On Wed, 24 Jul 2013, Takashi Iwai wrote:
> > I don't understand. Consider a simple playback example. Suppose the
> > user wants to keep the latency low, so he requests 2 periods per
> > buffer. snd-usb-audio ignores this value and decides to use 10 URBs,
> > which is equivalent to setting the buffer size to 10 periods.
>
> You don't have to fill all 10 URBs to start the stream...
Maybe you don't have to, but it looks the driver does just that. From
snd_usb_endpoint_start():
if (snd_usb_endpoint_implicit_feedback_sink(ep)) {
for (i = 0; i < ep->nurbs; i++) {
struct snd_urb_ctx *ctx = ep->urb + i;
list_add_tail(&ctx->ready_list, &ep->ready_playback_urbs);
}
return 0;
}
for (i = 0; i < ep->nurbs; i++) {
struct urb *urb = ep->urb[i].urb;
if (snd_BUG_ON(!urb))
goto __error;
if (usb_pipeout(ep->pipe)) {
prepare_outbound_urb(ep, urb->context);
} else {
prepare_inbound_urb(ep, urb->context);
}
err = usb_submit_urb(urb, GFP_ATOMIC);
if (err < 0) {
snd_printk(KERN_ERR "cannot submit urb %d, error %d: %s\n",
i, err, usb_error_string(err));
goto __error;
}
set_bit(i, &ep->active_mask);
}
In this case, I'm particularly considering what happens when
snd_usb_endpoint_implicit_feedback_sink(ep) is false.
Besides, what's the reason for allocating 10 URBs if you're not going
to submit more than 2 of them at a time?
Alan Stern
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