At Wed, 27 Sep 2006 09:58:53 -0400,
Lee Revell wrote:
>
> On Tue, 2006-09-19 at 17:15 +0200, Takashi Iwai wrote:
> > The workq task is a pseudo-DMA engine that runs in background.
> > When it's woken up, it feeds data from intermediate buffer to hardware
> > as much as possible. The available data can be found in
> >
> > If all data are fed, it sleeps again (or exit
> > the task). The timer wakes up or schedule the new workq task.
>
> I think something is missing - the available data can be found where?
We track the following two pointers (assume the playback direction):
- hw_ptr
points the current transferring position in the ring buffer
- appl_ptr
points the current position the data is filled
The available data size is calculated as (appl_ptr - hw_ptr).
The appl_ptr is the data size given by the application, i.e. it's
incremented at each time the app writes data chunk. So, the interest
for the driver is basically only hw_ptr.
The only access point that the driver provides hw_ptr is the pointer
callback. It returns a value between 0 and buffer_size-1, and the PCM
core layer expands to a linear position in 0 - pcm->boundary_size.
In case the hardware doesn't provide the exact DMA position... it's a
bit hard. If the hardware generates an irq at each period boundary,
the driver simply updates the hw_ptr at each interrupt (before calling
snd_pcm_period_elapsed()) such as
hw_ptr += frames_per_period;
hw_ptr %= buffer_size;
then pass this value in pointer callback.
If the hardware doesn't generate irq, we have to rely on the system
timer just like your code. In that case, the driver must keep
tracking the pointer by itself, such as
frames_per_period = rate / HZ;
frac_per_period = rate % HZ;
hw_ptr += frames_per_period;
frac += frac_per_period;
if (frac >= HZ) {
hw_ptr++;
frac -= HZ;
}
hw_ptr %= buffer_size;
Takashi
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