[display] Return the step from the reader again, as a plain enum with END = 0

The bool API cost flash at every call site (about 10 B per mipi_spi
instantiation on ESP8266); the unscoped enum keeps the old code shape and
lets callers loop with while (auto step = reader.next(entry)).

esphome/components/display/__init__.py

@@ -181,6 +181,16 @@ def get_all_display_metadata() -> dict[str, DisplayMetaData]:
     return {id_.id: meta for id_, meta in entries}
 
 
+def config_has_writer(config: ConfigType) -> bool:
+    """Return True when the display config draws anything."""
+    return (
+        config.get(CONF_AUTO_CLEAR_ENABLED) is True
+        or config.get(CONF_PAGES) is not None
+        or config.get(CONF_LAMBDA) is not None
+        or config.get(CONF_SHOW_TEST_CARD) is True
+    )
+
+
 def get_display_metadata(display_id: ID) -> DisplayMetaData:
     """Get display metadata by ID object
 

esphome/components/display/init_sequence.cpp

@@ -0,0 +1,28 @@
+#include "init_sequence.h"
+#include "esphome/core/hal.h"
+
+namespace esphome::display {
+
+InitSequenceStep InitSequenceReader::next(InitSequenceEntry &entry, uint8_t max_count) {
+  const size_t remaining = this->length_ - this->index_;
+  if (remaining == 0)
+    return INIT_SEQUENCE_STEP_END;
+  const uint8_t *pos = this->sequence_ + this->index_;
+  if (remaining >= 2) {
+    entry.value = progmem_read_byte(pos);
+    entry.count = progmem_read_byte(pos + 1);
+    entry.args = pos + 2;
+    if (entry.count == INIT_SEQUENCE_DELAY_FLAG) {
+      this->index_ += 2;
+      return INIT_SEQUENCE_STEP_DELAY;
+    }
+    if (entry.count <= max_count && remaining - 2 >= entry.count) {
+      this->index_ += 2 + entry.count;
+      return INIT_SEQUENCE_STEP_COMMAND;
+    }
+  }
+  this->index_ = this->length_;
+  return INIT_SEQUENCE_STEP_MALFORMED;
+}
+
+}  // namespace esphome::display

esphome/components/display/init_sequence.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+
+namespace esphome::display {
+
+/// Count byte that marks a delay entry: the byte before it is the delay in ms.
+static constexpr uint8_t INIT_SEQUENCE_DELAY_FLAG = 0xFF;
+
+/// Unscoped so `while (auto step = reader.next(entry))` stops at END.
+enum InitSequenceStep : uint8_t {
+  INIT_SEQUENCE_STEP_END = 0,
+  INIT_SEQUENCE_STEP_COMMAND,
+  INIT_SEQUENCE_STEP_DELAY,
+  INIT_SEQUENCE_STEP_MALFORMED,
+};
+
+/// `value` is the command or the delay in ms; `args` points into the (possibly PROGMEM) sequence.
+struct InitSequenceEntry {
+  const uint8_t *args;
+  uint8_t value;
+  uint8_t count;
+};
+
+/// Walks a mipi.flatten_sequence table (`cmd, count, args...` or `ms, 0xFF`) with progmem reads; resumable.
+class InitSequenceReader {
+ public:
+  InitSequenceReader(const uint8_t *sequence, size_t length) : sequence_(sequence), length_(length) {}
+
+  /// Fill `entry` with the next entry; a command with more than `max_count` args is MALFORMED.
+  InitSequenceStep next(InitSequenceEntry &entry, uint8_t max_count = 0xFE);
+
+ protected:
+  const uint8_t *sequence_;
+  size_t length_;
+  size_t index_{0};
+};
+
+}  // namespace esphome::display

