[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 18:59:31 +0800,
Shine Liu wrote:
> 
> On Wed, 2009-08-19 at 12:34 +0200, Takashi Iwai wrote:
> > 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.
> 
> Yes, the period_size and buffer_size I used was 0x800 and 0x3e80.
> 
> The log from the kernel is :
> 
> period_update: pcmC0D0p:0: pos=0x809/0x800/0x3e80, hwptr=0x1c, hw_base=0x0, hw_intr=0x0
> period_update: pcmC0D0p:0: pos=0x1008/0x800/0x3e80, hwptr=0x809, hw_base=0x0, hw_intr=0x800
> period_update: pcmC0D0p:0: pos=0x1808/0x800/0x3e80, hwptr=0x10e3, hw_base=0x0, hw_intr=0x1000
> period_update: pcmC0D0p:0: pos=0x2006/0x800/0x3e80, hwptr=0x1808, hw_base=0x0, hw_intr=0x1800
> period_update: pcmC0D0p:0: pos=0x2808/0x800/0x3e80, hwptr=0x203f, hw_base=0x0, hw_intr=0x2000
> period_update: pcmC0D0p:0: pos=0x3008/0x800/0x3e80, hwptr=0x2808, hw_base=0x0, hw_intr=0x2800
> period_update: pcmC0D0p:0: pos=0x3808/0x800/0x3e80, hwptr=0x30e9, hw_base=0x0, hw_intr=0x3000
> period_update: pcmC0D0p:0: pos=0x8/0x800/0x3e80, hwptr=0x3808, hw_base=0x0, hw_intr=0x3800
> PCM: Unexpected hw_pointer value (stream=0, pos=8, intr_ptr=16384)
> 
> It's true that irq was issued when hw_ptr_interrupt is
> 0x800,0x1000,0x1800,...,0x3800,0x3e80.
> 
> But the irq last issued should be at 0x3e80, but not 0x3800 + 0x800 =
> 0x4000. The DMA engine loaded 0x680 frame not 0x800 frame at the last
> time.

Then it's a driver bug.  If unaligned period size is allowed, it means
that the irq is really generated in that period, not at the buffer
boundary.  Otherwise, it must have a proper hw-constraint to align the
period size to the buffer size.

What hardware is it?


thanks,

Takashi