Re: period crossing page boundary

[Takashi Iwai <tiwai@suse.de>]

At Thu, 20 Mar 2003 15:24:20 +0100 (CET),
Jaroslav wrote:
> 
> On Thu, 20 Mar 2003, Giuliano Pochini wrote:
> 
> > 
> > >> Ok, but how do I build the sg list when bufsize!=sz*periods ? Take
> > >> the example above: have I to build 4 periods and let the last one
> > >> smaller than 22052 ?
> > >
> > > it depends on the hardware.
> > 
> > There are no hw constraints IFAIK, but the hw does not provide the
> > current dma address. It tells me how many bytes it transferred from
> > the trigger-start command, so I need a way to translate that number
> > into a frame pointer inside the buffer. To do that I must know
> > exactly how long is the buffer to build the sg list and to stranslate
> > the byte count in the pointer callback. (it's not enough see below)
> > What does alsa expect from .pointer() ? [0-bufsz] or [0-pe*sz] ?
> 
> 0 ... buffer_size
 
more accurately, 0 to (buffer_size - 1) :)

> > > if you need to set up a buffer-descriptor list like via82xx, the above
> > > h/w constraint (integer periods) will be most likely necessary.
> > > then the buffer size will be aligned to the period size
> > > automatically.
> > 
> > It's a quick and simple way to make things work, but it should't
> > be necessary.
> > The driver currently behaves correctly in two cases:
> > 
> > a - period size is a multiple of PAGE_SIZE
> > b - PAGE_SIZE is a multiple of period size
> >
> > In both cases periods*periodsize matches buffer_size. But if
> > periods*periodsize==buffer_size and !a and !b (eg. pe=4, psz=6K,
> > bufsz=24K), it doesn't work:
> > 
> > page0 per0[0-4095]
> > page1 per0[4096-6143] per1[0-2048]
> > page2 per1[2048-6143]
> > ...
> 
> Note that you'll have to call snd_pcm_period_elapsed() only after the 
> whole period is elapsed and not fragment of it.

and just make sure: if the buffer is linear, does this period size
work?


Takashi


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