[display] Test that both draw_pixels_at walks draw the same pixels

A recording display runs the row and column walks over a source with
nonzero offsets and padding for 332, 565 and 888 in both byte orders
and requires the same pixel set, plus a check that the orders differ.

esphome/components/display/display.cpp

@@ -76,7 +76,8 @@ static inline ESPHOME_ALWAYS_INLINE uint32_t read_packed_pixel(const uint8_t *pt
 
 // flatten keeps to_color() inlined in the pixel loop; in PSRAM builds GCC at
 // -Os stops inlining it once there are two call sites, which costs a call and
-// spills per pixel.
+// spills per pixel. Without PSRAM there is one call site and it changes
+// nothing, and draw_pixel_at() stays a virtual call either way.
 void __attribute__((flatten))
 Display::draw_pixels_at(int x_start, int y_start, int w, int h, const uint8_t *ptr, ColorOrder order,
                         ColorBitness bitness, bool big_endian, int x_offset, int y_offset, int x_pad) {

tests/components/display/__init__.py

@@ -0,0 +1,12 @@
+import esphome.codegen as cg
+from esphome.types import ConfigType
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # The column walk in Display::draw_pixels_at() is only built with PSRAM;
+    # define it so the host test can run both walk orders.
+    async def to_code_testing(config: ConfigType) -> None:
+        cg.add_define("USE_PSRAM")
+
+    manifest.to_code = to_code_testing

tests/components/display/test_draw_pixels_at.cpp

@@ -0,0 +1,82 @@
+#include <gtest/gtest.h>
+
+#include <algorithm>
+#include <tuple>
+#include <vector>
+
+#include "esphome/components/display/display.h"
+
+namespace esphome::display::testing {
+
+// Records every pixel write; pixel_axes_swapped() picks the walk order.
+class RecordingDisplay : public Display {
+ public:
+  void set_axes_swapped(bool swapped) { this->axes_swapped_ = swapped; }
+  bool pixel_axes_swapped() const override { return this->axes_swapped_; }
+  void draw_pixel_at(int x, int y, Color color) override { this->pixels_.emplace_back(x, y, color.raw_32); }
+  void update() override {}
+  DisplayType get_display_type() override { return DisplayType::DISPLAY_TYPE_COLOR; }
+  int get_width_internal() override { return 64; }
+  int get_height_internal() override { return 64; }
+
+  std::vector<std::tuple<int, int, uint32_t>> sorted_pixels() const {
+    auto sorted = this->pixels_;
+    std::sort(sorted.begin(), sorted.end());
+    return sorted;
+  }
+  size_t count() const { return this->pixels_.size(); }
+  const std::vector<std::tuple<int, int, uint32_t>> &pixels() const { return this->pixels_; }
+
+ protected:
+  bool axes_swapped_{false};
+  std::vector<std::tuple<int, int, uint32_t>> pixels_;
+};
+
+// A 7x5 region with a 2 pixel left offset, a 3 line top offset and 4 pixels of
+// right padding, so the source index arithmetic of both walks is exercised.
+static constexpr int W = 7, H = 5, X_OFFSET = 2, Y_OFFSET = 3, X_PAD = 4;
+static constexpr int LINE_STRIDE = X_OFFSET + W + X_PAD;
+static constexpr int LINES = Y_OFFSET + H;
+
+static std::vector<uint8_t> make_source(int bytes_per_pixel) {
+  std::vector<uint8_t> source(LINE_STRIDE * LINES * bytes_per_pixel);
+  uint8_t value = 1;
+  for (auto &byte : source)
+    byte = value++;
+  return source;
+}
+
+static void expect_walks_match(ColorBitness bitness, int bytes_per_pixel, bool big_endian) {
+  const auto source = make_source(bytes_per_pixel);
+  RecordingDisplay rows;
+  RecordingDisplay columns;
+  columns.set_axes_swapped(true);
+  rows.draw_pixels_at(10, 20, W, H, source.data(), COLOR_ORDER_RGB, bitness, big_endian, X_OFFSET, Y_OFFSET, X_PAD);
+  columns.draw_pixels_at(10, 20, W, H, source.data(), COLOR_ORDER_RGB, bitness, big_endian, X_OFFSET, Y_OFFSET, X_PAD);
+  EXPECT_EQ(rows.count(), static_cast<size_t>(W * H));
+  EXPECT_EQ(columns.count(), static_cast<size_t>(W * H));
+  EXPECT_EQ(rows.sorted_pixels(), columns.sorted_pixels());
+}
+
+TEST(DrawPixelsAt, ColumnWalkMatchesRowWalk332) { expect_walks_match(COLOR_BITNESS_332, 1, true); }
+TEST(DrawPixelsAt, ColumnWalkMatchesRowWalk565BigEndian) { expect_walks_match(COLOR_BITNESS_565, 2, true); }
+TEST(DrawPixelsAt, ColumnWalkMatchesRowWalk565LittleEndian) { expect_walks_match(COLOR_BITNESS_565, 2, false); }
+TEST(DrawPixelsAt, ColumnWalkMatchesRowWalk888BigEndian) { expect_walks_match(COLOR_BITNESS_888, 3, true); }
+TEST(DrawPixelsAt, ColumnWalkMatchesRowWalk888LittleEndian) { expect_walks_match(COLOR_BITNESS_888, 3, false); }
+
+// The column walk really is a different order, otherwise the tests above prove nothing.
+TEST(DrawPixelsAt, ColumnWalkChangesOrder) {
+  const auto source = make_source(1);
+  RecordingDisplay rows;
+  RecordingDisplay columns;
+  columns.set_axes_swapped(true);
+  rows.draw_pixels_at(0, 0, W, H, source.data(), COLOR_ORDER_RGB, COLOR_BITNESS_332, true, X_OFFSET, Y_OFFSET, X_PAD);
+  columns.draw_pixels_at(0, 0, W, H, source.data(), COLOR_ORDER_RGB, COLOR_BITNESS_332, true, X_OFFSET, Y_OFFSET,
+                         X_PAD);
+  ASSERT_GT(rows.count(), 1u);
+  ASSERT_GT(columns.count(), 1u);
+  EXPECT_EQ(std::get<0>(rows.pixels()[1]), 1);     // the row walk steps x first
+  EXPECT_EQ(std::get<1>(columns.pixels()[1]), 1);  // the column walk steps y first
+}
+
+}  // namespace esphome::display::testing