On 11/03/2016 05:45 PM, Lars-Peter Clausen wrote:
On 11/03/2016 10:11 AM, Alan Young wrote:
The code in the kernel, in /pcm_lib.c/ and /pcm_native.c/, uses fields buffer_size and boundary in struct snd_pcm_runtime. As far as I can see, there are always initialized to the same value. In what circumstance might they be different? There is clearly some relationship to hw_ptr_base, as illustrated by this repeated snipped from snd_pcm_update_hw_ptr0()
hw_base += runtime->buffer_size; if (hw_base >= runtime->boundary) { hw_base = 0; crossed_boundary++; } new_hw_ptr = hw_base + pos;boundary is actually set to the largest possible power-of-two multiple of buffer_size. This is done so that the hw_ptr that is reported to userspace is continuously increasing (up to boundary) and does not wrap around after just one buffer_size has elapsed. This is done so you can measure the distance between two hw_ptr that were captured at different points in time, but are more than half a buffer_size away from each other.
And the pointer that is reported to userspace is called appl_ptr, sorry for the confusion.