Added comments for pll_setting_t and pll_settings_str

Author: ericxu233

include/acl_pll.h

@@ -8,16 +8,19 @@
 
 #include "acl_bsp_io.h"
 
+// Settings for the Phase-Locked-Loop
+// note that we use double frequency of the kernel clock for some double-pumped
+// memories
 typedef struct {
-  unsigned int freq_khz;
-  unsigned int m;
-  unsigned int n;
-  unsigned int k;
-  unsigned int c0;
-  unsigned int c1;
-  unsigned int r;
-  unsigned int cp;
-  unsigned int div;
+  unsigned int freq_khz; /* output frequency in kHz */
+  unsigned int m;        /* multiplier factor */
+  unsigned int n;        /* 1st divider factor */
+  unsigned int k;        /* 2nd divider factor */
+  unsigned int c0;       /* output divider for kernel clock */
+  unsigned int c1;       /* output divider for double kernel clock */
+  unsigned int r;        /* lowpass filter setting */
+  unsigned int cp;       /* charge pump gain setting */
+  unsigned int div;      /* PLL mode */
 } pll_setting_t;
 
 typedef struct {

src/acl_device.cpp

@@ -708,6 +708,8 @@ clReleaseDevice(cl_device_id device) {
 
 ACL_EXPORT CL_API_ENTRY cl_int
 clReconfigurePLLIntelFPGA(cl_device_id device, const char *pll_settings_str) {
+  // To get the format of the second string argument please refer to the code
+  // comments specified for struct pll_setting_t in include/acl_pll.
   const acl_hal_t *hal;
   cl_int configure_status;
   acl_lock();