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bcm2711-rpi-ds: Switch to dma40 channel for hdmi audio
Also tweak the flags: Remove NO_WAIT_RESP (27) Add BURST_LENGTH (30) The AXI path from DMA controller to HDMI audio fifo is long, and may have considerable delay. When using DMA without waiting for responses it is very easy to overfill the fifo as when the fifo removes DREQ there may be large numbers of writes in flight. This means the DREQ fifo threshold must be set low enough to accommodate the maximum number of in flight writes (unknown by something like 24), which means the 32 element fifo only requests data when it contains fewer than 8 entries, making it susceptable to underflow. If we wait for write responses we can set the DREQ fifo threshold much higher as there are a controlled number of writes in flight. However the overall bandwidth is reduced by setting this, so also enable a burstsize of 4 to improve bandwidth. Signed-off-by: Dom Cobley <popcornmix@gmail.com>
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