Re: dsp mmap change

On Fri, May 06, 2011 at 04:16:40PM -0400, John Baldwin wrote:
> On Friday, May 06, 2011 10:04:28 am Kostik Belousov wrote:
> > On Fri, May 06, 2011 at 04:38:00PM +0300, Andriy Gapon wrote:
> > > on 06/05/2011 16:32 Kostik Belousov said the following:
> > > > On Fri, May 06, 2011 at 11:55:00AM +0300, Andriy Gapon wrote:
> > > >>
> > > >> I would like to ask for a review and/or testing of the following patch:
> > > >> http://people.freebsd.org/~avg/dev_dsp_mmap.diff
> > > >>
> > > >> It's supposed to fix an issue described here:
> > > >> http://lists.freebsd.org/pipermail/freebsd-multimedia/2011-
> February/011691.html
> > > >>
> > > >> In short, the following pseudo-code should do the right thing:
> > > >> fd = open(/dev/dsp, O_RDWR);
> > > >> mmap(PROT_READ, fd);
> > > >> mmap(PROT_WRITE, fd);
> > > >>
> > > >> Thank you!
> > > > 
> > > > I think that you have to call PCM_GIANT_LEAVE() when returning
> > > > EINVAL on the vm_pager_alloc() failure.
> > > 
> > > Yes, thank you.
> > > 
> > > > Your patch hardcodes an assumption that sndbufs are always
> > > > contiguous. I was unable to convince myself that this is true.
> > > 
> > > I think that this should be true for the case when DMA is used?
> > In the current driver, yes, but there is nothing that theoretically
> > prevents scatter-gather from be used.
> 
> You could "fix" this by creating an sglist (via sglist_build()) and an
> OBJT_SG VM object that the d_mmap_single callback returned.  I wish there
> was a cleaner way to just create a VM object and populate it with pages
> though, and then use vm_map_insert() to map it into the kernel rather
> than the more roundabout method of OBJT_SG.

You cannot have one page inserted into two vm objects. Contigmalloc()
inserts the allocated pages into kernel_object.