[alsa-devel] [RFC PATCH 3/4] ALSA: core: add report of max dma burst

[Pierre-Louis Bossart]

>> FIXME:
>> 1. this was copy/pasted/edited based on the fifo_size fields, not
>> sure why we would need this IOCTL1
>
> fifo_size would fit, but it means that it also changes the current
> behavior.  I don't believe that currently there are many programs
> relying on this value, but who knows.

I saw a mention of fifo_size in the VIA HDA controller, that's why I 
went ahead with a different field. The fifo_size could be useful as a 
max value for the internal hardware delay, when app use the 'delay' 
field in the status structure they get a very dynamic value that isn't 
necessarily straightforward to use.

>
>> 2. we also need the ability to set the actual fifo size, if suppported,
>> by the hardware, to negociate the prefetch amount between application
>> and driver. By default the default should be max buffering to allow
>> for lower power, but for low-latency use cases we may want to reduce
>> the amount of prefetching.
>
> Right.  So a hw_parmas field looks suitable for that purpose.

That 'set' capability is a lot more complicated, I am really having a 
hard time with all the constraints for hw_params and how to represent 
min,max and step values... If anyone is willing to help on that part I 
wouldn't mind, this is really a part of ALSA I never looked into...