From 4edddfa4aa5996ea2e313a853b0398cb924eb06b Mon Sep 17 00:00:00 2001
From: Petr Ledvina <ledvinap@gmail.com>
Date: Thu, 10 Nov 2022 14:53:06 +0000
Subject: [PATCH] improved GPIO slowdown

--led-slowdonw-gpio x
x > 0 - use dummy writes after seting GPIO
c == -1 - use `dsb st` to force AXI synchronization
---
 examples-api-use/minimal-example.cc |  2 +-
 lib/gpio.h                          | 27 +++++++++++++++++++--------
 lib/led-matrix.cc                   |  4 +++-
 3 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/examples-api-use/minimal-example.cc b/examples-api-use/minimal-example.cc
index 840e394d2..31f665cad 100644
--- a/examples-api-use/minimal-example.cc
+++ b/examples-api-use/minimal-example.cc
@@ -33,7 +33,7 @@ static void DrawOnCanvas(Canvas *canvas) {
   float angle_step = 1.0 / 360;
   for (float a = 0, r = 0; r < radius_max; a += angle_step, r += angle_step) {
     if (interrupt_received)
-      return;
+    return;
     float dot_x = cos(a * 2 * M_PI) * r;
     float dot_y = sin(a * 2 * M_PI) * r;
     canvas->SetPixel(center_x + dot_x, center_y + dot_y,
diff --git a/lib/gpio.h b/lib/gpio.h
index 2114b1834..00ccc28e4 100644
--- a/lib/gpio.h
+++ b/lib/gpio.h
@@ -47,26 +47,23 @@ class GPIO {
   inline void SetBits(gpio_bits_t value) {
     if (!value) return;
     WriteSetBits(value);
-    for (int i = 0; i < slowdown_; ++i) {
-      WriteSetBits(value);
-    }
+    delay();
   }
 
   // Clear the bits that are '1' in the output. Leave the rest untouched.
   inline void ClearBits(gpio_bits_t value) {
     if (!value) return;
     WriteClrBits(value);
-    for (int i = 0; i < slowdown_; ++i) {
-      WriteClrBits(value);
-    }
+    delay();
   }
 
   // Write all the bits of "value" mentioned in "mask". Leave the rest untouched.
   inline void WriteMaskedBits(gpio_bits_t value, gpio_bits_t mask) {
     // Writing a word is two operations. The IO is actually pretty slow, so
     // this should probably  be unnoticable.
-    ClearBits(~value & mask);
-    SetBits(value & mask);
+    WriteClrBits(~value & mask);
+    WriteSetBits(value & mask);
+    delay();
   }
 
   inline gpio_bits_t Read() const { return ReadRegisters() & input_bits_; }
@@ -75,6 +72,20 @@ class GPIO {
   static bool IsPi4();
 
 private:
+  inline void delay() const {
+    switch(slowdown_) {
+      case -1:
+#if __ARM_ARCH >= 7
+        asm volatile("dsb\tst");
+#endif
+        break;
+      case 0:
+        break;
+      default:
+        for (int n = 0; n < slowdown_; n++)
+          *gpio_clr_bits_low_ = 0;
+    }
+  }
   inline gpio_bits_t ReadRegisters() const {
     return (static_cast<gpio_bits_t>(*gpio_read_bits_low_)
 #ifdef ENABLE_WIDE_GPIO_COMPUTE_MODULE
diff --git a/lib/led-matrix.cc b/lib/led-matrix.cc
index abc830751..0812db0e7 100644
--- a/lib/led-matrix.cc
+++ b/lib/led-matrix.cc
@@ -618,7 +618,9 @@ RGBMatrix *RGBMatrix::CreateFromOptions(const RGBMatrix::Options &options,
   }
 
   // For the Pi4, we might need 2, maybe up to 4. Let's open up to 5.
-  if (runtime_options.gpio_slowdown < 0 || runtime_options.gpio_slowdown > 5) {
+  // on supproted architectures, -1 will emit memory barier (DSB ST) after GPIO write
+  if (runtime_options.gpio_slowdown < (__ARM_ARCH >= 7 ? -1 : 0)
+      || runtime_options.gpio_slowdown > 5) {
     fprintf(stderr, "--led-slowdown-gpio=%d is outside usable range\n",
             runtime_options.gpio_slowdown);
     return NULL;