[alsa-devel] RFC: PCM extra attributes

Jaroslav Kysela perex at perex.cz
Fri Jun 19 18:58:46 CEST 2009

On Fri, 19 Jun 2009, Takashi Iwai wrote:

> At Fri, 19 Jun 2009 13:14:15 +0200 (CEST),
> Jaroslav Kysela wrote:
>> On Fri, 19 Jun 2009, Takashi Iwai wrote:
>>> At Fri, 19 Jun 2009 12:45:04 +0200 (CEST),
>>> Jaroslav Kysela wrote:
>>>> On Fri, 19 Jun 2009, Jaroslav Kysela wrote:
>>>>> 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
>>>>> proposal.
>>>>> 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.
>>>> BTW: It's also question, if to divide TLVs to static/configuration ones.
>>>> TLV_READ might just return all TLVs and TLV_READONE filter only one, if
>>>> user space does not want to obtain all information.
>>>> I would like to preserve TLV_READ to obtain all TLVs for possible user
>>>> space enumeration (for example for debugging purposes) rather that do a
>>>> single query for all possible TLV types.
>>> I disagree with "all-in-on-TLV" strategy.  That makes life much harder.
>>> Sorry, it's no go.
>> Sorry, the implementation can be more simple than you think. Imagine just
>> a TLV callback in the lowlevel driver and switch/case statement in the
>> callback. We can define a special type in kernel that queries for all
>> present TLV types (bitmap) and the ioctl TLV_READ implementation can just
>> compose results from single queries. So, the code in the lowlevel driver
>> will grow only with 4 (or less) lines:
>>  	case TLV_TYPES:
>>  		tlv.length = 4;
>>  		tlv.data[0] = (1<<TLV_CHANNEL_MAP) | (1<<TLV_CONTROL_IDS);
>>  		return 0;
> Sorry I can't draw a picture from your explanation.
> How can it compose a "all-in-one" TLV at each time?

The tlv_read ioctl will be something like this (simplified without 
proper length and allocation handling):

 	tlv_types.type = TLV_TYPES;
 	idx = 0;
 	while (tlv_types[0] && (tlv_types[0] & (1 << idx)) != 0) {
 		tlv.type = idx;
 		merge_tlv(result, tlv);
 		tlv_types[0] &= ~(1 << idx);

> What I'm talking is like the scenario below:
> - app open PCM, do hw_params
> - app requires the channel map, get one
> - app changes hw_params again, then get channel map again
> With "all-in-one" TLV, you have to read all the information at each
> time you get channel maps because the channel map is to be generated
> dynamically.  At each time, the kernel needs to gather all
> information, compose into a single big TLV, and copy it.  Then
> user-space decomposes the big TLV, look for a specific tag, then picks
> up the value there.
> Why not simply query a value and get a value a la normal ioctl?

I'm talking about to have both "all-in-one" and single value ioctls. I see 
your optimization when one value is required to read multiple times. I 
just prefer to make things similar to the control interface (although 
single value read is not implemented in current control API, but it might 
be added later, if required). I would like to see also common naming in 
interfaces like:

SNDRV_PCM_IOCTL_TLV_COMMAND	# write one or more TLVs (COMPOUND type)
SNDRV_PCM_IOCTL_TLV_TREAD	# read only one TLV specified by type
 				# TREAD might be also READONE or so..


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

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