Subject: [PATCH v6 10/10] ASoC: rockchip_i2s: modify DMA max burst to 1

From: [email-masked] (Lars-Peter Clausen)
Date: Thu, 15 Oct 2015 10:53:36 +0200

On 10/15/2015 03:36 AM, Shawn Lin wrote:
[...]
> +
> +	if (snd_dmaengine_pcm_get_caps(&pdev->dev, &dma_caps) == 0) {
> +		if (dma_caps.max_burst > 4) {
> +			i2s->playback_dma_data.maxburst = 4;
> +			i2s->capture_dma_data.maxburst = 4;
> +		} else {
> +			i2s->playback_dma_data.maxburst = 1;
> +			i2s->capture_dma_data.maxburst = 1;

So this is what this is all about? I though you might have to program some
FIFO threshold registers in the peripheral itself.

But it seems all this does is to read the maximum burst length from the DMA
controller only to tell the DMA controller that this is the maximum burst
length it should use. That seems rather unnecessary.

The maxburst field of the dma_data indicates the maximum burst length that
the audio peripheral can handle. Typically this is the number of samples the
audio FIFO can receive without overflowing after sending the DMA request
signal. Since as the name suggests this is the maximum burst size the DMA
controller is free to choose a burst size smaller than this when writing
data to the peripheral.

So in your case instead of introducing all these facilities to query the
maximum burst size it should be OK to simply reduce the burst size in the
DMA controller itself when it gets a request with a burst size larger than
it can handle, or is there a reason why this is not possible?

- Lars