Re: [RFC PATCH 08/16] soundwire: bus: add bpt_stream pointer

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

On 12/18/23 05:55, Vinod Koul wrote:
> On 07-12-23, 16:29, Pierre-Louis Bossart wrote:
>> Add a convenience pointer to the 'sdw_bus' structure. BPT is a
>> dedicated stream which will typically not be handled by DAIs or
>> dailinks. Since there's only one BPT stream per link, storing the
>> pointer at the link level seems rather natural.
>>
>> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
>> ---
>>  include/linux/soundwire/sdw.h | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/include/linux/soundwire/sdw.h b/include/linux/soundwire/sdw.h
>> index e54c5bbd2b91..8db0cd7d0d89 100644
>> --- a/include/linux/soundwire/sdw.h
>> +++ b/include/linux/soundwire/sdw.h
>> @@ -965,6 +965,7 @@ struct sdw_master_ops {
>>   * @stream_refcount: number of streams currently using this bus
>>   * @btp_stream_refcount: number of BTP streams currently using this bus (should
>>   * be zero or one, multiple streams per link is not supported).
>> + * @bpt_stream: pointer stored for convenience.
>>   */
>>  struct sdw_bus {
>>  	struct device *dev;
>> @@ -996,6 +997,7 @@ struct sdw_bus {
>>  	int hw_sync_min_links;
>>  	int stream_refcount;
>>  	int bpt_stream_refcount;
>> +	struct sdw_stream_runtime *bpt_stream;
> 
> So we are limiting to single stream? Can we have multiple transfers
> queued up, which I guess might imply multiple streams?


Yes for now there is a BTP/BRA single stream active when there are no
audio transfers taking place. This is the only way to guarantee
predictable download/resume times.

There is no mechanism to queue up transfers, be it from the same
peripheral device or different ones. It would be up to the peripheral
driver to wait for the BTP stream to be available.

Adding a queuing mechanism is a bridge too far for now, most platforms
only have 1 or 2 devices only, and a peripheral driver may or may not be
ok with delayed downloads. For now the main ask is to reduce download
times, a single stream is already a good start. There are other
refinements we need to add as well, such as changing clocks to use the
fastest gear. I'd like to proceed with baby steps...