[alsa-devel] [PATCH] pcm_lib.c: Fixed inaccurate calculation of hw_ptr_interrupt in snd_pcm_update_hw_ptr_interrupt function

[Takashi Iwai]

At Wed, 19 Aug 2009 18:22:19 +0800,
Shine Liu wrote:
> 
> There's no restriction on that the runtime->period_size should be the
> multiplicator of the runtime->buffer_size in current kernel code.
> 
> So "hw_ptr_interrupt = runtime->hw_ptr_interrupt +
> runtime->period_size;" is not always the trueth.

The hw_ptr_interrupt is the expected position where the current irq is
issued.  So, the current code should be correct -- it's simply
increased in the size of period_size.

> I've encounter the problem of this bug, the kernel told me "PCM:
> Unexpected hw_pointer value (stream=0, pos=8, intr_ptr=16384)".
> 
> The period_size and buffer_size I used was 0x800 and 0x3e80 when the bug
> occured.
> 
> Following patch will fix the bug.

Does the problem happen with the latest driver code (as found in
sound git tree or alsa-driver-snapshot)?  If yes, we'd need more
further analysis, e.g. by enabling some xrun_debug flags.


thanks,

Takashi


> Signed-off-by: Shine Liu <shinel at foxmail.com>
> -----------------------------------------------------------------------
> 
> --- a/sound/core/pcm_lib.c	2009-08-14 06:43:34.000000000 +0800
> +++ b/sound/core/pcm_lib.c	2009-08-19 17:59:00.000000000 +0800
> @@ -248,6 +248,12 @@
>  	hw_base = runtime->hw_ptr_base;
>  	new_hw_ptr = hw_base + pos;
>  	hw_ptr_interrupt = runtime->hw_ptr_interrupt + runtime->period_size;
> +	/* if the period_size is not the multiplicator of the 
> + 	 * buffer_size, the hw_ptr_interrupt calculated above 
> + 	 * may not be correct, should be fixed. 
> + 	 */
> +	if(hw_ptr_interrupt > hw_base + runtime->buffer_size)
> +		hw_ptr_interrupt = hw_base + runtime->buffer_size;
>  	delta = new_hw_ptr - hw_ptr_interrupt;
>  	if (hw_ptr_interrupt >= runtime->boundary) {
>  		hw_ptr_interrupt -= runtime->boundary;
> 
> 
>