Re: [PATCH v2] soundwire: stream: Prepare ports in parallel to reduce stream start latency

[Richard Fitzgerald <rf@opensource.cirrus.com>]

On 25/11/2025 10:13 am, Richard Fitzgerald wrote:
> Issue DP prepare to all ports that use full CP_SM. Then wait for the
> prepare to complete. This allow all the DP to prepare in parallel to
> reduce the latency of starting an audio stream.
> 
> On a system with six CS35L56 amps, this reduces the startup latency,
> from runtime_resume to all amps ready to play, from ~160 ms to ~60 ms.
> 
> (Test hardware: UpXtreme i14, BIOS v1.2, Core Ultra 7 155H, 3x CS35L56
> on link 0, 3x CS35L56 on link 1).
> 
> An initial read of DPn_PREPARESTATUS is done before dropping into the wait,
> so that a quick exit can be made if the port is already prepared. Currently
> this is essential because the wait deadlocks - the stream setup takes
> bus_lock, which blocks the interrupt handler - so the wait for completion
> will always timeout.
> 
> However, an experiment of removing the bus_lock from stream setup, so that
> the interrupt will work, shows that wait for completion takes ~700..800 us
> but the quick-exit read takes 50..200 us. So the quick exit is still
> valuable even if the stream.c code was rewritten to allow the completion
> interrupt to work. Rewriting the code so it doesn't take bus_lock is risky.
> The deadlock only lasts until the wait times out so it's not a serious
> problem now that the DP prepare happens in parallel.
> 
> Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
> ---
> Changes in V2:
> - Fixed missing initialization of dp0_prop.
Sorry, ignore this version. I misunderstood what the kernel robot build
was complaining about. The actual problem was that I'd intended to
remove the local dp0_prop as unnecessary but it was still in V1.

I've sent a V3.