WritingAnAlsaDriver

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There is a shameful lack of information on the Alsa website about how to write a driver for Alsa. This is becoming a frequently-asked question on the alsa-dev mailing list.

The best advice seems to be:

  1. Read the book "Linux Device Drivers" (It is available online at http://www.xml.com/ldd/chapter/book/ )
  2. Look at the half-finished documentation for the woefully out-of-date 0.4 kernel API linked from http://alsa-project.org/documentation.php3
  3. Or look at another oudated Alsa 0.5 Developer document at http://www.math.TU-Berlin.DE/\~sbartels/alsa/index.html (Isn't this the same document as #2?)
  4. Find a driver for a card that is similar to the one you're writing for. Read the source code for it. Best to take a widely-used driver as your template, since the more esoteric drivers may be broken without anyone noticing for some time.
  5. Install a journaling filesystem before you begin development! You'll be crashing and fsck'ing your machine many times.

Update: Takashi has helped fill this gaping void: http://www.alsa-project.org/\~iwai/writing-an-alsa-driver/index.html


Alfons Adriaensen asked on the alsa-user mailing-list about the API change for v1.0: Is there any document that describes this new API and the rationale for it ?

Takashi responded: ... the doxygen output, i.e. online manual on web, is already based on the new API. Basically, the difference is found only in some snd_pcm_hw_params_xxx and snd_pcm_sw_params_xxx functions, such as snd_pcm_hw_params_get_access(). These functions returned non-integer value as the value itself, e.g.

int snd_pcm_hw_params_get_access(const snd_pcm_hw_params_t *params);

with the new API, they return always the error code and store the value in the given pointer uniformly, e.g.

int snd_pcm_hw_params_get_access(const snd_pcm_hw_params_t *params, snd_pcm_access_t *access);

so that you can check the error condition safely. btw, you can still use the old codes by defining

#define ALSA_PCM_OLD_HW_PARAMS_API
#define ALSA_PCM_OLD_SW_PARAMS_API

before inclusion of asound/asoundlib.h. Takashi 20031123

Retrieved from "http://alsa.opensrc.org/WritingAnAlsaDriver"

Category: Howto

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