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Message-ID: <CAG5mAdy553q4Te5755575WaiLxGRp43h=WVH6QzF7idsaCnhbg@mail.gmail.com>
Date: Fri, 15 Jan 2016 09:03:28 -0800
From: Caleb Crome <caleb@...me.org>
To: Timur Tabi <timur@...i.org>
Cc: Mark Brown <broonie@...nel.org>,
Nicolin Chen <nicoleotsuka@...il.com>,
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 5:46 AM, Timur Tabi <timur@...i.org> wrote:
> Mark Brown wrote:
>>>
>>> >I admit it's a grey area, but the hardware doesn't work if you use the
>>> > wrong
>>> >value, and it is a fixed value per device. A p1022ds would use a
>>> > different
>>> >value than in in i.MX6, and once you pick a value, it's the same no
>>> > matter
>>> >which sample rate, buffer size, etc you choose.
>
>
>> Caleb's original message suggested this was rate dependant.
>
>
> Yeah, I just noticed that. In that case, I agree that a device tree
> property is inappropriate, unless it's an array that contains tuples of
> sample rates and watermark/maxburst settings. That would get unwieldy very
> easily, though.
The rate dependance is only a *potential* issue. I suspect that a
value of 4 should be functional for all rates and chips. The only
trade off is more DMA requests/bursts.
In a typical 15 word fifo, 48kHz, stereo, single fifo DMA system, the
old value was 15-2 = 13, which would mean 7385 13-word DMA
bursts/second. A new value of 4 would mean 24,000 4-word DMA
bursts/second.
Is that consequential for anybody? It's about the same total
bandwidth on the system, but just broken up into smaller chunks (I
don't know what the overhead is for a DMA burst)
In a high channel count system (16 channels @ 48kHz), the old value
doesn't work, and the new value would mean 192,000 4-word DMA
bursts/second, which works on my MX6. So given that 192000 works
fine, I'm not sure that the difference in a typical system would
matter at all.
If nobody objects, we can just set the value to 4 and be done with it.
Another question: is the watermark ever going to be different than
maxburst? Is there any reason to have them different?
-Caleb
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