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
>>>
>>> So, SNDRV_CTL_IOCTL_TLV_COMMAND == SNDRV_PCM_IOCTL_SET_ATTR in your
>>> 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;
Jaroslav
-----
Jaroslav Kysela <perex@xxxxxxxx>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.
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