linux-kernel - Re: [PATCH RFC 1/1] ASoC: fsl_ssi: Make fifo watermark and maxburst settings device tree options

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Date:	Fri, 15 Jan 2016 10:38:24 -0800
From:	Nicolin Chen <nicoleotsuka@...il.com>
To:	Caleb Crome <caleb@...me.org>
Cc:	Timur Tabi <timur@...i.org>, Mark Brown <broonie@...nel.org>,
	Xiubo Li <Xiubo.Lee@...il.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Jaroslav Kysela <perex@...ex.cz>,
	Takashi Iwai <tiwai@...e.com>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>
Subject: Re: [PATCH RFC 1/1] ASoC: fsl_ssi: Make fifo watermark and maxburst
 settings device tree options

On Fri, Jan 15, 2016 at 09:03:28AM -0800, Caleb Crome wrote:

> If nobody objects, we can just set the value to 4 and be done with it.

I agree. And we may apply it only to i.MX platforms with DMA if
other platform owners feel comfortable with the previous settings.

> Another question:  is the watermark ever going to be different than
> maxburst?  Is there any reason to have them different?

The watermark is merely a threshold to trigger a DMA request. The
only relationship with the burst size is that each burst transfer
should not carry more data than the number of empty slots; FIFO
under/overflow occurs otherwise. So it's just more efficient and
safer to set an identical value to both of them. I don't think
it will cause functional problems to set TFWM to 4 and burst size
to 1 -- It just lets DMA operate in a single data transfer mode.