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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