tests/components/display/test_init_sequence.cpp

@@ -0,0 +1,82 @@
+#include <gtest/gtest.h>
+
+#include <cstdint>
+
+#include "esphome/components/display/init_sequence.h"
+
+namespace esphome::display::testing {
+
+TEST(InitSequenceReader, WalksCommandsAndDelays) {
+  static const uint8_t SEQ[] = {0x11, 0x00, 120, 0xFF, 0x3A, 0x01, 0x55, 0x2A, 0x02, 0x00, 0xEF};
+  InitSequenceReader reader(SEQ, sizeof(SEQ));
+  InitSequenceEntry entry{};
+
+  ASSERT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_COMMAND);
+  EXPECT_EQ(entry.value, 0x11);
+  EXPECT_EQ(entry.count, 0);
+
+  ASSERT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_DELAY);
+  EXPECT_EQ(entry.value, 120);
+
+  ASSERT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_COMMAND);
+  EXPECT_EQ(entry.value, 0x3A);
+  ASSERT_EQ(entry.count, 1);
+  EXPECT_EQ(entry.args, &SEQ[6]);
+
+  ASSERT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_COMMAND);
+  EXPECT_EQ(entry.value, 0x2A);
+  ASSERT_EQ(entry.count, 2);
+  EXPECT_EQ(entry.args[0], 0x00);
+  EXPECT_EQ(entry.args[1], 0xEF);
+
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_END);
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_END);
+}
+
+TEST(InitSequenceReader, EmptySequenceEnds) {
+  InitSequenceReader reader(nullptr, 0);
+  InitSequenceEntry entry{};
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_END);
+}
+
+// A count above 127 is a real length, not a flag bit.
+TEST(InitSequenceReader, HonoursFullCount) {
+  uint8_t seq[2 + 253 + 2] = {0x32, 253};
+  seq[2 + 253] = 0x29;
+  InitSequenceReader reader(seq, sizeof(seq));
+  InitSequenceEntry entry{};
+
+  ASSERT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_COMMAND);
+  EXPECT_EQ(entry.value, 0x32);
+  EXPECT_EQ(entry.count, 253);
+  ASSERT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_COMMAND);
+  EXPECT_EQ(entry.value, 0x29);
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_END);
+}
+
+TEST(InitSequenceReader, TruncatedHeaderIsMalformed) {
+  static const uint8_t SEQ[] = {0x11, 0x00, 0x29};
+  InitSequenceReader reader(SEQ, sizeof(SEQ));
+  InitSequenceEntry entry{};
+  ASSERT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_COMMAND);
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_MALFORMED);
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_END);
+}
+
+TEST(InitSequenceReader, ShortArgsAreMalformed) {
+  static const uint8_t SEQ[] = {0x2A, 0x04, 0x00, 0x00, 0x00};
+  InitSequenceReader reader(SEQ, sizeof(SEQ));
+  InitSequenceEntry entry{};
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_MALFORMED);
+  EXPECT_EQ(reader.next(entry), INIT_SEQUENCE_STEP_END);
+}
+
+TEST(InitSequenceReader, CountOverMaxIsMalformed) {
+  static const uint8_t SEQ[] = {10, 0xFF, 0x2A, 0x02, 0x00, 0xEF};
+  InitSequenceReader reader(SEQ, sizeof(SEQ));
+  InitSequenceEntry entry{};
+  ASSERT_EQ(reader.next(entry, 1), INIT_SEQUENCE_STEP_DELAY);
+  EXPECT_EQ(reader.next(entry, 1), INIT_SEQUENCE_STEP_MALFORMED);
+}
+
+}  // namespace esphome::display::testing

tests/unit_tests/components/test_display.py

@@ -1,4 +1,4 @@
-"""Tests for display.add_writer."""
+"""Tests for display.add_writer and display.config_has_writer."""
 
 from unittest.mock import AsyncMock, MagicMock, patch
 
@@ -6,7 +6,8 @@ import pytest
 
 from esphome import codegen as cg
 from esphome.components import display
-from esphome.const import CONF_LAMBDA
+from esphome.components.display import CONF_SHOW_TEST_CARD
+from esphome.const import CONF_AUTO_CLEAR_ENABLED, CONF_LAMBDA, CONF_PAGES
 
 
 @pytest.mark.parametrize(
@@ -39,3 +40,20 @@ async def test_add_writer_without_lambda_does_nothing() -> None:
 
     process.assert_not_awaited()
     add.assert_not_called()
+
+
+@pytest.mark.parametrize(
+    ("config", "expected"),
+    [
+        ({}, False),
+        ({CONF_AUTO_CLEAR_ENABLED: True}, True),
+        ({CONF_AUTO_CLEAR_ENABLED: False}, False),
+        ({CONF_AUTO_CLEAR_ENABLED: "unspecified"}, False),
+        ({CONF_PAGES: []}, True),
+        ({CONF_LAMBDA: "x"}, True),
+        ({CONF_SHOW_TEST_CARD: True}, True),
+        ({CONF_SHOW_TEST_CARD: False}, False),
+    ],
+)
+def test_config_has_writer(config: dict, expected: bool) -> None:
+    assert display.config_has_writer(config) is expected