From 46875049363ff26540ab0a9f7a1bbb2349b18f58 Mon Sep 17 00:00:00 2001
From: chrysn <chrysn@fsfe.org>
Date: Wed, 30 Sep 2020 23:55:29 +0200
Subject: [PATCH] drivers/saul/auto_init: Support bool and percent for dimmers

Booleans make sense for dimmers when accessed from a Sith application
that deals in absolutes.

Percent values are also a widespread interpretation of brightness
levels, and are thus supported as well.

The bit arithmetic makes sure that the arithmetic operation value / 100
* saul_pwm_resolution is done efficiently (by expression as a
multiplication followed by shifting) and accurately (by maximizing the
number of usable bits) while still being flexible both with respect to
integer sizes and to changes of saul_pwm_resolution.

Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
---
 drivers/saul/pwm_saul.c | 54 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/drivers/saul/pwm_saul.c b/drivers/saul/pwm_saul.c
index ca2db36859..0a84425e45 100644
--- a/drivers/saul/pwm_saul.c
+++ b/drivers/saul/pwm_saul.c
@@ -22,6 +22,42 @@
 #include "phydat.h"
 #include "periph/pwm.h"
 #include "saul/periph.h"
+#include "bitarithm.h"
+
+/**
+ * Find factor and shiftback such that for each value entry in the phydat, the
+ * resulting PWM duty cycle would be (value * factor) >> shiftback.
+ */
+static int extract_scaling(const phydat_t *state, int *factor, int *shiftback)
+{
+    if (state->scale != 0) {
+        return -ECANCELED;
+    }
+
+    /** Number of bits i by which we can shift the calculation (value * (255 << i)/100) >> i
+     * to get a better result than value * 2 (which would otherwise happen in integers) */
+    int shift100 = bitarithm_msb(INT_MAX) - bitarithm_msb(saul_pwm_resolution);
+
+    switch (state->unit) {
+    case UNIT_UNDEF:
+    case UNIT_NONE:
+        *factor = 1;
+        *shiftback = 0;
+        break;
+    case UNIT_BOOL:
+        *factor = saul_pwm_resolution;
+        *shiftback = 0;
+        break;
+    case UNIT_PERCENT:
+        *factor = ((int)saul_pwm_resolution << shift100) / 100;
+        *shiftback = shift100;
+        break;
+    default:
+        return -ECANCELED;
+    }
+
+    return 0;
+}
 
 static inline void setchan(const saul_pwm_channel_t *chan, uint16_t value)
 {
@@ -34,7 +70,14 @@ static int write_dimmer(const void *dev, phydat_t *state)
 {
     const saul_pwm_dimmer_params_t *p = dev;
 
-    setchan(&p->channel, state->val[0]);
+    int factor, shiftback;
+
+    int err = extract_scaling(state, &factor, &shiftback);
+    if (err < 0) {
+        return err;
+    }
+
+    setchan(&p->channel, (state->val[0] * factor) >> shiftback);
     return 3;
 }
 
@@ -48,8 +91,15 @@ static int write_rgb(const void *dev, phydat_t *state)
 {
     const saul_pwm_rgb_params_t *p = dev;
 
+    int factor, shiftback;
+
+    int err = extract_scaling(state, &factor, &shiftback);
+    if (err < 0) {
+        return err;
+    }
+
     for (int i = 0; i < 3; ++i) {
-        setchan(&p->channels[i], state->val[i]);
+        setchan(&p->channels[i], (state->val[i] * factor) >> shiftback);
     }
     return 3;
 }