Re: [RFC PATCH 1/6] ALSA: compress: add Sample Rate Converter codec support

Jaroslav Kysela
2024-08-12T13:31:41Z

On 12. 08. 24 12:24, Shengjiu Wang wrote:
> On Fri, Aug 9, 2024 at 10:01 PM Jaroslav Kysela <perex@perex.cz> wrote:
>>
>> On 09. 08. 24 14:52, Pierre-Louis Bossart wrote:
>>
>>>> And metadata
>>>> ioctl can be called many times which can meet the ratio modifier
>>>> requirement (ratio may be drift on the fly)
>>>
>>> Interesting, that's yet another way of handling the drift with userspace
>>> modifying the ratio dynamically. That's different to what I've seen before.
>>
>> Note that the "timing" is managed by the user space with this scheme.
>>
>>>> And compress API uses codec as the unit for capability query and
>>>> parameter setting, So I think need to define "SND_AUDIOCODEC_SRC'
>>>> and 'struct snd_dec_src', for the 'snd_dec_src' just defined output
>>>> format and output rate, channels definition just reuse the snd_codec.ch_in.
>>>
>>> The capability query is an interesting point as well, it's not clear how
>>> to expose to userspace what this specific implementation can do, while
>>> at the same time *requiring* userpace to update the ratio dynamically.
>>> For something like this to work, userspace needs to have pre-existing
>>> information on how the SRC works.
>>
>> Yes, it's about abstraction. The user space wants to push data, read data back
>> converted to the target rate and eventually modify the drift using a control
>> managing clocks using own way. We can eventually assume, that if this control
>> does not exist, the drift cannot be controlled. Also, nice thing is that the
>> control has min and max values (range), so driver can specify the drift range,
>> too.
>>
>> And again, look to "PCM Rate Shift 100000" control implementation in
>> sound/drivers/aloop.c. It would be nice to have the base offset for the
>> shift/drift/pitch value standardized.
>
> Thanks.
>
> But the ASRC driver I implemented is different, I just register one sound
> card, one device/subdevice. but the ASRC hardware support 4 instances
> together, so user can open the card device 4 times to create 4 instances
> then the controls can only bind with compress streams.

It's just a reason to add the subdevice code for the compress offload layer
like we have in other APIs for overall consistency. I'll try to work on this.

> I think I can remove the 'SNDRV_COMPRESS_SRC_RATIO_MOD',

Yes.

> Only define a private type for driver, which means only the ASRC driver
> and its user application know the type.

The control API should be used for this IMHO.

> For the change in 'include/uapi/sound/compress_params.h", should I
> keep them, is there any other suggestion for them?

I don't see a problem there.

Jaroslav

--
Jaroslav Kysela <perex@perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.