# Re: Memory sharing issue by application on V4L2 based device

- From: Michel Dänzer \<hidden\>
- Date: 2011-01-07 09:04:25 
- List: linux-fbdev
- Page: https://ratatoskr.run/linux-fbdev/2011/01/8337906
- Thread: https://ratatoskr.run/linux-media/2011/01/8338067/t.md

```text
On Fre, 2011-01-07 at 11:17 +0900, InKi Dae wrote: 
> thank you for your comments.
> 
> your second comment has no any problem as I said before, user virtual
> addess could be translated in page unit. but the problem, as you said,
> is that when cpu access to the memory in user mode, the memory
> allocated by malloc, page fault occurs so we can't find pfn to user
> virtual address. I missed that. but I think we could resolve this one.
> 
> as before, user application allocates memory through malloc function
> and then send it to device driver(using userptr feature). if the pfn
> is null when device driver translated user virtual address in page
> unit then it allocates phsical memory in page unit using some
> interface such as alloc_page() and then mapping them. when pfn is
> null, to check it and allocate physical memory in page unit could be
> processed by videobuf2.
> 
> of course, videobuf2 has no any duty considered for system mmu. so
> videobuf2 just provides callback for 3rd party and any platform with
> system mmu such as Samsung SoC C210 implements the function(allocating
> physical memory and mapping it) and registers it to callback of
> videobuf2. by doing so, I think your first comment could be cleared.

FWIW, TTM (drivers/gpu/drm/ttm, include/drm/ttm) is designed and used
for managing memory between CPU/GPU and kernel/userspace access. I
haven't looked at your requirements in detail, but if you haven't looked
at TTM yet, it sounds like it might be worth a look.


-- 
Earthling Michel Dänzer           |                http://www.vmware.com
Libre software enthusiast         |          Debian, X and DRI developer
```