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

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

On 2020/1/17 下午4:00, Takashi Iwai wrote:
> On Fri, 17 Jan 2020 06:37:16 +0100,
> Keyon Jie wrote:
>>
>> On 2020/1/17 上午1:40, 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.
>>
>> For S24_LE, it is 512KB, the point is that if we can't re-allocate
>> buffer at hw_params() stage, then we need follow a BKM that we have to
>> preallocate the largest DMA buffer that we claim to support at
>> pcm_new(), I think this is actually another kind of wast with these
>> largest pinned buffer that can't be swapped out...
> 
> Well, that's the case you'd need a larger preallocation.
> I guess many distros already set it to a higher value for PulseAudio.
> The default 64kB is just from historical and compatibility reason, and
> we may extend it to 1MB or so now.

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));


Thanks,
~Keyon

> 
> 
> Takashi
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