[alsa-devel] [RESEND][PATCH v4 1/3] ALSA: core: let low-level driver or userspace disable rewinds

[Pierre-Louis Bossart]

On 3/28/18 10:20 AM, Takashi Iwai wrote:
> On Wed, 28 Mar 2018 16:30:09 +0200,
> Pierre-Louis Bossart wrote:
>>
>> On 3/25/18 9:58 AM, Takashi Iwai wrote:
>>> On Sun, 25 Mar 2018 12:46:43 +0200,
>>> Sriram Periyasamy wrote:
>>>>
>>>> On Tue, Mar 20, 2018 at 05:17:35PM +0100, Takashi Iwai wrote:
>>>>> On Tue, 20 Mar 2018 17:01:06 +0100,
>>>>> Sriram Periyasamy wrote:
>>>>>>
>>>>>> From: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
>>>>>>
>>>>>> Add new hw_params flag to explicitly tell driver that rewinds will never
>>>>>> be used. This can be used by low-level driver to optimize DMA operations
>>>>>> and reduce power consumption. Use this flag only when data written in
>>>>>> ring buffer will never be invalidated, e.g. any update of appl_ptr is
>>>>>> final.
>>>>>>
>>>>>> Note that the update of appl_ptr include both a read/write data
>>>>>> operation as well as snd_pcm_forward() whose behavior is not modified.
>>>>>>
>>>>>> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
>>>>>> Signed-off-by: Ramesh Babu <ramesh.babu at intel.com>
>>>>>> Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty at intel.com>
>>>>>> Signed-off-by: Sriram Periyasamy <sriramx.periyasamy at intel.com>
>>>>>
>>>>> Well, I'm still not convinced with this flag.
>>>>>
>>>>> First off, does it really need to be per PCM stream?  The introducing
>>>>
>>>> Flag per PCM stream helps where each stream in given system may have
>>>> different requirement such as low power or low latency based on the
>>>> use case. For example in case of low power stream, driver can perform
>>>> required optimizations at hardware level based on the no_rewind flag.
>>>
>>> Yes, but does it really need to be PCM stream, i.e. per application?
>>> Certainly the system will be using some sound backend (like PA).  In
>>> which scenario can such behavior change -- some application uses a
>>> different backend on a phone or a tablet?
>>
>> This is intended for the case where the system server exposes a
>> 'deep-buffer' PCM device for music playback in low-power mode and a
>> separate one for system sounds or anything that requires
>> interactivity.
>> The need for rewinding is really for the case where the interactive
>> system sounds are mixed with music, when you have separation between
>> types of sounds and hardware/firmware mixing then the rewinds are
>> unnecessary.
> 
> Yes, but why application must tell no-rewind flag if it wants to
> save a bit of power?  IOW, how each application can know it needs to
> set no-rewind flag *for saving power*?
> 
> Or, put in another way: you want to make all applications running in
> lower power generically.  What would you do?  Adding no-rewind flag to
> all calls?  It makes no sense if the application runs on non-Intel
> chips, so can't be hard-coded.
> 
>> If there are multiple applications using different PCM devices each
>> (which is a bit hypothetical to me) there is no way to know ahead of
>> time when the modules are loaded if the application will perform
>> rewinds due to its interactive nature or will just stream without ever
>> invalidating the ring buffer. So yes it's per stream.
> 
> Fair enough, per stream is a requirement.
> 
> But still my argument below applies: what you really want to set
> is to make the stream low-power.  It's not about to make the stream
> non-rewindable.  And this makes me feel uneasy.
> 
> 
>>>>> something to hw_parms implies that it varies per application.  But I
>>>>> can't imagine that a system requires different behavior per stream
>>>>> regarding such a thing.
>>>>>
>>>>> Second, the driver can implement a check in PCM ack callback to
>>>>> prevent the rewind, too.  Then there is no need to touch the PCM
>>>>> core.
>>>>>
>>>>
>>>> As per the previous discussion at [1],
>>>>
>>>> [1]
>>>> https://patchwork.kernel.org/patch/9795233/
>>>>
>>>> from Pierre,
>>>>
>>>> "The application (which is in most cases an audio server) *knows* if it
>>>> requires rewinds or not. It's part of its design, with rewinds typically
>>>> disabled if period interrupts are required. It's been that way for a
>>>> number of years now. The use of rewinds is typically associated with the
>>>> combination of a large buffer and no interrupts (having either of the
>>>> two would not require rewinds).
>>>>
>>>> So the idea is that the application makes a statement that rewinds will
>>>> not be used, and the low-level driver makes use of the information to
>>>> enable whatever optimizations are available at the hardware level.
>>>>
>>>> Exposing more information to userspace would quickly lead to a confusing
>>>> decision-making and would require more than just a flag."
>>>
>>> And, requiring *that* information is already confusing, IMO.
>>> Think from the application writer POV: what is the relation between
>>> the power-saving and no_rewind behavior in application level at all?
>>> If you have no idea about the hardware details, they are totally
>>> irrelevant.
>>
>> I feel like disabling IRQs or disabling rewinds is the same level of
>> information, you set the flags without necessary knowing all the power
>> savings down to the mW level. But it provides an opportunity to save
>> power with additional degrees of freedom for implementations.
>   
> Sure, I do understand this will bring the merit.  But the question is
> the API design.
> 
>> An additional benefit of using the underlying SPIB register on Intel
>> hardware is that the DMA hardware will not wrap-around, which can lead
>> to better detection of real-time issues and a guarantee that stale
>> data will not be played.
> 
> So, again, the purpose of no-rewind isn't the rewind thing itself.
> It's set for obtaining other benefits.
> 
>>> Or, think like this way: imagine a hardware that requires a different
>>> constraint, e.g. the power-of-two buffer size, for power-efficient
>>> operation.  What would you do?  Adding a new power_of_two bit flag
>>> into hw_params?  Likely not.
>>
>> we've added the noIRQ mode in the past using flags, if now you are
>> saying that flags is a bad idea then fine, but let's be consistent...
> 
> The no-IRQ is rather a more drastic behavior change.  The ALSA PCM
> mandated the period update per definition, and setting this flag
> really switches to a different mode, hence it deserves for an API
> extension.  And, the flag itself is self-explaining: the less IRQ is
> less power.  But no-rewind is...?
> 
>>> In such a case, I would expect some operation mode switch
>>> (e.g. power-saving vs low latency or whatever) instead of a very
>>> specific hw_parmas flag.  It might be a module option, via ALSA
>>> control, or something else.  But it's clearer for which purpose it's
>>> present, at least, and it can be implemented well without changing the
>>> existing API.
>>
>> We have no way of predicting what the application will do so the
>> module option is not possible.
>>
>> Using an ALSA control is possible, but it's odd to me.
>>
>> I really don't see what's so problematic about adding flags. I uses an
>> existing capability of the API, it's consistent with the previous
>> usages. There is no change in behavior for existing apps, only newer
>> can benefit for better use of the hardware. There is no complicated
>> decision making, you set the flags if you don't use IRQ or rewinds.
>> And it's not like we will have new flags every week, we've been
>> talking about this SPIB capability since Skylake which is 3 years old
>> already.
> 
> Again, my concern is that you swapped between the purpose and the
> method.  The no-irq isn't any purpose, per se.  It's just a
> requirement some hardware casually applies for power saving.
> 
> The real need isn't about each detailed hardware-specific flag, but
> rather some API to give a hint for the preferred operation mode.

