linux-kernel - Re: [PATCH] Video/UVC: Port mainlined uvc video driver to NOMMU

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Date:	Tue, 18 Nov 2008 22:39:09 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Laurent Pinchart <laurent.pinchart@...net.be>
Cc:	"Hennerich, Michael" <Michael.Hennerich@...log.com>,
	Bryan Wu <cooloney@...nel.org>,
	linux-uvc-devel@...ts.berlios.de, video4linux-list@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Video/UVC: Port mainlined uvc video driver to NOMMU

On Fri, Nov 14, 2008 at 12:52:10AM +0100, Laurent Pinchart wrote:
> On Monday 10 November 2008, Hennerich, Michael wrote:
> > >On Thursday 06 November 2008, Bryan Wu wrote:
> > > > @@ -1071,7 +1072,20 @@ static int uvc_v4l2_mmap(struct file *file,
> > > > struct vm_area_struct *vma) addr += PAGE_SIZE;
> > > >  		size -= PAGE_SIZE;
> > > >  	}
> > > > +#else
> > > > +	if (i == video->queue.count ||
> > > > +		PAGE_ALIGN(size) != video->queue.buf_size) {
> > >
> > > Just out of curiosity, why do you need to PAGE_ALIGN size for non-MMU
> > > platforms ?
> >
> > Size and video->queue.buf_size is not the 100% same size (off by a few
> > bytes < pagesize), I think it's because on NOMMU the kernel calls
> > kmalloc() to allocate the buffer, not get_free_page().
> 
> That's right, but only for private mappings. It makes little sense to create a 
> private mapping on a V4L2 device as the kernel will read() device data into 
> the buffer when mapping the device (at least on NOMMU platforms). Only shared 
> mappings make sense in this case.
> 
The change itself is crap anyways, since it is just working around the
fact that vm_insert_page() presently -EINVAL's out on nommu. The
vmalloc_to_page()/vm_insert_page() pair is used extensively across the
v4l drivers, and it's unrealistic to expect to patch every call site with
this sort of a workaround.

While we can't support vm_insert_page() in most cases, these sorts of
use cases where it is just iterating over a contiguous block of pages on
a VMA that has already been established is something that can at least be
handled fairly easily (in this case, even just doing nothing in
vm_insert_page() would likely give the desired result, although we ought
to fix up vm_insert_page() to do something more useful instead). Working
out the bounds of the VMA isn't exactly difficult either, since we have
all of that already filled in the VMA by the time vma_insert_page() is
called.

Regarding the VMA flag, this can be rectified by providing a dummy
get_unmapped_area() for the v4l devices which will set some default
capabilities. After that, determine_vm_flags() should take care of
setting up the proper VMA flags without hacking every driver's mmap()
routine. This sort of thing has absolutely no place in the driver,
though.
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