Re: Does get_user_pages_fast lock the user pages in memory in my case?

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, 20 Apr 2009 11:28:00 +0800
Huang Shijie <shijie8@gmail.com> wrote:

I will summarize your method. 
Is right ?


kernel(driver)					application 

						posix_memalign(buffer)
						ioctl(buffer)

ioctl handler
get_user_pages(pages);
/* This pages are mapped at user's vma' 
address space */
vaddr = vmap(pages);
/* This pages are mapped at vmalloc space */
.
.
<after sometime, 
It may change to other process context>
.
.
interrupt handler in your driver 
memcpy(vaddr, src, len); 
notify_user();

						processing(buffer);

It's rather awkward use case of get_user_pages. 

If you want to share one big buffer between kernel and user, 
You can vmalloc and remap_pfn_range.
You can refer cpia_mmap in drivers/media/video/cpia.c
 


> Minchan Kim a??e??:
> > On Mon, 20 Apr 2009 10:15:55 +0800
> > Huang Shijie <shijie8@gmail.com> wrote:
> >
> >   
> >> Minchan Kim a??e??:
> >>     
> >>> On Fri, 17 Apr 2009 15:01:01 +0800
> >>> Huang Shijie <shijie8@gmail.com> wrote:
> >>>
> >>>   
> >>>       
> >>>>    I'm writting a driver for a video card with the V4L2 interface .
> >>>>    V4L2 interface supports the USER-POINTER method for the video frame 
> >>>> handling.
> >>>>
> >>>>    VLC player supports the USER-POINTER method,while MPALYER does not.
> >>>>
> >>>>    In the USER-POINTER method, VLC will call the posix_memalign() to 
> >>>> allocate
> >>>> 203 pages in certain PAL mode (that is 720*576*2) for a single frame.
> >>>>    In my driver , I call the get_user_pages_fast() to obtain the pages 
> >>>> array,and then call
> >>>> the vmap() to map the pages to VMALLOC space for the memcpy().The code 
> >>>> shows below:
> >>>>    ....................
> >>>>    get_user_pages_fast();
> >>>>    ...
> >>>>    f->data = vmap();
> >>>>    .......................
> >>>>     
> >>>>         
> >>> What I understand is that you get the pages of posix_memalign by get_user_pages_fast 
> >>> and then that pages are mapped at kernel vmalloc space by vmap. 
> >>>
> >>> Is it for removing copy overhead from kernel to user ?
> >>>
> >>>   
> >>>       
> >> I need a large range of virtual contigous memory to store my video 
> >> frame(about 203 pages). When I received a full frame ,I will queue the 
> >> buffer in
> >> a VIDIOC queue,which will be remove by the VIDIOC_DQBUF.'
> >>     
> >
> > I can't understand your point. 
> > Sorry for that. 
> >
> > Could you explain more detail relation (user buffer which is allocated by posix_memalign) and (kernel buffer which is mapped by vmap) ?
> >
> >   
> :) sorry for my poor english.
> [1] VLC uses the posix_memalign to allocate a big buffer for a single 
> frame(203 pages).
> [2] vmap sets up the mapping of virtual contigous address for gup()'s 
> pages array(the pages are not consecutive).
>    memcpy() needs a contiguous address to copy in kernel mode.
> [3] my driver do some specail operations  to  received data, then 
> memcopy the data to the buffer get in step [2].
> [4] when the buffer is full ,I will give the the user process (VLC).
> 
> That's all.


-- 
Kinds Regards
Minchan Kim

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