On Tue, 18 Feb 2003, Abramo Bagnara wrote:
> Jaroslav Kysela wrote:
> >
> > On Mon, 17 Feb 2003, Jaroslaw Sobierski wrote:
> >
> > > >> I see, the read/saturate/write must be atomic, too. In this case, it would
> > > >> be better to use a global (or a set of) mutex(es) to lock the hardware
> > > >> ring buffer. The futexes are nice.
> > > >
> > > >They are nice indeed, but definitely not the right solution here.
> > > >
> > > >Although I don't know if it's the absolute best solution, the 'retry'
> > > >approach I've proposed is far better and much more efficient.
> > >
> > > I have to agree with Abramo. A global mutex would cause long and unnecessary
> > > waits for the processes trying to write to the plugin. Locking access to
> > > individual parts of the buffer is messy. Notice that concurrent writes
> > > to the same sample in the buffer will occur sporadically, and the "re-read"
> > > in the loop costs almost nothing, while synchronization mechanisms could
> > > block often.
> >
> > Note that your all nice ideas go to some blind alley. Who will silence the
> > sum buffer? Driver silences only hardware buffer which will not be used
> > for the calculation in your algorithm.
>
>
> Not so blind ;-)
>
> v = *src;
> if (cmpxchg(hw, 0, 1) == 0)
> v -= *sw;
> xadd(sw, v);
> do {
> v = *sw;
> if (v > 0x7fff)
> s = 0x7fff;
> else if (v < -0x8000)
> s = -0x8000;
> else
> s = v;
> *hw = s;
> } while (unlikely(v != *sw));
>
> I've convinced you?
>
> However as I've written in the my first message the evil of dmix
> approach lies in details: they might destroy efficiency of approach
> rather easily.
I've implemented the whole transfer and mix loop in assembly and it works
without any drastic impact on CPU usage. I tried to optimize the assembler
part as much as I can, but if some assembler guru want to give a glance,
I'll appreciate it. The function is named mix_areas1() in
alsa-lib/src/pcm/pcm_dmix.c.
Jaroslav
-----
Jaroslav Kysela <[EMAIL PROTECTED]>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs
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