Re: DMA feature question.

[Takashi Iwai <tiwai@suse.de>]

At Mon, 28 Jun 2004 18:32:58 +0100,
James Courtier-Dutton wrote:
> 
> Takashi Iwai wrote:
> >>If not, I will just have to make (1) a bit bigger, and place the "table 
> >>of periods" in there.
> > 
> > 
> > I don't think it's a good idea.
> > The buffer will be mmapped to the user-space, and obviously you don't
> > want to expose it.
> 
> The terms "Massive security hole!" comes to mind!
> 
> > 
> > I'd suggest to allocate a table (one page should be large enough) via
> > snd_dma_alloc_pages() statically at the driver initialization, and
> > sets the max. number of periods according to the page size.
> > 
> > 
> > Takashi
> > 
> > 
> 
> Is a page 1024 bytes or 4096 bytes?

4k on i386.

>  Does it vary depending on which type 
> of CPU is in use?

Yes, but 4k is minimal, so far.

>  Is there a sensible minimum size to set it.
> 
> I only need to allow for a max of 16 periods per buffer. (does anyone 
> need >16 periods per buffer ? )
> There are 4 channels, with 8 bytes per period, that gives 16*4*8 = 512 
> bytes.

Then a single page should suffice.
(of course you can add a code to check the page size to be sure.)


Takashi


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