Re: Scatter Gather

[Manuel Jander <manuel.jander@mat.utfsm.cl>]

Hi,

On Mon, 2004-03-01 at 08:52, Clemens Ladisch wrote:
> Manuel Jander wrote:
> > AFAIK, it seems that when using a scatter gather buffer, not every
> > period is located inside of a page. ...
> > The problem is that a DMA transfer can not go accross 2 different pages.
> > On Aureal hardware, even on contiguous buffers this causes all sort of
> > odd problems. Each DMA chunk must be on the same page.
> > ...
> > That means that i would have to split each period that crosses a page
> > boundary into 2 DMA chunks, with all the complications involved because
> > of irregular sizes, offsets... ARghhh!
> 
> Yes.  Every driver using SG must do this.  Look into bt87x.c or
> via82xx.c for examples.

Well, i guess in that case the period size loses any relevance in terms
of efficiency. Is there any gain if DMA chunks and period sizes are
related in any way ?

> > I could just leave the period size as a power of 2, on that way, a
> > period would never cross any page boundary, but most ALSA programs are
> > too picky to accept that kind of restrictions in most cases.
> 
> Devices like usb and ymfpci have a fixed period size, but their
> drivers emulate arbitrary period sizes.

That means that those hwparam restrictions "should not be used" ?? After
realizing that 99% of ALSA apps don't work when using them, renders them
quiet useless.

By the way, is it really really necesary that "snd_pcm_period_elapsed()"
is called exactly when one period has been playback ? Must that callback
be called for every period ?
After looking at the bt87x driver it seems that it doesn't matter after
all (is that right ?). I was thinking in calling
snd_pcm_period_elapsed() just after each page crossing, instead of
period crossing, for simplicity.

Any comments ?

Best Regards

Manuel




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