Re: [PATCH v3] soundwire: stream: Prepare ports in parallel to reduce stream start latency

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

On 12/10/25 10:59, Richard Fitzgerald wrote:
> On 9/12/25 16:41, Pierre-Louis Bossart wrote:
>>
>>>>> Changes in V2:
>>>>> +    if (simple_ch_prep_sm)
>>>>> +        return 0;
>>>>> +
>>>>> +    /*
>>>>> +     * Check if already prepared. Avoid overhead of waiting for interrupt
>>>>> +     * and port_ready completion if we don't need to.
>>>>> +     */
>>
>> 1.
>>
>>>>> +    val = sdw_read_no_pm(s_rt->slave, SDW_DPN_PREPARESTATUS(p_rt->num));
>>>>> +    if (val < 0) {
>>>>> +        ret = val;
>>>>> +        goto err;
>>>>> +    }
>>>>> +
>>>>> +    if (val & p_rt->ch_mask) {
>>>>
>>>> Can you explain why we don't use the ch_mask in the already-prepared case? I am missing something.
>>>>
>>> I'm not sure what you mean here. The if() immediately above your comment
>>> uses ch_mask to check the already-prepared state.
>>
>> I was referring to the 1. above, you read the prepare status without checking for ch_mask first.
>>
> 
> What would be the purpose of checking ch_mask before the read?

I don't know - why do we need to read it in the second case and not the first is all I am asking.
 
>>>>> +        /* Wait for completion on port ready */
>>>>> +        port_ready = &s_rt->slave->port_ready[p_rt->num];
>>>>> +        wait_for_completion_timeout(port_ready, msecs_to_jiffies(ch_prep_timeout));
>>>>
>>>> I understand the code is the same as before but would there be any merit in checking the timeout before starting a read? If the device is already in the weeds, doing another read adds even more time before reporting an error.
>>>>
>>> Do you mean save the system time when the DPN_PREPARE was written to
>>> that peripheral and then check here whether the timeout period has
>>> already elapsed?
>>
>> I meant testing the return value of wait_for_completion_timeout(). If you already timed out at this point with a return value of zero, there's no point in checking the status any more, the system is in the weeds.
>>
> 
> Wait completion will _always_ timeout because this code is holding the
> bus lock, which blocks the ALERT handler from running and signalling
> the completion. The wait_for_completion_timeout() is effectively
> msleep(msecs_to_jiffies(ch_prep_timeout));
> So we have to read the register afterwards to see whether the peripheral
> actually prepared.
> 
> I've left the useless wait_for_completion_timeout() in the code so this
> commit is only changing what it says it is changing, and nothing else.
> 
> What to do about the deadlocked wait_for_completion_timeout() is a
> separate problem.

Humm, what happens if you have a single peripheral, does this result in an increase of the prepare time all the way to the timeout value?
I can see how preparing all ports in parallel would reduce the total time compared to a serial approach, even with a timeout, but if we end-up always timing out even in the case where there is a single device it'd be quite odd.

In other words does this patch reduce the start latency only in the case of multiple devices, but adds a 'tax' for all other cases?