linux-kernel - Re: Correct location for ADC/DAC drivers

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Date:	Sun, 6 May 2007 18:01:04 +0200
From:	Robert Schwebel <r.schwebel@...gutronix.de>
To:	Bodo Eggert <7eggert@....de>
Cc:	Robin Getz <rgetz@...ckfin.uclinux.org>,
	Stefan Roese <ml@...fan-roese.de>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org,
	Marc Kleine-Budde <mkl@...gutronix.de>
Subject: Re: Correct location for ADC/DAC drivers

On Sun, May 06, 2007 at 02:19:59PM +0200, Bodo Eggert wrote:
> Since you ask for random thoughts:
> 
> IO of data streams from or to a DAC/ADC is essentially what soundcards do.
> I'm wondering if these cards are similar enough to use alsa, and if using
> that interface would ease or hinder programming the driver.

I had a short look at ALSA and it looks like it is a little bit too
focussed towards "normal" audio. For example, if follows an asynchronous
model in the sense that an application can read/write to a buffer at any
time, taken that the ringbuffer doesn't overflow. That's not enough for
example for control applications; you need the option for real
synchronous operation there.

IMHO we need an API which offers

- kernel ring buffers (to make it sure no data gets lost)
- generic data types (streaming plain ADC values is just the simplest
  case; we also have for example streaming FPGA data containing 
  preprocessed data sets)
- optional timestamping for each sample
- zero-copy from hardware into userspace memory, with DMA support
- application notification every N samples (N >= 1)

v4l2 may also be worth a look; but in the end I suppose they are all
specialized sub-cases of what we'd need for fast DAQ.

Robert
-- 
 Dipl.-Ing. Robert Schwebel | http://www.pengutronix.de
 Pengutronix - Linux Solutions for Science and Industry
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