Re: [Xenomai-help] rtdm_iomap_to_user on PPC

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Alexis Berlemont wrote:
> Hi,
> 
> On Friday 17 October 2008 06:46, thomas.debes@domain.hid wrote:
>> No comments or ideas? Providing this feature is essential for our CPCI
>> drivers. We have to port some Linux applications to Xenomai which use that
>> mmap stuff intensely.
>>
>> Thanks again!
>>
>> Thomas
>>
>>> -----Ursprüngliche Nachricht-----
>>> Von: xenomai-help-bounces@domain.hid
>>> [mailto:xenomai-help-bounces@domain.hid] Im Auftrag von
>>> thomas.debes@domain.hid
>>> Gesendet: Mittwoch, 8. Oktober 2008 11:49
>>> An: xenomai@xenomai.org
>>> Betreff: [Xenomai-help] rtdm_iomap_to_user on PPC
>>>
>>> Hello,
>>>
>>> in the following thread I was asking for a solution to use
>>> rtdm_iomap_to_user() on PPC (MPC5200, Denx 2.4.25)
> 
> 
> In 2.4 kernel, rtdm_iomap_to_user() is based on the function remap_page_range 
> (with vma->vm_flags |= VM_RESERVED).
> 
> Once, I had to develop a classical Linux driver (for PPC) which provided mmap 
> functionalites so as to let a user application mmap a buffer allocated thanks 
> to __get_free_pages().
> 
> On a 2.6 kernel, I used remap_pfn_range() and it works great but on 2.4 kernel 
> the function remap_page_range() did not work as I expected. I had a quick 
> look on its implementation and I found that instead of mapping my buffer it 
> mapped newly allocated zeroed pages (if my memories are correct).
> 
> If I remember well, these pages were allocated in lazy mode. That could 
> explain the freeze of your whole system: in case a RT application in primary 
> mode tries to access the mmapped buffer.

Well, normally, the fault should cause the RT application to switch to
secondary mode and be handled gracefully from there, unless there is a
bug hidden in Xenomai or I-pipe. Besides, RT applications usually use
"mlockall", so the kernel should make all the pages present and not rely
on further faults (at least, is it how it works on 2.6).

-- 
					    Gilles.