use descriptor chain in SGDMA

the align attribute will ensure the different buffers will not end up o the same cache line, but there could still be some parts of video_buf that are cached and you can get the wrong contents.

Try to call

alt_dcache_flush ((void*)video_buff,769*sizeof(alt_u32));

before the SGDMA transfer. This will write down the cache contents to video_buff, but most importantly will invalidate the cache for this area. That way you will be sure the cache won't interfere with the data.

AFAIK Altera doesn't provide a macro to just invalidate the cache without flushing it, even if the instruction exists in assembly.