Ensure the video capture DMA can't be enabled/disabled mid-frame.

Fix #30.

include/gba/ppu.h

@@ -111,18 +111,19 @@ union oam_entry {
 static_assert(sizeof(union oam_entry) == 3 * sizeof(uint16_t));
 
 struct ppu {
-    // The emulator's screen as it is being rendered.
+    /* The emulator's screen as it is being rendered. */
     uint32_t framebuffer[GBA_SCREEN_WIDTH * GBA_SCREEN_HEIGHT];
 
-    // Internal registers used for affine backgrounds
+    /* Internal registers used for affine backgrounds */
     int32_t internal_px[2];
     int32_t internal_py[2];
 
-    // Set when the registers above must be reloaded at the next H-Draw.
-    bool reload_internal_affine_regs;
+    bool reload_internal_affine_regs;       // Set when the registers above must be reloaded at the next H-Draw.
 
     bool win_masks[2][GBA_SCREEN_WIDTH];
-    uint32_t win_masks_hash[2];                /* The min/max for that windows. Kept to avoid rebuilding the mask across scanlines. */
+    uint32_t win_masks_hash[2];             // The min/max for that windows. Kept to avoid rebuilding the mask across scanlines. */
+
+    bool video_capture_enabled;             // Set when the DMA video capture is enabled
 };
 
 /* gba/ppu/background/bitmap.c */

source/gba/memory/dma.c

@@ -278,9 +278,14 @@ mem_schedule_dma_transfers_for(
     enum dma_timings timing
 ) {
     struct dma_channel *channel;
+    bool enabled;
 
     channel = &gba->io.dma[channel_idx];
-    if (channel->control.enable && channel->control.timing == timing) {
+
+    /* Bypass the usual enable check for DMA3's video mode */
+    enabled = (channel_idx == 3 && timing == DMA_TIMING_SPECIAL) ? gba->ppu.video_capture_enabled : channel->control.enable;
+
+    if (enabled && channel->control.timing == timing) {
         channel->enable_event_handle = sched_add_event(
             gba,
             NEW_FIX_EVENT_ARGS(

source/gba/ppu/ppu.c

@@ -397,9 +397,20 @@ ppu_hblank(
         mem_schedule_dma_transfers(gba, DMA_TIMING_HBLANK);
     }
 
-    if (io->vcount.raw >= 2 && io->vcount.raw < GBA_SCREEN_HEIGHT + 2) {
+    if (gba->ppu.video_capture_enabled && io->vcount.raw >= 2 && io->vcount.raw < GBA_SCREEN_HEIGHT + 2) {
         mem_schedule_dma_transfers_for(gba, 3, DMA_TIMING_SPECIAL);  // Video DMA
     }
+
+    /*
+    ** Video mode is evaluated once at the beginning of the frame and can't be enabled/disabled mid-frame.
+    **
+    ** Reference:
+    **   - https://github.com/mgba-emu/mgba/issues/2017
+    **   - https://github.com/skylersaleh/SkyEmu/issues/104
+    */
+    if (io->vcount.raw == GBA_SCREEN_HEIGHT + 2) {
+        gba->ppu.video_capture_enabled = gba->io.dma[3].control.enable && gba->io.dma[3].control.timing == DMA_TIMING_SPECIAL;
+    }
 }
 
 /*