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

[Keyon Jie <yang.jie@linux.intel.com>]

On 2020/1/19 下午3:09, Takashi Iwai wrote:
> On Sun, 19 Jan 2020 04:52:55 +0100,
> Keyon Jie wrote:
>>
>>
>> On 2020/1/17 下午7:12, Takashi Iwai wrote:
>>> On Fri, 17 Jan 2020 11:43:24 +0100,
>>> Keyon Jie wrote:
>>>>
>>>> In SOF driver, we don't use kernel config item like
>>>> CONFIG_SND_HDA_PREALLOC_SIZE for HDA, the code for it is:
>>>>
>>>> 	snd_pcm_lib_preallocate_pages(pcm->streams[stream].substream,
>>>> 				      SNDRV_DMA_TYPE_DEV_SG, sdev->dev,
>>>> 				le32_to_cpu(caps->buffer_size_min),
>>>> 				le32_to_cpu(caps->buffer_size_max));
>>>>
>>>> So the preallocated size is configured via topology file, that is
>>>> caps->buffer_size_min, no chance for PulseAudio to reconfigure it.
>>>>
>>>> So, it looks like we have to change it to this if we don't change the
>>>> ALSA core:
>>>>
>>>> 	snd_pcm_lib_preallocate_pages(pcm->streams[stream].substream,
>>>> 				      SNDRV_DMA_TYPE_DEV_SG, sdev->dev,
>>>> -				le32_to_cpu(caps->buffer_size_min),
>>>> +				le32_to_cpu(caps->buffer_size_max),
>>>> 				le32_to_cpu(caps->buffer_size_max));
>>>
>>> Yes, passing buffer_size_min for the preallocation sounds already
>>> bad.  The default value should be sufficient for usual operations, not
>>> the cost-cutting minimum.  Otherwise there is no merit of
>>> preallocation.
>>>
>>> Alternatively, we may pass 0 there, indicating no limitation, too.
>>> But, this would need a bit other adjustment, e.g. snd_pcm_hardware
>>> should have lower buffer_bytes_max.
>>
>> Thank you Takashi, then let's follow it to pre-allocate with
>> caps->buffer_size_max, as we don't specify any limitations in
>> snd_pcm_hardware today, we want to leave it configurable to each
>> specific topology file for different machines.
> 
> How big is caps->buffer_size_max?  Passing the value there means
> actually trying to allocate the given size as default, and it'd be a
> lot of waste if a too large value (e.g. 32MB) is passed there.

It varies for each stream, most of them are 65536 Bytes only, whereas 
one for Wake-On-Voice might need a > 4 Seconds buffer could be up to 
about 1~2MBytes, and another one for deep-buffer playback can be up to 
about 8MBytes.

> 
> I think we can go for passing zero as default, which means skipping
> preallocation.  In addition, we may add an upper limit of the total

Just did an experiment and this works for me, I believe we still need to 
call snd_pcm_set_managed_buffer() though the preallocation is skipped in 
this, right?

> amount of allocation per card, controlled in pcm_memory.c, for
> example.  This logic can be applied to the legacy HDA, too.
> 
> This should be relatively easy, and I'll provide the patch in the next
> week.

OK, that's fine for me also, thank you.

~Keyon

> 
> 
> Takashi
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