prealloc_buffer_size and buffer_bytes_max
Peter Rosin
peda at lysator.liu.se
Thu May 21 05:35:32 PDT 2015
On 2015-05-21 14:28, Peter Rosin wrote:
> Hi!
>
> I got my hopes up when I read the commit message for c14e2591bf54
> ASoC: atmel-pcm-dma: increase buffer_bytes_max. Quoting it:
>
> atmel-pcm-dma is not limited to a buffer size of 64kB like
> atmel-pcm-pdc. Increase buffer_bytes_max to 512kB to allow
> for higher bit rates (i.e. 32bps at 192kHz) to work correctly.
> By default, keep the prealloc at 64kB.
>
> However, as I (think I) request a bigger buffer it still caps out at 64kB.
> I'm using the latency argument of snd_pcm_set_params to control the buffer:
>
> snd_pcm_set_params(pcm,
> SND_PCM_FORMAT_S32_LE,
> SND_PCM_ACCESS_RW_INTERLEAVED,
> 2, /* channels */
> 250000, /* rate */
> 0, /* do not resample */
> 90000); /* latency in us */
>
> But snd_pcm_hw_params_get_buffer_size "only" returns 8192 frames (64kB)
> even if I request 9us * 250kHz * 2 * 4 = 180kB. If I change the prealloc
Bzzzt, s/9us/90ms/
> from 64kB to 256kB I get a bigger buffer (and it works better too!).
>
> Admittedly I backported this patch on top of the linux-3.18-at91
> branch from the Atmel git repo, so there might be some support
> missing that has gone in after 3.18?
>
> Or have I completely misunderstood, and these are unrelated buffers?
>
> Any insight in how I can get a big enough buffer without hacking the
> prealloc is appreciated!
>
> Cheers,
> Peter
>
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