Shrink WS2811/WS2812 LED strip DMA buffer with a chunked circular refill

src/main/common/colorconversion.c

@@ -16,6 +16,7 @@
  */
 
 #include "stdint.h"
+#include "stdbool.h"
 
 #include "color.h"
 #include "colorconversion.h"
@@ -24,9 +25,35 @@
  * Source below found here: http://www.kasperkamperman.com/blog/arduino/arduino-programming-hsb-to-rgb/
  */
 
+// Small direct-mapped cache of recent HSV->RGB conversions. LED strip
+// patterns tend to reuse a handful of distinct colors across many LEDs and
+// updates, so a few cached entries catch most repeats without the cost of a
+// full RGB-native color store. Round-robin replacement is enough here: the
+// working set per frame is normally <= 4 distinct colors, so eviction order
+// doesn't matter much. Not safe for concurrent callers (e.g. one from an
+// ISR, one from task context) — currently fine since the sole caller
+// (light_ws2811strip.c) never does that.
+static struct {
+    hsvColor_t in;
+    rgbColor24bpp_t out;
+    bool valid;
+} hsvToRgbCache[4];
+static uint8_t hsvToRgbCacheNextSlot = 0;
+
+static bool hsvColorEqual(const hsvColor_t *a, const hsvColor_t *b)
+{
+    return a->h == b->h && a->s == b->s && a->v == b->v;
+}
+
 rgbColor24bpp_t* hsvToRgb24(const hsvColor_t* c)
 {
-    static rgbColor24bpp_t r;
+    for (int i = 0; i < 4; i++) {
+        if (hsvToRgbCache[i].valid && hsvColorEqual(&hsvToRgbCache[i].in, c)) {
+            return &hsvToRgbCache[i].out;
+        }
+    }
+
+    rgbColor24bpp_t r;
 
     uint16_t val = c->v;
     uint16_t sat = 255 - c->s;
@@ -79,6 +106,13 @@ rgbColor24bpp_t* hsvToRgb24(const hsvColor_t* c)
 
         }
     }
-    return &r;
+
+    hsvToRgbCache[hsvToRgbCacheNextSlot].in = *c;
+    hsvToRgbCache[hsvToRgbCacheNextSlot].out = r;
+    hsvToRgbCache[hsvToRgbCacheNextSlot].valid = true;
+    rgbColor24bpp_t *cached = &hsvToRgbCache[hsvToRgbCacheNextSlot].out;
+    hsvToRgbCacheNextSlot = (hsvToRgbCacheNextSlot + 1) % 4;
+
+    return cached;
 }
 

src/main/drivers/light_ws2811strip.c

@@ -37,10 +37,14 @@
 
 #include "common/color.h"
 #include "common/colorconversion.h"
+#include "common/maths.h"
+#include "common/time.h"
 
 #include "drivers/dma.h"
 #include "drivers/io.h"
+#include "drivers/time.h"
 #include "drivers/timer.h"
+#include "drivers/timer_impl.h"
 #include "drivers/light_ws2811strip.h"
 
 #include "fc/runtime_config.h"
@@ -49,7 +53,20 @@
 #define WS2811_BIT_COMPARE_1 ((WS2811_PERIOD * 2) / 3)
 #define WS2811_BIT_COMPARE_0 (WS2811_PERIOD / 3)
 
-static DMA_RAM timerDMASafeType_t ledStripDMABuffer[WS2811_DMA_BUFFER_SIZE];
+// Circular DMA buffer: 2 halves, 1 LED group each. ws2811DMARefillCallback
+// refills whichever half DMA just finished sending, so this only needs to
+// hold a couple of LEDs regardless of strip length.
+#define WS2811_LEDS_PER_GROUP 4
+#define WS2811_GROUP_BITS (WS2811_LEDS_PER_GROUP * WS2811_BITS_PER_LED)
+#define WS2811_CHUNK_BUFFER_SIZE (2 * WS2811_GROUP_BITS)
+
+// WS2812 reset/latch is a *minimum* low duration with no upper bound, so
+// this is a threshold to check against, not a preamble to budget for.
+#define WS2811_RESET_US 60
+
+// CCR is a 16-bit register on TIM3/TIM4; DMA must write it at that width or
+// the high byte is left stale.
+static DMA_RAM uint16_t ledStripDMABuffer[WS2811_CHUNK_BUFFER_SIZE];
 
 static IO_t ws2811IO = IO_NONE;
 static TCH_t * ws2811TCH = NULL;
@@ -93,16 +110,37 @@ void setStripColors(const hsvColor_t *colors)
     }
 }
 
