Re: ALSA lib application compatibility [was] Re: mixer device

[James Courtier-Dutton <James@superbug.demon.co.uk>]

Manuel Jander wrote:
> 
> As the main author of the Aureal Vortex driver, its very stupid having
> to handle arbitrary period sizes, introducing a lot of overhead and
> complexity in the driver, while the hardware just is not designed to
> handle period sizes that are not powers of two, due to page boundary
> overlapping trouble. 

You don't have to handle arbitary period sizes in the alsa-driver.
Here is an example from the emu10k1 driver, that restricts what period 
sizes the application can use for caputure.

static unsigned int capture_period_sizes[31] = {
         384,    448,    512,    640,
         384*2,  448*2,  512*2,  640*2,
         384*4,  448*4,  512*4,  640*4,
         384*8,  448*8,  512*8,  640*8,
         384*16, 448*16, 512*16, 640*16,
         384*32, 448*32, 512*32, 640*32,
         384*64, 448*64, 512*64, 640*64,
         384*128,448*128,512*128
};

static snd_pcm_hw_constraint_list_t hw_constraints_capture_period_sizes = {
         .count = 31,
         .list = capture_period_sizes,
         .mask = 0
};

static int snd_emu10k1_capture_open(snd_pcm_substream_t * substream)
{
<snip>
snd_pcm_hw_constraint_list(runtime, 0, SNDRV_PCM_HW_PARAM_PERIOD_SIZE, 
&hw_constraints_capture_period_sizes);

<snip>

Has a driver writer, you have full control over these sorts of things.

Cheers
James


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