prealloc_buffer_size and buffer_bytes_max

Peter Rosin peda at
Thu May 21 05:28:25 PDT 2015


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:

		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
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!


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