[alsa-devel] hw_ptr_interrupt removal broke interrupt pointer updates

Jaroslav Kysela perex at perex.cz
Wed Jan 27 18:21:10 CET 2010

On Wed, 27 Jan 2010, Clemens Ladisch wrote:

> Jaroslav Kysela wrote:
>> On Wed, 27 Jan 2010, Clemens Ladisch wrote:
>>> A somewhat unrelated issue:  Both old and new code assume that
>>> hw_ptr==0 is a period boundary, but that is not true if the boundary
>>> is not an integer multiple of the period size, and the pointer wraps.
>>> I'm not sure what happens then.
>> I'm not exactly sure what you're talking about. Where is the
>> hw_ptr==0 assumption?
> This code, which tries to align hw_ptr_interrupt to a period boundary:
> 	runtime->hw_ptr_interrupt = new_hw_ptr -
> 			(new_hw_ptr % runtime->period_size);

I see. It is really problem, because if hw_ptr_interrupt shifts, then the 
 		delta = runtime->hw_ptr_interrupt + runtime->period_size;
                 if (delta > new_hw_ptr) {

is not accurate and might cause unwanted issues.

The simple fix for 64-bit archs is to use "boundary = buffer_size * 
period_size" expression to setup the boundary variable properly.

I added code to find the lowest common multiple for 32-bit archs.

The patch is:



Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

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