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

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 8/20/24 13:53, Mark Brown wrote:
> On Tue, Aug 20, 2024 at 09:48:05AM +0200, Pierre-Louis Bossart wrote:
> 
>>> 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.
> 
>> Do I get this right that these async capabilities could be used to
>> enable this faster SoundWire protocol, but the regular regmap_write()
>> could still happen in parallel with these async transfers? This could be
>> useful if e.g. there's a jack detection while downloading firmware for
>> another function.
> 
> As far as the regmap core is concerned, yes - with SPI we do wind up
> with ordering but that's because the SPI sync API is a thin wrapper
> around the SPI async API rather than anything regmap does.

ok. I am struggling a bit to adjust the plumbing that was obviously
defined for SPI.

The regmap async_write() doesn't have any wait, but there's a notifier
that needs to be called by *something* that waits. I think the SPI layer
has a set of kthreads but in our case we could just rely on a a
workqueue after 1ms or something and do the wait then for the DMAs to
complete and finally call the notifier to wake-up the regmap stuff.

>> The only thing that would need to be extended is that we'd need
>> additional callbacks to check if the protocol is supported on a given
>> hardware/firmware platform, and if there's no audio stream. In these
>> cases the async writes would be demoted to regular writes. Otherwise the
>> bus would be locked to make sure no reconfiguration takes place while
>> the firmware download happens, and unlocked when the transfer ends.
> 
> Those callbacks seem reasonable.  We already do the demotion to sync for
> buses that don't have async, it'd just need to be made dynamic.

sounds good.