Re: difference between OSS mmap and alsa mmap? [alsa-oss]

[Florian Schmidt <mista.tapas@gmx.net>]

On Mon, 05 Jul 2004 15:19:57 +0200
Takashi Iwai <tiwai@suse.de> wrote:

> At Mon, 05 Jul 2004 09:13:46 -0400,
> Paul Davis wrote:
> > 
> > >the last thing i can see not working correctly in alsa-oss is the
> > >mmap emulati on [may closed source apps use OSS with mmap emu].
> > >jcdutton told me on irc tha t this is due to the differences in OSS
> > >mmaping and ALSA mmaping and it might not be possible to resolve
> > >these issues..
> > 
> > i don't think its possible. the OSS mmap API provides raw ptr access
> > to the DMA area of the h/w. you cannot intercede when an app just
> > uses pointer indirection and advance to read/write data. ALSA's API
> > does the same, of course, but access is bounded by two calls that
> > allow correct handling for devices where the DMA area is a bit more
> > complex than the OSS model allows for (e.g. 2 devices, or a device
> > that can't support mmap in hardware, or a dmix device, or ...)
> 
> A hack would be to call mmap_avail() in oss's GETSPACE (or similar)
> ioctls.  But, right, I don't think that the "perfect" emulation is
> possible on all hardwares as it is now, too.

btw: i might prove my ignorance with this question, but i have to ask:

what makes the kernel oss emu handle mmap so good? it's an emulation,
too, i suppose.. so what is the big difference in these two emulations,
that in one case it works well [kernel oss emu] and in the other it
seems almost impossible [libaoss.so]..

flo


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