# Re: Problem trying to address custom hardware

- From: Matt Porter \<mporter@kernel.crashing.org\>
- Date: 2004-08-25 23:05:25 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2004/08/9394814
- Thread: https://ratatoskr.run/linuxppc-dev/2004/08/9394814/t.md

```text

On Wed, Aug 25, 2004 at 03:29:20PM -0700, Oliver King-Smith wrote:
>
> Things are working much better.  Both the driver and the main code work
> without crashing now.
>
> > Others addressed your core problem, but you should know that this
> > mmap request won't produce the desired result. You need MAP_SHARED
> > since MAP_PRIVATE will result in a COW when you attempt to modify
> > your register.
>
> I am not sure I understand why this needs to be MAP_SHARED (it sure does
> though).  Is there a good resource I can read to understand this a bit
> better?

The mmap(2) manpage explains the difference pretty well.  MAP_PRIVATE
indicates that any modification are local to the process. So, as
soon as you write to it, it makes a copy of the page for your process
to modify. MAP_SHARED indicates that any modifications are seen by
any processes that also map the region.

googling will probably yield many threads on lkml on this topic.

-Matt

** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
```