[alsa-devel] [RFC PATCH 1/4] ALSA: core: let low-level driver or userspace disable rewinds

[Pierre-Louis Bossart]

On 7/28/15 10:43 AM, Alexander E. Patrakov wrote:
> 28.07.2015 19:19, Pierre-Louis Bossart wrote:
>> On 7/11/15 12:06 PM, Alexander E. Patrakov wrote:
>
>>> 3. I have not seen any justification for the drastic measure of making a
>>> DMA-based device completely unrewindable. Maybe a more polite "please
>>> make this a batch/blocktransfer card" request, thus disallowing only
>>> sub-period rewinds, would still be useful for powersaving, without
>>> killing dmix.
>>>
>>> 4. If this "no rewinds" mode is not made the default, then exactly
>>> nobody will use it. Everyone except sound servers opens the default
>>> device with the default flags. I understand the potential to break
>>> existing userspace, especially PulseAudio, but we really need to think
>>> more here.
>>
>> Not sure I understand the issue. when a new functionality is added it
>> takes time to be adopted. If we can push it in sound servers first then
>> it creates a wide pool of users from day1.
>
> The issue is that the proposed functionality, in the currently proposed
> "I promise to never rewind" form, is nearly useless by its very nature
> for any sound server that cares about power consumption significantly
> more than dmix does. It is indeed usable by JACK (by its design, it
> doesn't rewind, and uses low latency) and CRAS (which currently doesn't
> rewind, but I am not sure whether this is a bug or a deliberate decision
> based on non-public measurements of extra power savings that "rewinds +
> high latency" would allow).
>
> As I have already explained, dmix, when mixing, writes to the hardware
> buffer multiple times, which is equivalent to rewinding. PulseAudio uses
> rewinds for a very specific purpose - to avoid CPU wakeups in the common
> "nothing unexpected happened" case, i.e. to allow very high average
> latency while keeping the latency of reaction to unexpected events low.
> So, by convincing PulseAudio to never rewind and to tell the driver
> about this fact, you can save some power in the card, but (if the
> proponents of rewinds are right - see my earlier request for non-public
> information) will waste way more power due to the need for much more
> frequent CPU wakeups ("cannot rewind but have to react to unexpected
> events within 20 ms" = "must wake up every 20 ms").
>
> The only cases where this flag can be useful for sound servers are:
>
> 1. Sound servers that already, by design, always waste CPU power by
> running at low latency;

Your classification is not exhaustive enough. You need to take into 
account sound servers that have two outputs per endpoint, one for 
low-latency and one for low-power/deep-buffer with a DSP/hardware mixer.
Power is also no longer directly linked to wakeup rates only but also to 
DDR access patterns.
When a larger on-chip buffer is available, disabling rewinds does let 
the hardware know it can safely fetch data in bigger chunks rather than 
use small data bursts.
This sort of capabilities is becoming prevalent these days, and not just 
on Intel platforms - see e.g. Nexus5/9 devices -, and maybe PulseAudio 
and friends need to evolve to make use of these resources rather than 
stay the course with single output and rewind mechanisms that prevent 
power optimizations on newer platforms.

>
> 2. Sound cards like USB audio where the kernel already contains a
> low-latency thread that copies data to the card in the serialized way.
>
> For PulseAudio on Intel hardware and HDA-compatible card, it would (if
> the situation on my Sony laptop is an exception and not the rule), as I
> have already mentioned, only trade some power consumption improvement in
> the card for bigger losses in the CPU.
>
> And with (2), I am not sure whether this is a win at all: you are likely
> trading a kernel thread that wakes up, say, every 6 ms (with userspace
> waking up very rarely), to a userspace wakeup that happens every 6 or
> maybe 20 ms (as the sound server chooses), but incurs more context
> switches. It does shift the policy decision about the wakeup rate to
> userspace, though.
>