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...
thanks,
Takashi
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