Frame buffer memory allocation needs to be tolerant of fragmented memory space

[Roger-random]

The default code path begin() will allocate 61KB of memory.

• 60KB for the frame buffer itself
• Another ~1KB (240 32-bit pointers) for the lines array.

This is fine for small simple sketches, but some developers may want control over this memory.

• It is conceivable to have 61KB of memory free but not contiguous, which would cause memory allocation call to fail. A developer could allocate memory in smaller chunks and assemble them via the lines array.
• The developer may prefer to statically allocate an array at compile time.
• The developer may prefer to control memory allocation for any other reason.

To support this scenario, add an alternate begin() that accepts a pointer to caller-allocated memory. The caller would be responsible for freeing this memory as well.

There was an early draft of this feature, but it was removed before v1.0.0 release in an effort to keep API surface area simple in accordance with Arduino API Style Guide.

This feature can be reintroduced if there is demand for it.

[Roger-random]

Design change: In the interest of keeping API surface simple for beginners to use, will not place burden of frame buffer memory allocation on caller.

However, we still want to be tolerant of situations when 60KB of contiguous memory is not available. The ESP_8_BIT architecture can easily handle non-contiguous memory. The frame buffer is addressed by an array of 240 horizontal lines, each line is 256 bytes of memory. But adjacent lines need not be adjacent in memory!

Investigation with heap_caps_print_heap_info() indicate 16-byte overhead for each allocation, so allocating each line individually would mean a lot of overhead: 239 16 double buffer=7648 bytes!

As a compromise, the frame buffer will be allocated in 4kB chunks. This translates to 16 lines per chunk, and 15 chunks per frame to make up the total of 240 lines. This fits frame buffer into more fragmented memory space while imposing 14 16 double buffer=448 bytes of extra overhead, which is much more reasonable tradeoff for the benefits.

Some raw dumpes from heap_caps_print_heap_info()

Before allocating frame buffer memory:

Heap summary for capabilities 0x00000004:
  At 0x3ffbdb28 len 52 free 4 allocated 0 min_free 4
    largest_free_block 4 alloc_blocks 0 free_blocks 1 total_blocks 1
  At 0x3ffb8000 len 6688 free 5872 allocated 688 min_free 5872
    largest_free_block 5872 alloc_blocks 5 free_blocks 1 total_blocks 6
  At 0x3ffb0000 len 25480 free 17172 allocated 8228 min_free 17172
    largest_free_block 17172 alloc_blocks 2 free_blocks 1 total_blocks 3
  At 0x3ffae6e0 len 6192 free 6092 allocated 36 min_free 6092
    largest_free_block 6092 alloc_blocks 1 free_blocks 1 total_blocks 2
  At 0x3ffaff10 len 240 free 0 allocated 128 min_free 0
    largest_free_block 0 alloc_blocks 5 free_blocks 0 total_blocks 5
  At 0x3ffb6388 len 7288 free 0 allocated 6784 min_free 0
    largest_free_block 0 alloc_blocks 29 free_blocks 1 total_blocks 30
  At 0x3ffb9a20 len 16648 free 5784 allocated 10208 min_free 284
    largest_free_block 4980 alloc_blocks 37 free_blocks 5 total_blocks 42
  At 0x3ffc1f78 len 123016 free 122968 allocated 0 min_free 122968
    largest_free_block 122968 alloc_blocks 0 free_blocks 1 total_blocks 1
  At 0x3ffe0440 len 15072 free 15024 allocated 0 min_free 15024
    largest_free_block 15024 alloc_blocks 0 free_blocks 1 total_blocks 1
  At 0x3ffe4350 len 113840 free 113792 allocated 0 min_free 113792
    largest_free_block 113792 alloc_blocks 0 free_blocks 1 total_blocks 1
  Totals:
    free 286708 allocated 26072 min_free 281208 largest_free_block 122968

Allocating double frame buffers in single 60KB chunks:

