linux-kernel - Re: [PATCH 04/32] dmaengine: ste_dma40: Amalgamate DMA source and destination channel numbers

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Message-ID: <CACRpkdZj-3DbcX1LREGKf2RZs4FqbDn6sWkaZ-t67KUKJhZaow@mail.gmail.com>
Date:	Thu, 25 Apr 2013 15:21:06 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Lee Jones <lee.jones@...aro.org>, Rabin Vincent <rabin@....in>,
	Linus WALLEIJ <linus.walleij@...ricsson.com>,
	Arnd Bergmann <arnd@...db.de>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Per Forlin <per.forlin@...ricsson.com>,
	Dan Williams <djbw@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 04/32] dmaengine: ste_dma40: Amalgamate DMA source and
 destination channel numbers

On Thu, Apr 25, 2013 at 3:09 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:

> There's a problem with device to device transfers though - you have to
> consider the rate at which the devices produce and consume data, and
> whether they both can cope with differing data rates.
>
> Take for instance your audio in to audio out idea - even if they are
> both operating at the same bits per sample and sample rate, if they are
> independently clocked, chances are that the clocks are not exactly the
> same, which means you will either underrun or overrun one of the FIFOs
> in the system.

This magically works for us, but yes, I think they had to use the same
clock (simplest solution, anyway).

It is/was a very real usecase, IIRC coming from roundtrip constraints
in the GSM 3GPP spec, wanting to keep the latency as low as possible.
With this scheme there is no more than a FIFO on each end of delay,
which is nice.

Yours,
Linus Walleij
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