[alsa-devel] RFC: PCM extra attributes

Jaroslav Kysela perex at perex.cz
Fri Jun 19 12:29:17 CEST 2009

On Fri, 19 Jun 2009, Takashi Iwai wrote:

> At Fri, 19 Jun 2009 10:47:30 +0200 (CEST),
> Jaroslav Kysela wrote:
>> On Fri, 19 Jun 2009, Takashi Iwai wrote:
>>> Hi,
>>> this is yet another topic I'm (currently) working on -- the addition
>>> of PCM ioctls to get/set some extra attributes.  Basically, it adds
>>> two simple ioctls for getting/setting extra attributes to the PCM
>>> substream.  The attribute has a sort of TLV form,
>>>  /* PCM extra attributes */
>>>  struct snd_pcm_attr {
>>>  	unsigned int type;	/* SNDRC_PCM_TYPE_ATTR_XXX */
>>>  	unsigned int len;	/* GET R: the max elements in value array
>>>  				 *     W: the actually written # elements
>>>  				 * SET R/W: # elements to store
>>>  				 */
>>>  	unsigned int value[0];	/* value(s) to read / write */
>>>  };
>>> And corresponding two ioctls
>>>  #define SNDRV_PCM_IOCTL_GET_ATTR	_IOWR('A', 0x14, struct snd_pcm_attr)
>>>  #define SNDRV_PCM_IOCTL_SET_ATTR	_IOWR('A', 0x15, struct snd_pcm_attr)
>> I would prefer to implement similar TLV implementation as for the control
>> API. The amount of information for reading (get) will be small, so
>> filtering in this direction is not necessary. Also, common parts of
>> implementation (future merging of more TLVs to compounds) can be shared.
> Actually it's a sort of TLV.  You see exactly it in snd_pcm_attr
> struct, no? :)
> And, thinking twice after posting (that's a good effect of posting to
> ML, BTW), I feel that using a callback would be a better way, such as
> re-using the existing ops->ioctl with a new cmd tag rather than the
> statically assigned thing.
> A similar method like control TLV can be used, too.  However, a
> distinct from the existing control TLV is that this is intended for
> just one type of information while the control TLV is supposed to
> contain everything in a single shot.
> That is, this is a query with a key.  In that sense, sharing a small
> amount of control TLV code (about 10 lines) doesn't give a big
> benefit.  In anyways, it's a implementation detail, so one could
> optimize somehow, though...

I don't mean current implementation. TLVs can be nested. In this case, we 
need a set of functions which operates with TLVs (merging). These 
functions can be shared. It's also possible to share TLV code in 
the user space (search). But it's really implementation detail. We should 
focus on ioctl definitions now.

I would defined 'struct snd_pcm_attr' as 'struct snd_tlv' - it's same as 
for control API.

The control API has:

SNDRV_CTL_IOCTL_TLV_READ	- read all static information
SNDRV_CTL_IOCTL_TLV_WRITE	- write static information (userspace controls)
SNDRV_CTL_IOCTL_TLV_COMMAND	- change some setup


SNDRV_CTL_IOCTL_TLV_WRITE is not probably useable unless we have virtual 
user-space PCM interface kernel implementation.

SNDRV_CTL_IOCTL_TLV_READ might make sense for static-only information 
which don't change between open()/close() syscalls for given substream.

SNDRV_PCM_IOCTL_GET_ATTR cannot be mapped at this time. Might be something 
like TLV_READONE, TLV_CONFIG, TLV_SETUP, TLV_GET or so - what's better 
for COMMAND word, if we agree on common names for all kernel interfaces.


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

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