Re: [Xenomai-help] user mode vs. kernel module

[Philippe Gerum <rpm@xenomai.org>]

Philippe Gerum wrote:
> Ignacio García Pérez wrote:
> 
>> Hi,
>>
>> So far I've had my application implemented entirely as a kernel module 
>> with some user mode helper programs. I've had in mind changing to user 
>> mode but never felt compelled enough.
>>
>> Now, having a new requirement to implement, I think it's the moment to 
>> give it a try, and I'm porting it. I have a couple of questions:
>>
>> 1- How significative, in real world terms, is the overhead involved in 
>> operaing in user mode?. As far as I know all operations are executed 
>> via "pods" which involve one or more system calls and context 
>> switches. I'm using a Geode board right now but there are plans to 
>> move to a more low-end platform (486).
>>
>> 2- ioperm. This is a true headache. I've spent countless hours 
>> debugging this and banging my head agaist the wall. I'm not 100% sure 
>> of what's going on here, but it looks like the ioperm call must be 
>> issued from primary mode if you are doing the I/O from primary mode. 
>> Something like this would segfault:
>>
>> ioperm(0x443, 0x01, 1);
>> rt_mutex_lock(...)
>> outb(0x01, 0x443)
>>
>> While this works fine:
>>
>> rt_mutex_lock(...)
>> ioperm(0x443, 0x01, 1);
>> outb(0x01, 0x443);
>>
>> While leads me to think that the rt_mutex_lock puts the task in 
>> primary mode and then both ioperm and outb are executed in the same 
>> mode so everything works.
>>
>> Is this right?
> 
> 
> No.
> 
> . If so, I find this extremely inconvenient...
> 
>>
> 
> Fixed in the repository. Use iopl(3) as a work-around, or resync.
> 

Btw, the fix concerns the active i/o bitmap ownership in the TSS for kernel 
versions >= 2.6.15, but the segfault you encounter is likely an application error: 
ioperm() cannot be used for ports above 0x3ff. You must resort to using iopl() to 
access port address 0x443.

>> Nacho.
>>
>>
>>
>>
>>
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>>
> 
> 


-- 

Philippe.