DVDInterface: Don't evict cache block i unless block i + 2 was read
Intends to fix https://bugs.dolphin-emu.org/issues/12279. I have hardware tested the behavior, but I haven't tested the game.
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@ -1512,27 +1512,38 @@ static void ScheduleReads(u64 offset, u32 length, const DiscIO::Partition& parti
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dvd_offset += DVD_ECC_BLOCK_SIZE;
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} while (length > 0);
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// Update the buffer based on this read. Based on experimental testing,
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// we will only reuse the old buffer while reading forward. Note that the
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// buffer start we calculate here is not the actual start of the buffer -
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// it is just the start of the portion we need to read.
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// Evict blocks from the buffer which are unlikely to be used again after this read,
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// so that the buffer gets space for prefetching new blocks. Based on hardware testing,
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// the blocks which are kept are the most recently accessed block, the block immediately
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// before it, and all blocks after it.
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//
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// If the block immediately before the most recently accessed block is not kept, loading
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// screens in Pitfall: The Lost Expedition are longer than they should be.
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// https://bugs.dolphin-emu.org/issues/12279
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const u64 last_block = dvd_offset;
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if (last_block == buffer_start + DVD_ECC_BLOCK_SIZE && buffer_start != buffer_end)
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constexpr u32 BUFFER_BACKWARD_SEEK_LIMIT = DVD_ECC_BLOCK_SIZE * 2;
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if (last_block - buffer_start <= BUFFER_BACKWARD_SEEK_LIMIT && buffer_start != buffer_end)
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{
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// Special case: reading less than one block at the start of the
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// buffer won't change the buffer state
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// Special case: reading the first two blocks of the buffer doesn't change the buffer state
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}
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else
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{
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// Note that the s_read_buffer_start_offset value we calculate here is not the
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// actual start of the buffer - it is just the start of the portion we need to read.
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// The actual start of the buffer is s_read_buffer_end_offset - STREAMING_BUFFER_SIZE.
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if (last_block >= buffer_end)
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{
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// Full buffer read
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s_read_buffer_start_offset = last_block;
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}
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else
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{
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// Partial buffer read
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s_read_buffer_start_offset = buffer_end;
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}
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s_read_buffer_end_offset = last_block + STREAMING_BUFFER_SIZE - DVD_ECC_BLOCK_SIZE;
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// Assume the buffer starts reading right after the end of the last operation
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s_read_buffer_end_offset = last_block + STREAMING_BUFFER_SIZE - BUFFER_BACKWARD_SEEK_LIMIT;
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// Assume the buffer starts prefetching new blocks right after the end of the last operation
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s_read_buffer_start_time = current_time + ticks_until_completion;
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s_read_buffer_end_time =
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s_read_buffer_start_time +
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