Re: [RFC PATCH 01/17] ALSA: pcm: Introduce MSBITS subformat interface

[Cezary Rojewski <cezary.rojewski@intel.com>]

On 2023-08-22 9:03 PM, Jaroslav Kysela wrote:
> On 22. 08. 23 17:38, Takashi Iwai wrote:
>> On Tue, 22 Aug 2023 17:29:47 +0200,
>> Jaroslav Kysela wrote:
>>>
>>> On 22. 08. 23 17:07, Takashi Iwai wrote:
>>>> On Tue, 22 Aug 2023 17:03:02 +0200,
>>>> Jaroslav Kysela wrote:
>>>>>
>>>>> On 11. 08. 23 18:48, Cezary Rojewski wrote:
>>>>>
>>>>>> +#define SNDRV_PCM_SUBFMTBIT_MSBITS_32    
>>>>>> _SNDRV_PCM_SUBFMTBIT(MSBITS_32)
>>>>>
>>>>> What was reason to add 32/32 format ? Subformat STD + msbits == 32
>>>>> should already handle the maximal resolution. Until we do not have 64
>>>>> bit formats, it seems like an useless extension.
>>>>
>>>> My understanding is to distinguish the cases "we do fully support
>>>> 32bit" and "we don't care".  But, the end effect is same for both,
>>>> user-space would handle 32bit in both cases, so this difference won't
>>>> help much, indeed.
>>>
>>> I don't think that we have a "do not care" situation. The applications
>>> currently expects to use the maximal msbits for STD subformat. The
>>> subformat should be used only to refine (downgrade) the resolution on
>>> the driver / hw side on demand. I would just add only necessary API
>>> extensions and save one bit for now.
>>
>> Well, the current behavior (with STD) is to choose whatever 32bit
>> format the driver supports, and the driver may set a different value
>> of hw_params.msbits at hw_params.  The explicit MSBITS_32 would
>> enforce the hw_params.msbits to be 32, otherwise hw_refine would
>> fail.  So I see a potential difference.
> 
> I see. But if our target is to create a complete query/set msbits API, 
> we should cover all cases also for other formats.
> 
> I vote to replace SUBFMTBIT_MSBITS_32 to SUBFMTBIT_MSBITS_MAX as the 
> second bit (right after STD). The format hw parameter already defines 
> the maximal width. We can add SUBFMTBIT_MSBITS_32 when it's really 
> required. Note that MAX should be handled for all cases (not only for 
> S32_LE or so).

In my opinion STD already states "max". The word is not explicit either 
- max in the eyes of whom? The driver'? Then the driver may reply: max 
allowed e.g.: 24/32. And that translates to: fallback to STD.


Kind regards,
Czarek