From mboxrd@z Thu Jan  1 00:00:00 1970
From: Takashi Iwai <tiwai@suse.de>
Subject: Re: difference between OSS mmap and alsa mmap? [alsa-oss]
Date: Mon, 05 Jul 2004 15:19:57 +0200
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To: Paul Davis <paul@linuxaudiosystems.com>
Cc: Florian Schmidt <mista.tapas@gmx.net>, alsa-devel@lists.sourceforge.net
List-Id: alsa-devel@alsa-project.org

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.


Takashi


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