+static void ws2811DMARefillCallback(TCH_t * tch, bool transferComplete);
+
 bool ledConfigureDMA(void) {
     /* Compute the prescaler value */
     uint8_t period = WS2811_TIMER_HZ / WS2811_CARRIER_HZ;
 
     timerConfigBase(ws2811TCH, period, WS2811_TIMER_HZ);
     timerPWMConfigChannel(ws2811TCH, 0);
 
-    return timerPWMConfigChannelDMA(ws2811TCH, ledStripDMABuffer, sizeof(ledStripDMABuffer[0]), WS2811_DMA_BUFFER_SIZE);
+    return timerPWMConfigChannelDMA(ws2811TCH, ledStripDMABuffer, sizeof(ledStripDMABuffer[0]), WS2811_CHUNK_BUFFER_SIZE);
 }
 
+// Written from both task context and the DMA refill ISR (never truly
+// concurrently — the ISR only runs while a transfer is active, and task
+// context only touches these once it isn't — but volatile documents that
+// and guards against the compiler assuming otherwise).
+
+// Number of LEDs in the most recent transfer; bounds the DMA transfer (and
+// ws2811SetIdleHigh's target) to what's actually configured.
+static volatile uint16_t activeLedCount = WS2811_LED_STRIP_LENGTH;
+
+// groupInHalf[i]: group index currently in half i, so the refill callback
+// can tell when the group it just finished sending was the last one.
+static volatile uint16_t totalGroups;
+static volatile uint16_t nextGroupToAssign;
+static volatile uint16_t groupInHalf[2];
+
+// Shared between normal transfer completion and ws2811SetIdleHigh (PINIO).
+static volatile bool lineIdleLow = true;
+static volatile timeUs_t lastLowAtUs = 0;
+
 void ws2811LedStripInit(void)
 {
     const timerHardware_t * timHw = timerGetByTag(IO_TAG(WS2811_PIN), TIM_USE_ANY);
@@ -120,6 +158,8 @@ void ws2811LedStripInit(void)
         return;
     }
 
+    impl_timerPWMSetDMARefillCallback(ws2811TCH, ws2811DMARefillCallback);
+
     ws2811IO = IOGetByTag(timHw->tag); //IOGetByTag(IO_TAG(WS2811_PIN));
     IOInit(ws2811IO, OWNER_LED_STRIP, RESOURCE_OUTPUT, 0);
     IOConfigGPIOAF(ws2811IO, IOCFG_AF_PP_FAST, timHw->alternateFunction);
@@ -132,65 +172,131 @@ void ws2811LedStripInit(void)
 
     // Zero out DMA buffer — LED pin idles LOW between WS2812 bursts
     memset(&ledStripDMABuffer, 0, sizeof(ledStripDMABuffer));
+    lineIdleLow = true;
+    lastLowAtUs = micros();
     ws2811Initialised = true;
 
-    ws2811UpdateStrip();
+    ws2811UpdateStrip(WS2811_LED_STRIP_LENGTH);
 }
 
 bool isWS2811LedStripReady(void)
 {
     return !timerPWMDMAInProgress(ws2811TCH);
 }
 
-STATIC_UNIT_TESTED uint16_t dmaBufferOffset;
-static int16_t ledIndex;
-
-STATIC_UNIT_TESTED void fastUpdateLEDDMABuffer(rgbColor24bpp_t *color)
+static void writeLedBits(uint16_t *dest, const rgbColor24bpp_t *color)
 {
     uint32_t grb = (color->rgb.g << 16) | (color->rgb.r << 8) | (color->rgb.b);
 
     for (int8_t index = 23; index >= 0; index--) {
-        ledStripDMABuffer[WS2811_DELAY_BUFFER_LENGTH + dmaBufferOffset++] = (grb & (1 << index)) ? WS2811_BIT_COMPARE_1 : WS2811_BIT_COMPARE_0;
+        *dest++ = (grb & (1 << index)) ? WS2811_BIT_COMPARE_1 : WS2811_BIT_COMPARE_0;
     }
 }
 
