[alsa-devel] [PATCH 1/3] add API to allow disabling period interrupts

Jassi Brar jassisinghbrar at gmail.com
Tue May 18 11:11:07 CEST 2010

On Mon, May 17, 2010 at 11:42 PM, Jaroslav Kysela <perex at perex.cz> wrote:
> On Mon, 17 May 2010, Jassi Brar wrote:
>> On Mon, May 17, 2010 at 8:16 PM, Clemens Ladisch <clemens at ladisch.de>
>> wrote:
>>> Jassi Brar wrote:
>>>> I am in favor of support variable hw_interrupt at lowest level, i.e, in
>>>> ring buffer driver, instead of disabling hw_interrupt altogether.
>>> Removing the constant-sized periods restriction would certainly be
>>> useful.  However, it doesn't look as if anybody has the time to redesign
>>> the ALSA API, the kernel framework and all the drivers.
>> No need to redesign ALSA API and certainly no need to change _any_ driver,
>> just like this interrupt disable call is optional so would period resize
>> be.
>> I only ask to make this newly added call as period-resize rather than
>> a special case of period-disable.
> This is very good point. But I have two comments:
> 1) Period-disable function is OK, but it should not have a name "no
>   better. This change does imply to set the period size automatically in
>   the driver - probably to highest value (applications cannot choose/set
>   the period size in this operation mode - it's useless anyway).
> 2) The avail_min parameter in sw_params was overlooked. The lowlevel
>   drivers can use this value to compute the wake-up point and set hw
>   appropriately, to do wake-up at requested time. We can add a support
>   functions like "return how many samples are expected to be transferred
>   for next wake-up point" to linux/sound/pcm.h. In case when this value
>   is high, no interrupts (wake ups) will be processed in the driver. If
>   hardware cannot do the precise transfers, we can program a system
>   timer as the wake-up source.
Sounds good, though I had a different implementation in mind...
A new _optionally_ supported call to set the ratio of h/w period
to ring buffer.
The fields are returned with actually set ratio by the low
level driver upon call return. The boundary case of say 0/0
can be interpreted as intr disable and n/n as period := ring
The call can be used to query current ratio by asking for
a/b where a > b i.e, invalid ratio.
Depending upon the capability of the h/w and it's driver,
fine-tuning can be achieved to max possible extent.

something like....

snd_pcm_uframes_t prd, ring;

/* Get current ratio */
prd = 257, ring = 256; /* Invalid ratio */
snd_pcm_set_ratio(&prd, &ring);

/* Increase the period by desirable amount */
prd += incr;
snd_pcm_set_ratio(&prd, &ring);

/* Disable period interrupts */
prd = 0, ring = 0;
snd_pcm_set_ratio(&prd, &ring);
if (!prd && !ring) {
   Interrupts are successfull disabled;
} else {
    prd/ring ratio is max supported by h/w

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