Heap summary for capabilities 0x00000004:
  At 0x3ffbdb28 len 52 free 4 allocated 0 min_free 4
    largest_free_block 4 alloc_blocks 0 free_blocks 1 total_blocks 1
  At 0x3ffb8000 len 6688 free 3920 allocated 2608 min_free 3824
    largest_free_block 3920 alloc_blocks 7 free_blocks 1 total_blocks 8
  At 0x3ffb0000 len 25480 free 17172 allocated 8228 min_free 17172
    largest_free_block 17172 alloc_blocks 2 free_blocks 1 total_blocks 3
  At 0x3ffae6e0 len 6192 free 6092 allocated 36 min_free 6092
    largest_free_block 6092 alloc_blocks 1 free_blocks 1 total_blocks 2
  At 0x3ffaff10 len 240 free 0 allocated 128 min_free 0
    largest_free_block 0 alloc_blocks 5 free_blocks 0 total_blocks 5
  At 0x3ffb6388 len 7288 free 0 allocated 6784 min_free 0
    largest_free_block 0 alloc_blocks 29 free_blocks 1 total_blocks 30
  At 0x3ffb9a20 len 16648 free 5784 allocated 10208 min_free 284
    largest_free_block 4980 alloc_blocks 37 free_blocks 5 total_blocks 42
  At 0x3ffc1f78 len 123016 free 56 allocated 122880 min_free 56
    largest_free_block 56 alloc_blocks 2 free_blocks 1 total_blocks 3
  At 0x3ffe0440 len 15072 free 15024 allocated 0 min_free 15024
    largest_free_block 15024 alloc_blocks 0 free_blocks 1 total_blocks 1
  At 0x3ffe4350 len 113840 free 113792 allocated 0 min_free 113792
    largest_free_block 113792 alloc_blocks 0 free_blocks 1 total_blocks 1
  Totals:
    free 161844 allocated 150872 min_free 156248 largest_free_block 113792

Allocating double frame buffers in 4KB chunks:

Heap summary for capabilities 0x00000004:
  At 0x3ffbdb28 len 52 free 4 allocated 0 min_free 4
    largest_free_block 4 alloc_blocks 0 free_blocks 1 total_blocks 1
  At 0x3ffb8000 len 6688 free 784 allocated 5744 min_free 784
    largest_free_block 784 alloc_blocks 7 free_blocks 1 total_blocks 8
  At 0x3ffb0000 len 25480 free 724 allocated 24612 min_free 724
    largest_free_block 724 alloc_blocks 6 free_blocks 1 total_blocks 7
  At 0x3ffae6e0 len 6192 free 1004 allocated 5092 min_free 1004
    largest_free_block 1004 alloc_blocks 3 free_blocks 1 total_blocks 4
  At 0x3ffaff10 len 240 free 0 allocated 128 min_free 0
    largest_free_block 0 alloc_blocks 5 free_blocks 0 total_blocks 5
  At 0x3ffb6388 len 7288 free 0 allocated 6776 min_free 0
    largest_free_block 0 alloc_blocks 29 free_blocks 1 total_blocks 30
  At 0x3ffb9a20 len 16648 free 1672 allocated 14304 min_free 264
    largest_free_block 868 alloc_blocks 38 free_blocks 5 total_blocks 43
  At 0x3ffc1f78 len 123016 free 28392 allocated 94208 min_free 28392
    largest_free_block 28392 alloc_blocks 23 free_blocks 1 total_blocks 24
  At 0x3ffe0440 len 15072 free 15024 allocated 0 min_free 15024
    largest_free_block 15024 alloc_blocks 0 free_blocks 1 total_blocks 1
  At 0x3ffe4350 len 113840 free 113792 allocated 0 min_free 113792
    largest_free_block 113792 alloc_blocks 0 free_blocks 1 total_blocks 1
  Totals:
    free 161396 allocated 150864 min_free 159988 largest_free_block 113792