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 11:10:29 -0800
From:	Caleb Crome <caleb@...me.org>
To:	Nicolin Chen <nicoleotsuka@...il.com>
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 10:57 AM, Nicolin Chen <nicoleotsuka@...il.com> wrote:
> On Fri, Jan 15, 2016 at 10:49:04AM -0800, Caleb Crome wrote:
>
>> > 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.
>>
>> If there is no penalty for setting maxburst to 1 (or 2 in the case of
>> dual fifo I think), then should we just set both the watermark and
>> maxburst to 1?
>>
>> I guess the real difference would be when you're in FIQ mode.  In FIQ
>> mode, the penalty of an interrupt per word would be pretty bad, but in
>> DMA mode, if we just set both to 1, we should be fine, right?
>
> There will be much more overhead drawn by frequent DMA transfers.
> I believe you understand the idea -- less burst size then more DMA
> request. Each DMA transfer contains a pair of handshaking overhead
> according to the bus protocol. Apparently the bus will be wasted
> with lots of handshaking section instead of keep dedicated to data
> transfer truly. It might work if SSI is the only user of the bus,
> which we shouldn't assume.

Right, I knew there must be a trade off.  So, how about I submit a
patch with a fixed value of 4 for whichever platforms you think is
correct.

I see the 4 compatibles are:  fsl,mpc8610-ssi, fsl,imx51-ssi,
fsl,imx35-ssi, and fsl,imx21-ssi.

Shall I submit a patch that sets the value to 4 for for any/all of the
above?  Should it be different based on whether fiq is enabled.  Maybe
set it to 4 for whenever DMA is used, and to 13 whenever FIQ is used?

-Caleb