lin16: factorize code for exp5

Centralize the lin16 code. It should be used to better apply the
specifications based on PMBus-Specification-Rev-1-3-1-Part-II-20150313.pdf
from https://pmbus.org/specification-archives/

Author: vjardin

src/TODO

@@ -1,7 +1,4 @@
 * use getopt_long() for each subcommand
 * add a usage_long() for some complex commands
 * improve main() using a table of token -> function
-* factorize the ldexp() from double to uint16_t, XXX libm is not needed when:
-  double scaled = (exp5 < 0)
-                  ? v * (double)(1u << (-exp5))   /* exp negative -> multiply */
-                  : v / (double)(1u << exp5);     /* exp zero/positive -> divide */
+* compile using 'warning_level=everything',

src/pgood_cmd.c

@@ -1,13 +1,13 @@
 /* SPDX-License-Identifier: AGPL-3.0-or-later */
 #include "pmbus_io.h"
 #include "util_json.h"
+#include "util_lin.h"
 
 #include <jansson.h>
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>
-#include <math.h>
 
 static void
 usage_pgood(void) {
@@ -104,13 +104,7 @@ cmd_pgood(int fd, int argc, char *const *argv, int pretty) {
         return 2;
       }
       double v = strtod(on_v, NULL);
-      /* N = v * 2^(−exp5) */
-      double scaled = ldexp(v, -exp5);
-      if (scaled < 0.0)
-        scaled = 0.0;
-      if (scaled > 65535.0)
-        scaled = 65535.0;
-      won = (uint16_t) (scaled + 0.5);
+      won = units_to_lin16u(v, exp5);
 
       set_on = 1;
     }
@@ -122,13 +116,7 @@ cmd_pgood(int fd, int argc, char *const *argv, int pretty) {
         return 2;
       }
       double v = strtod(off_v, NULL);
-      /* N = v * 2^(−exp5) */
-      double scaled = ldexp(v, -exp5);
-      if (scaled < 0.0)
-        scaled = 0.0;
-      if (scaled > 65535.0)
-        scaled = 65535.0;
-      wof = (uint16_t) (scaled + 0.5);
+      wof = units_to_lin16u(v, exp5);
 
       set_off = 1;
     }

src/pmbus_io.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: AGPL-3.0-or-later */
 
 #include "pmbus_io.h"
+#include "util_lin.h"
 
 #include <linux/i2c-dev.h>
 #include <i2c/smbus.h>
@@ -71,6 +72,7 @@ pmbus_send_byte(int fd, uint8_t cmd) {
   return i2c_smbus_write_byte(fd, cmd);
 }
 
+/* see PMBus-Specification-Rev-1-3-1-Part-II-20150313.pdf, section 8.3 */
 int
 pmbus_get_vout_mode_exp(int fd, int *exp_out) {
   int v = pmbus_rd_byte(fd, PMBUS_VOUT_MODE);
@@ -105,8 +107,7 @@ pmbus_lin11_to_double(uint16_t raw) {
 
 double
 pmbus_lin16u_to_double(uint16_t raw, int exp5) {
-  /* value = raw * 2^exp5 */
-  return ldexp((double)raw, exp5);
+  return lin16u_to_units(raw, exp5);
 }
 
 uint16_t

src/util_lin.h

@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later */
+
+#pragma once
+
+#include <stdint.h>
+#include <math.h> /* ldexp: no need of libm */
+
+/* TODO: align with PMBus-Specification-Rev-1-3-1-Part-II-20150313.pdf */
+
+/* Round-to-nearest and saturate into [0, 65535] for non-negative doubles. */
+static inline uint16_t
+u16_round_sat_pos(double x) {
+  if (isnan(x))
+    return 0;
+
+  if (x >= 65535.0)
+    return 65535;
+
+  return (uint16_t)(x + 0.5);
+}
+
+/* Linear16-Unsigned: units = y * 2^N */
+static inline double
+lin16u_to_units(uint16_t y, int expN) {
+  return ldexp((double)y, expN);
+}
+
+/* lin16u = round(units * 2^{-N}) */
+static inline uint16_t units_to_lin16u(double units, int expN) {
+  return u16_round_sat_pos(ldexp(units, -expN));
+}

src/vin_cmd.c

@@ -1,13 +1,13 @@
 /* SPDX-License-Identifier: AGPL-3.0-or-later */
 #include "pmbus_io.h"
 #include "util_json.h"
+#include "util_lin.h"
 
 #include <jansson.h>
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>
-#include <math.h>
 
 /* Uses LIN16U coding with user-provided exponent (exp5), or raw words. */
 static void
@@ -107,13 +107,7 @@ cmd_vin(int fd, int argc, char *const *argv, int pretty) {
         return 2;
       }
       double v = strtod(on_v, NULL);
-      /* N = v * 2^(−exp5) */
-      double scaled = ldexp(v, -exp5);
-      if (scaled < 0.0)
-        scaled = 0.0;
-      if (scaled > 65535.0)
-        scaled = 65535.0;
-      won = (uint16_t) (scaled + 0.5);
+      won = units_to_lin16u(v, exp5);
 
       set_on = 1;
     }
@@ -125,13 +119,7 @@ cmd_vin(int fd, int argc, char *const *argv, int pretty) {
         return 2;
       }
       double v = strtod(off_v, NULL);
-      /* N = v * 2^(−exp5) */
-      double scaled = ldexp(v, -exp5);
-      if (scaled < 0.0)
-        scaled = 0.0;
-      if (scaled > 65535.0)
-        scaled = 65535.0;
-      wof = (uint16_t) (scaled + 0.5);
+      wof = units_to_lin16u(v, exp5);
 
       set_off = 1;
     }

src/vout_cmd.c

@@ -2,6 +2,7 @@
 
 #include "pmbus_io.h"
 #include "util_json.h"
+#include "util_lin.h"
 #include <jansson.h>
 #include <string.h>
 #include <stdlib.h>
@@ -21,20 +22,13 @@
 static inline double
 lin16u_to_volts(uint16_t y, int exp5) {
   /* V = Y * 2^N */
-  return (double) y *ldexp(1.0, exp5);
+  return lin16u_to_units(y, exp5);
 }
 
 static inline uint16_t
 volts_to_lin16u(double v, int exp5) {
   /* Y = round(V * 2^{-N}) */
-  double scaled = v * ldexp(1.0, -exp5);
-  if (scaled < 0.0)
-    scaled = 0.0;
-  if (scaled > 65535.0)
-    scaled = 65535.0;
-
-  /* scaled is clamped to [0, 65535] already: llround(scaled), avoid libm */
-  return (uint16_t)(scaled + 0.5);
+  return units_to_lin16u(v, exp5);
 }
 
 static int