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

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

On 12/20/23 16:16, Charles Keepax wrote:
> On Tue, Dec 19, 2023 at 06:08:15PM +0100, Pierre-Louis Bossart wrote:
>> On 12/19/23 17:53, Mark Brown wrote:
>>> On Tue, Dec 19, 2023 at 05:50:30PM +0100, Pierre-Louis Bossart wrote:
>>>>> 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.
>>>
>>> The only actual async implementation is SPI which processes things in
>>> order of submission, the sync API wraps the async API.
>>>
>>>> Also is this just me spacing out or there is no regmap_raw_read_async()?
>>>
>>> Right, there was never any need.
>>
>> ok. I am starting to think that we could have a new type of regmap, say
>> "regmap-sdw-bra", where the use of write_raw_async() would rely on the
>> send/wait bus primitives, and write_raw() would fallback to the regular
>> read/write commands. We'd need a mutual exclusion to prevent parallel
>> async/sync access to the same regmap.
>>
>> In other words, "memory" areas that are used for firmware downloads
>> would be moved to a different regmap with async capabilities and no
>> caching support.
> 
> I would be a little inclined to say leave adding a regmap for a
> follow up series, whether we add it to the existing regmap or add
> a new one, or whatever, it should all sit happily on top of the
> API being added in this series. Makes it a little more contained
> to focus on one area at a time, and leave this series as adding
> core support for BRA. But that said, if we really want to I don't
> feel mega strongly on this one.

Right, I was probably going too far down in the details for a December
20 post.

The point I was trying to make it seems there's consensus that regmap
with the async parts would be the API used by SoundWire/ASoC codecs, and
the regmap implementation would rely on the bus/host send/wait routines.

The regmap stuff will need joint work with codec driver folks so it
should indeed be done in a second step when the SoundWire bus/host parts
are available.

Put differently: is there any sustained objection to the proposal of
extending regmap with async BRA transfers?