Re: [RFC PATCH 01/16] Documentation: driver: add SoundWire BRA description

[Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>]

>>> This feels a lot like it could map onto the regmap async interface, it
>>> would need a bit of a rework (mainly that currently they provide
>>> ordering guarantees with respect to both each other and sync I/O) but
>>> those could be removed with some care) but it's got the "here's a list
>>> of I/O, here's another call to ensure it's all actually happened" thing.
>>> It sounds very much like how I was thinking of the async API when I was
>>> writing it and the initial users.
> 
>>> It's this bit that really got me thinking it could fit well into regmap.
> 
>> I really don't know anything about this async interface, if you have
>> pointers on existing examples I am all ears....I am not aware of any
>> Intel platform or codec used on an Intel platform making use of this API.
> 
> grep for regmap_.*async - cs_dsp.c is the upstream example in a driver,
> or there's the rbtree cache sync code which uses a back door to go into
> an async mode.  Basically just variants of all the normal regmap I/O
> calls with a _complete() call you can use to wait for everything to
> happen.  The implementation is a bit heavyweight since it was written to
> work with fairly slow buses.

I spent a fair amount of time this afternoon trying to understand the
regmap_async parts, and I am not following where in the code there is an
ordering requirement/enforcement between async and sync usages.

I do see this comment in the code

 * @async_write: Write operation which completes asynchronously,
optional and must serialise with respect to non-async I/O.

But that 'must' is a requirement on the codec side, isn't it?

When using regmap_raw_write_async(), the lock is released immediately. I
don't see anything at the regmap level that would prevent a codec driver
from using regmap_raw_write() immediately.

That's probably a violation of the API to do so, but it's the same
problem I was referring earlier in the conversation where 'regular'
read/writes cannot happen in parallel with BTP/BRA transactions.

Also is this just me spacing out or there is no regmap_raw_read_async()?