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

[Keyon Jie]

On 2020/1/17 下午3:57, Takashi Iwai wrote:
> On Fri, 17 Jan 2020 06:30:18 +0100,
> Keyon Jie wrote:
>>
>> On 2020/1/17 上午4:37, Takashi Iwai wrote:
>>
>> 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?
> 
> No, in general, all sound hardware buffers are pinned.

Sorry, I must have been wrong here, what I was focusing on is those 
allocated SG DMA buffers, I am not sure if they are those you called 
"hardware buffers" here.

My understanding was like this:

1. in pcm_new() stage, the device PCM driver should call 
snd_pcm_lib_preallocate_pages()->
	snd_pcm_lib_preallocate_pages()->
		preallocate_pcm_pages()
and then the substream->dma_buffer is initialized with the preallocated 
buffer.

2. in pcm_open() stage, the device PCM driver should call
snd_pcm_lib_malloc_pages()->
	snd_dma_alloc_pages() //if we need to reallocate bigger buffer. *The 
substream->dma_buffer won't be freed, Takashi, this is what I thought 
you named "pinned" buffer.* And those reallocated bigger buffer via 
snd_dma_alloc_pages() will be freed at pcm_close() per my understanding?

Thanks,
~Keyon

> 
> 
> Takashi
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