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

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Message-Id: <200705061533.36705.rgetz@blackfin.uclinux.org>
Date:	Sun, 6 May 2007 15:33:36 -0400
From:	Robin Getz <rgetz@...ckfin.uclinux.org>
To:	"Robert Schwebel" <r.schwebel@...gutronix.de>
Cc:	"Bodo Eggert" <7eggert@....de>,
	"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 6 May 2007 12:01, Robert Schwebel pondered:
> 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.

Yes - there are too many applications which require tight/sync connection with 
the data - control loops, software radios, machine control, etc - all have 
much different needs than audio, hwmon, or touchscreen.

> 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

I am assuming that the driver takes care of all DMA/coherency issues that 
might pop up? (and handles all interrupts)?

> - 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.

I would guess that something like v4l might be the best bet - the driver 
provides a few mmapped buffers that you can read/write into, and some 
mechanism to sync in/out of the buffers.

I was actually looking at UIO, but thought that since we really desire the 
kernel ring buffers, that a think model might be better.?

http://www.kernel.org/pub/linux/kernel/people/gregkh/gregkh-2.6/gregkh-01-driver/uio-documentation.patch

-Robin
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