Re: [Xenomai-core] [PATCH] provide rtdm_mmap_to_user / rtdm_munmap

[Rodrigo Rosenfeld Rosas <lbocseg@domain.hid>]

Jan Kiszka escreveu:
> Rodrigo Rosenfeld Rosas wrote:
>   
>> Hi Jan, it just happened once and I couldn't reproduce (I didn't want to 
>> reproduce it too since I would need to restart my computer because the driver 
>> wouldn't unload)...
>>
>> When it happened I forgot to start the timer running the latency program and 
>>     
>
> Already running latest SVN version?
Almost there ;) That is why your patch didn't apply cleany, but I just 
needed to include two "#include" and "#define" lines to drvlib.c or 
something like...

>  rt_timer_start&friends became
> deprecated last weekend, so you can't forget this step anymore.
>   
I didn't use rt_timer_start at all. I was doing as you suggested, 
calling another program to start the timer, like "latency".

>> my driver failed to load and due to some mistake I've made (I have not 
>> indentified it yet), it crashed on rmmoding. I need to check this, but I 
>> still think it is a good idea to make the sanity checks...
>>     
>
> We need some XENO_ASSERT that is only active when CONFIG_XENO_OPT_DEBUG
> is set. I don't want to put such checks in production code, but I see
> that they may help debugging early drivers.
>   
I understand your concernings but I really don't think they are 
relevant... This checks will be much faster then the procedure itself 
and it would conform to normal munmap behaviour. From man page:

"The address start must be a multiple of the page size. All pages 
containing a part of the indicated range are unmapped, and subsequent 
references to these pages will generate SIGSEGV. It is not an error if 
the indicated range does not contain any mapped pages."

I think that if there was an extra parameter for user_info, it would 
also verify for validity. BTW, I think there is missing some 
documentation about the user_info parameter. I had to remember our 
conversation and look at the code to understand that I should record 
"current" on this parameter on the moment I called mmap and passing it 
again on munmap. And it would be good to see the rtdm_user_info_t 
defined as struct task_struct on the documentation.

>> I have not written the user-space program yet, so you'll have to wait until 
>> monday, when I'll be able to test it, hopefully. But it seems to be 
>> working... I changed my driver design. I do the mmap's on driver 
>> initialization and just pass the returned addresses on the IOCTL, so I can do 
>> them in a RT-context. The problem is that even if the user call an IOCTL to 
>>     
>
> Hmm, I guess there is still some lacking documentation about what is
> possible with RTDM. If you call an IOCTL from RT context, you end up in
> the _rt-handler the driver of a device may have registered (if there is
> no _rt-handler at all, the _nrt one is invoked, but this is likely not
> relevant here).
>
> I assume that you were wondering how to call rtdm_mmap_to_user from this
> real-time handler, right?
No. I know it is not possible from the moment. I think I did not explain 
myself very well. I was wondering how to define a RT mmap like ioctl. As 
I know I could not use rtdm_mmap_to_user then, I thought in another way 
of doing it. So I mmaped on driver initialization. On the IOCTL I just 
passed the already known addresses to the user requesting it. I would 
have to explain you how these buffers work on V4L2. It is a bit long 
explanation but I can explain it on other message if you wish.

> Well, the trick is to return -ENOSYS for those
> IOCTL codes that can only be handled by the _nrt-handler. Xenomai will
> then switch your RT task to secondary mode, restart the IOCTL, and the
> mmap can safely be executed.
>   
But as I've said, it is not the behaviour I want :)

> Well, maybe you do not have any arguments for rtdm_mmap_to_user that
> should be influenced by the user's IOCTL.
That is my case.

> In this case your current driver design is ok as well. I just wanted to underline that it is not necessarily the only way.
>   
But I couldn't find other way of doing it in a RT-context.

>> munmap, it will still be possible to him to continue using the provided 
>> address and this would result in a problem. But, as in all situations, there 
>>     
>
> When rtdm_munmap is executed, the virtual address range becomes invalid
> for the user. Thus any further access to it will raise a segfault.
> That's the only problem, but it will not influence the driver integrity.
>   
Yes, that is the problem. Since I only mark as unused on the munmap 
IOCTL, it would be possible to the user to continue using that address 
even after the munmap IOCTL call. It I was using a really rtdm_munmap, 
it wouldn't be possible. It would be a better behaviour, but it would 
not be run on RT-context. That is the trade-off.

>> are trade-offs and I prefer to rely on the user, while providing a 
>> RT-MMAP-IOCTL. Of course it isn't really a mmap, but if the user don't mess 
>> with the pointers, it will work like if it was...
>>     
>
> The user can only access the window you mapped in and only as long as it
> is mapped.
In my case, it is always mapped to make possible the RT-IOCTLs.
> And if you map it read-only, there is even no chance to
> destroy potential management structures of the hardware or the driver
> within this range.
>   
I do not want to make it read-only because it will probably be very 
usefull to the user to write on it. The user may want to capture a frame 
and do some image processing routines on the same memory area when it is 
possible, avoiding to copy that memory region.
>> Hope you understood me, I wrote it a little confusing... :)
>>     
>
> We will see...
>   
:)
> Happy WE,
>   
Happy weekend for you too,

Rodrigo.


		
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