Re: [PATCH update6] drivers: add LCD support

[David Miller <davem@davemloft.net>]

From: Andrew Morton <akpm@osdl.org>
Date: Thu, 2 Nov 2006 12:04:12 -0800

> On Thu, 2 Nov 2006 19:33:48 +0000
> "Miguel Ojeda" <maxextreme@gmail.com> wrote:
> 
> > May 2.6.18-new vmalloc
> > related functions help correlating userspace & kernel addresses? I
> > will try them and come with an answer tomorrow.
> > 
> > Quoting http://lwn.net/Articles/2.6-kernel-api/
> > 
> > "Some functions have been added to make it easy for kernel code to
> > allocate a buffer with vmalloc() and map it into user space. They are:
> > 
> >      void *vmalloc_user(unsigned long size);
> >      void *vmalloc_32_user(unsigned long size);
> >      int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
> >                              unsigned long pgoff);
> > 
> > The first two functions are a form of vmalloc() which obtain memory
> > intended to be mapped into user space; among other things, they zero
> > the entire range to avoid leaking data. vmalloc_32_user() allocates
> > low memory only. A call to remap_vmalloc_range() will complete the
> > job; it will refuse, however, to remap memory which has not been
> > allocated with one of the two functions above."
> 
> No, it doesn't look like those helper functions are designed to handle this.
> 
> I'm really not the person to be asking about this.  I can poke around in
> arch/sparc64/kernel/sys_sparc.c:arch_get_unmapped_area() as well as the
> next guy, and it seems to be doing the right thing for MAP_FIXED, but
> how/whether it handles !MAP_FIXED I do not know.  Ask davem ;)

Unfortunately that code never gets called for MAP_FIXED :-)

I'll comment on these issues and explain what needs to occur,
we have several things that want to do this kind of user/kernel
sharing of ring buffers and similar, so best to get the
infrastructure going to get it right.

As a first approximation, getting remap_vmalloc_range() to do the
right thing is the best way to start this stuff off.