On Fri, 17 Jan 2020 11:13:31 +0100, Keyon Jie wrote: > > > > 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? What I meant as "pinned" is that the pages are not swapped out by swapper process like the user-space or anonymous pages. So if you open all streams (say 16 streams) on a machine with 32MB buffers, it'll cost a half GB. And, we have no restriction about which user may do it, so all normal users who have the access to the sound device can consume a half GB kernel space pages easily. For a big server it's no problem, but for a small system, it's costing. Takashi _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx https://mailman.alsa-project.org/mailman/listinfo/alsa-devel
