[alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max constrained by preallocated bytes issue

[Keyon Jie]

On 2020/1/17 上午4:37, Takashi Iwai wrote:
> On Thu, 16 Jan 2020 18:40:26 +0100,
> Pierre-Louis Bossart wrote:
>>
>>
>>>>>> So, do you suggest not doing preallocation(or calling it with 0
>>>>>> size) for all
>>>>>> driver with TYPE_SG? I am fine if this is the recommended method,
>>>>>> I can try
>>>>>> this on SOF I2S platform to see if it can work as we required for
>>>>>> very large
>>>>>> buffer size.
>>>>
>>>> Keyon, for the rest of us to follow this patch, would you mind
>>>> clarifying what drives the need for a 'very large buffer size', and
>>>> what order of magnitude this very large size would be.
>>>>
>>>> FWIW, we've measured consistently on different Windows/Linux
>>>> platforms, maybe 10 years ago, that once you reach a buffer of 1s
>>>> (384 kB) the benefits from increasing that buffer size further are
>>>> marginal in terms of power consumption, and generate all kinds of
>>>> issues with volume updates and deferred routing changes.
>>>>
>>> We need bigger buffer on host side to compensate the wake up time
>>> from d0ix to d0 which takes ~2 seconds on my setup. So, wiith
>>> smaller buffer sizes like < 2 seconds we overwrite data since FW
>>> keeps copping while host doesn't read until its up and running
>>> again.
>>
>> Right, that's a valid case, but that's 256 kB, not 'very large' or
>> likely to ever trigger an OOM case.
> 
> That size shouldn't matter, and would work even with the
> preallocation.
> 
> My concern is that removing the limitation would allow the allocation
> of too large sizes.  Even with dma_max limit, it can go up to 32MB
> physical pages per stream for HDA.  Depending on the hardware setup,
> there can be a lot of streams assignment (e.g. HDMI codecs) and
> multiple codecs / controllers, and imagine that all those allocated
> pages are pinned and can't be swapped out...

Hi Takashi, I get your concern here, but if we switch to use dma_max 
limit, we won't change the preallocated buffer, it will be still 64KB 
for each stream, user space can ask for re-allocate buffer for each 
stream up to 32MB, but those pinned and can't be swapped out ones are 
the 64KB preallocated ones only, am I wrong?

Thanks,
~Keyon

> 
> 
> Takashi
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