let me try a different explanation (don't want to be pedantic but try to 
explain the line of thought).

There are two needs in terms of application/driver interaction.

The first need is to let the application know about hardware 
capabilities or restrictions. This is handled today with the .info 
fields. We have quite a few of them that provide information to 
userspace and let the application use the ALSA API in different ways 
(e.g. BATCH, BLOCK_TRANSFER, PAUSE, SYNC_START, WALL_CLK). To take the 
example above, if a specific hardware could handle optimizations for 
powers of two, it could add a flag in the .info field and let the 
application make that decision.

The second need is to establish a contract between application and 
driver, set in stone and non modifiable dynamically while the stream is 
open. We are using hw_params for this, and the flags are one way to 
extend the API to new capabilities. the no-irq flag is one example of 
this contract which fundamentally changes the way the application is 
written. It's not limited to power savings but can also be used to 
reduce the latency as done by Android/AAudio, and it's not a 'casual' 
way of doing things but a fundamental design decision.

The proposal in this patchset is to restrict the use of rewinds which 
can have two known benefits
1. better DMA handling with opportunistic/bursty transfers
2. no wrap-around and handling of stale data.
This capability may be used for low-power and low-latency, in a similar 
way to the no-irq mode. Whether it makes sense for a system is not the 
debate here, we want to leave the decision making to system integrators. 
Since we use the hw_params flags in the past, we chose the same 
solution. We also did not add any flag in the .info field since the 
application doesn't really need to know what hardware will do or not. If 
it doesn't use rewinds, it just states it and lets the hardware enable 
new capabilities.

I am really struggling to see how the proposal is viewed as different 
from previous ones and where we confused 'purpose and method'. We used 
the same hooks for the same purposes.

I also don't think that either the no-irq and no-rewinds can be assigned 
to low-power or low-latency usages. I don't think we are in a position 
to make that call and I don't think we want to add a 'low-latency' or 
'low-power' flag to the ALSA API - this would be extremely confusing to 
applications.

If your position is that we can no longer report or enable new 
capabilities with the .info and hw_params fields, then what are the options?
a) do nothing and leave ALSA handle hardware designed prior to 2013
b) define a new API and transition both the info and hw_params to 
something else