Scatter Gather

Scatter Gather

[Manuel Jander]

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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Re: Scatter Gather

[Clemens Ladisch]

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.

> 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.


Regards,
Clemens




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Re: Scatter Gather

[Manuel Jander]

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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Re: Scatter Gather

[Jaroslav Kysela]

On Tue, 2 Mar 2004, Manuel Jander wrote:

> 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.

It will create extra latency, because application will be woken up later
than expected. The driver should try to call this ack immediately when the
period transfer is finished.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs


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Re: Scatter Gather

[Giuliano Pochini]

On 02-Mar-2004 Manuel Jander wrote:

>> 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.

Efficiency about what ?  If the hardware can be programmed to use
any period size, then the application is free to choose the size
it prefers. It's not a bad thing.  NB: the interrupt should be
generated at the end of the period, not at the end of each block,
eg. when it crosses the page boundary.


> Is there any gain if DMA chunks and period sizes are
> related in any way ?

No, unless the hardware benefits from it.


--
Giuliano.


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Re: Scatter Gather

[Takashi Iwai]

At Tue, 02 Mar 2004 11:16:33 +0100 (CET),
Giuliano Pochini wrote:
> 
> 
> On 02-Mar-2004 Manuel Jander wrote:
> 
> >> 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.
> 
> Efficiency about what ?  If the hardware can be programmed to use
> any period size, then the application is free to choose the size
> it prefers. It's not a bad thing.  NB: the interrupt should be
> generated at the end of the period, not at the end of each block,
> eg. when it crosses the page boundary.

well, it's ok to generate irqs at each page boundary, too, as long as
the driver processes it properly, i.e. calling
snd_pcm_period_elapsed() only at the irq of the end of period.
of course, less irqs are better from the performance viewpoint.


Takashi


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scatter gather

[Round Robinjp]

Hi,

Is scatter gather supported on ARM linux?
I am trying to find out how to use it, but can find
only old info. Can I find uptodate info anywhere
which can be used with newer kernels?

Thanks
RR

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scatter gather

[Linus Walleij]

2010/8/18 Round Robinjp <roundrobinjp@yahoo.co.jp>:

> Is scatter gather supported on ARM linux?

What do you mean by this? There is plenty of generic kernel
code using <linux/scatterlist.h>, and the implementation
is in lib/scatterlist.c, it's available on all archs AFAICT.

Yours,
Linus Walleij