From: Manuel Jander <manuel.jander@mat.utfsm.cl>
To: Alsa Devel list <alsa-devel@lists.sourceforge.net>
Subject: Scatter Gather
Date: Fri, 27 Feb 2004 14:17:43 -0400 [thread overview]
Message-ID: <1077905854.6881.28.camel@localhost> (raw)
Hi,
AFAIK, it seems that when using a scatter gather buffer, not every
period is located inside of a page. If for example i use a period size
of 3000 bytes and a Scatter Gather buffer, i would have this:
pages : page0, page1, page2 ...
periods: p0, p1, p2, p3, p4 ...
|...page0...||...page1...||...page3...|
|...p0...||...p1...||...p2...||...p3...|
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.
So i would have to do this:
d0, d1, d2, d3 : DMA chunks programmed in hardware. There are 4 for each
DMA channel.
|...page0...||...page1...||...page3...|
|...p0...||..|.p1...||...p2...||...p3...|
|...d0...||d1||.d2..||d3||.d0.||..d1..|
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!
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.
----------------------------------------------
Regarding the "Writing a ALSA driver" Docbook:
Regarding the Non contiguous buffers, that the sgbuf struct can be
derived as:
snd_pcm_sgbuf_t *sgbuf = (snd_pcm_sgbuf_t*)substream->dma_private;
But there is a problem: The substream->dma_private is only valid after
snd_pcm_lib_malloc_pages() has been called. Maybe it would be nice to
hint about this small detail in the document.
Best Regards
Manuel
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next reply other threads:[~2004-02-27 18:17 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-02-27 18:17 Manuel Jander [this message]
2004-03-01 12:52 ` Scatter Gather Clemens Ladisch
2004-03-02 4:09 ` Manuel Jander
2004-03-02 8:00 ` Jaroslav Kysela
2004-03-02 10:16 ` Giuliano Pochini
2004-03-02 10:23 ` Takashi Iwai
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