-/*
- * This method is non-blocking unless an existing LED update is in progress.
- * it does not wait until all the LEDs have been updated, that happens in the background.
- */
-void ws2811UpdateStrip(void)
+// Slots at or beyond activeLedCount get a direct "off" write instead of a
+// color lookup, so a group is self-contained regardless of where the
+// configured strip actually ends.
+static void ws2811FillGroup(uint8_t halfIndex, uint16_t groupIndex)
 {
-    static rgbColor24bpp_t *rgb24;
+    uint16_t *half = &ledStripDMABuffer[halfIndex * WS2811_GROUP_BITS];
+    uint16_t baseLed = groupIndex * WS2811_LEDS_PER_GROUP;
+
+    for (uint8_t slot = 0; slot < WS2811_LEDS_PER_GROUP; slot++) {
+        uint16_t *dest = &half[slot * WS2811_BITS_PER_LED];
+        uint16_t ledIdx = baseLed + slot;
+
+        if (ledIdx < activeLedCount) {
+            writeLedBits(dest, hsvToRgb24(&ledColorBuffer[ledIdx]));
+        } else {
+            for (uint8_t bit = 0; bit < WS2811_BITS_PER_LED; bit++) {
+                dest[bit] = WS2811_BIT_COMPARE_0;
+            }
+        }
+    }
+}
 
-    // don't wait - risk of infinite block, just get an update next time round
-    if (timerPWMDMAInProgress(ws2811TCH)) {
+static void ws2811RefillHalf(uint8_t halfIndex)
+{
+    ws2811FillGroup(halfIndex, nextGroupToAssign);
+    groupInHalf[halfIndex] = nextGroupToAssign;
+    nextGroupToAssign++;
+}
+
+// CCR is preload/shadow-buffered, so the direct write below takes effect
+// cleanly at the next period boundary without needing further DMA.
+static void ws2811StopTransfer(void)
+{
+    timerPWMStopDMA(ws2811TCH);
+    *timerCCR(ws2811TCH) = 0;
+    lineIdleLow = true;
+    lastLowAtUs = micros();
+}
+
+// transferComplete: true = half 1 just finished (DMA wrapped to half 0),
+// false = half 0 just finished (DMA moved on to half 1).
+static void ws2811DMARefillCallback(TCH_t * tch, bool transferComplete)
+{
+    (void)tch;
+
+    uint8_t finishedHalf = transferComplete ? 1 : 0;
+
+    if (groupInHalf[finishedHalf] == totalGroups - 1) {
+        ws2811StopTransfer();
         return;
     }
 
-    dmaBufferOffset = 0;                // reset buffer memory index
-    ledIndex = 0;                       // reset led index
+    ws2811RefillHalf(finishedHalf);
+}
 
-    // fill transmit buffer with correct compare values to achieve
-    // correct pulse widths according to color values
-    while (ledIndex < WS2811_LED_STRIP_LENGTH)
-    {
-        rgb24 = hsvToRgb24(&ledColorBuffer[ledIndex]);
-        fastUpdateLEDDMABuffer(rgb24);
-        ledIndex++;
+static void ws2811EnsureResetGap(void)
+{
+    // A gap before real data is always safe regardless of length — only a
+    // mid-frame gap (prevented by true circular DMA) risks looking like a
+    // premature reset — so usually this is just a compare, not a wait.
+    if (!lineIdleLow || cmpTimeUs(micros(), lastLowAtUs) < WS2811_RESET_US) {
+        *timerCCR(ws2811TCH) = 0;
+        lineIdleLow = true;
+        delayMicroseconds(WS2811_RESET_US);
+        lastLowAtUs = micros();
     }
+}
 
-    // Initiate hardware transfer
+// Non-blocking except when the line was left idle-high by PINIO or updates
+// are requested faster than the reset window allows. LEDs are transmitted
+// in the background via the DMA refill callback.
+void ws2811UpdateStrip(uint16_t usedLedCount)
+{
     if (!ws2811Initialised || !ws2811TCH) {
         return;
     }
 
