -8,4 +8,12 @@ void apss_set_ffdc(GpeErrorStruct *o_error, uint32_t i_addr, uint32_t i_rc, uint
 
 int wait_spi_completion(GpeErrorStruct *error, uint32_t reg, uint8_t timeout);
 
+// Read decrementer register
+#define MFDEC(reg_var) \
+asm volatile \
+    ( \
+    " mfdec %[dec_var] \n" \
+    : [dec_var]"=r"(reg_var) \
+    );
+
 #endif //_APSS_UTIL_H