-    timerPWMPrepareDMA(ws2811TCH, WS2811_DMA_BUFFER_SIZE);
-    timerPWMStartDMA(ws2811TCH);
+    // don't wait - risk of infinite block, just get an update next time round
+    if (timerPWMDMAInProgress(ws2811TCH)) {
+        return;
+    }
+
+    activeLedCount = MIN(usedLedCount, (uint16_t)WS2811_LED_STRIP_LENGTH);
+    if (activeLedCount == 0) {
+        return;
+    }
+
+    ws2811EnsureResetGap();
+
+    totalGroups = (activeLedCount + WS2811_LEDS_PER_GROUP - 1) / WS2811_LEDS_PER_GROUP;
+    nextGroupToAssign = 0;
+    ws2811RefillHalf(0);
+    ws2811RefillHalf(1);
+
+    impl_timerPWMSetDMACircular(ws2811TCH, true, WS2811_CHUNK_BUFFER_SIZE);
 }
 
 void ws2811SetIdleHigh(bool high)
 {
-    ledStripDMABuffer[WS2811_DMA_BUFFER_SIZE - 1] = high ? 255 : 0;
+    lineIdleLow = !high;
+    *timerCCR(ws2811TCH) = high ? 255 : 0;
+    if (!high) {
+        lastLowAtUs = micros();
+    }
 }
 
 #endif

src/main/drivers/light_ws2811strip.h

@@ -23,19 +23,14 @@
 
 #define WS2811_LED_STRIP_LENGTH 128
 #define WS2811_BITS_PER_LED 24
-#define WS2811_DELAY_BUFFER_LENGTH 42 // for 50us delay 
-
-#define WS2811_DATA_BUFFER_SIZE (WS2811_BITS_PER_LED * WS2811_LED_STRIP_LENGTH)
-
-#define WS2811_DMA_BUFFER_SIZE (WS2811_DELAY_BUFFER_LENGTH + WS2811_DATA_BUFFER_SIZE + 1)   // leading bytes (reset low 302us) + data bytes LEDS*3  + 1 byte(keep line high optionally)
 
 #define WS2811_TIMER_HZ         2400000
 #define WS2811_CARRIER_HZ       800000
 
 void ws2811LedStripInit(void);
 void ws2811SetIdleHigh(bool high);
 
-void ws2811UpdateStrip(void);
+void ws2811UpdateStrip(uint16_t usedLedCount);
 
 void setLedHsv(uint16_t index, const hsvColor_t *color);
 void getLedHsv(uint16_t index, hsvColor_t *color);

src/main/drivers/timer.h

@@ -156,6 +156,15 @@ typedef struct timerCallbacks_s {
     timerCallbackFn * callbackOvr;
 } timerCallbacks_t;
 
+// Circular-DMA refill callback: invoked from the DMA IRQ while
+// dmaState == TCH_DMA_CIRCULAR, once per half-cycle, so the consumer can
+// refill the half that was just transmitted. transferComplete is true for
+// the TC (second-half-just-sent) event, false for the HT
+// (first-half-just-sent) event. Optional (NULL) for circular DMA consumers
+// that don't need refilling (e.g. motor DShot idle-packet repeat during
+// EEPROM writes) — those get no HT/TC IRQs at all.
+typedef void timerDmaRefillFn(struct TCH_s * tch, bool transferComplete);
+
 // Run-time TCH (Timer CHannel) context
 typedef struct TCH_s {
     struct timHardwareContext_s *   timCtx;         // Run-time initialized to parent timer
@@ -164,6 +173,7 @@ typedef struct TCH_s {
     DMA_t                           dma;            // Timer channel DMA handle
     volatile tchDmaState_e          dmaState;
     void *                          dmaBuffer;
+    timerDmaRefillFn *              dmaRefillCallback; // optional, see typedef above
 } TCH_t;
 
 // Run-time timer context (dynamically allocated), includes 4x TCH

src/main/drivers/timer_impl.h

@@ -85,6 +85,7 @@ void impl_timerPWMPrepareDMA(TCH_t * tch, uint32_t dmaBufferElementCount);
 void impl_timerPWMStartDMA(TCH_t * tch);
 void impl_timerPWMStopDMA(TCH_t * tch);
 void impl_timerPWMSetDMACircular(TCH_t * tch, bool circular, uint32_t dmaBufferSize);
+void impl_timerPWMSetDMARefillCallback(TCH_t * tch, timerDmaRefillFn * callback);
 
 #ifdef USE_DSHOT_DMAR
 bool impl_timerPWMConfigDMABurst(burstDmaTimer_t *burstDmaTimer, TCH_t * tch, void * dmaBuffer, uint8_t dmaBufferElementSize, uint32_t dmaBufferElementCount);