Re: [Xenomai-help] Problem accesing I2C without using Linux kernel

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

On 06/19/2011 03:16 PM, Andrey Nechypurenko wrote:
> Hi,
> 
> I am working on real-time control application using BeagleBoard
> and Xenomai. Some sensors are connected to the second i2c bus. It
> is necessary to collect sensor data strictly periodically and
> that is why I would prefer to query the sensors from xenomai
> thread.
> 
> I can read sensor data from user space using kernel interface,
> i.e. via /dev/i2c-2 file. However, as I understand, doing it from
> the xenomai thread will lead to the context switch and degrade
> the real-time performance (introduce unpredictable delays and all
> other badnesses :-) ). That is why I decide to try to access
> OMAP's i2c controller directly without using kernel driver and
> /dev file system.
> 
> For this purposes, I simply poke the relevant parts from u-boot
> sources and thought that it should work. Unfortunately it is
> not. I am getting the "Bus error" message which is coming not
> from my test program and then the program quits.
> 
> I am suspecting that there is some kind of conflict with
> kernel. I am not experienced enough with kernel internals and
> that is why the questions I would like to ask are: a) whether my
> assumption about possible conflict with kernel is reasonable, b)
> would it help to recompile the kernel with i2c support completely
> disabled and c) if such kernel will work at all on BB? Please
> note, that for this application I do not need any power
> management and no display output.
> 
> Just for the sake of completeness, and if somebody is interested
> in more details, here is the brief outline of what I did and how
> it fails. I was doing the following steps:
> 
> 1. Open /dev/mem and memory map the I2C 2 region at
> 0x48072000 (as defined in the TRM which is in meanwhile become
> my most popular reading :-) ):
> 
> fd = open("/dev/mem", O_RDWR | O_SYNC);
> i2c_base = mmap(NULL, sizeof(i2c_base),
>                PROT_READ | PROT_WRITE,
>                MAP_SHARED, fd, I2C_DEFAULT_BASE);

What is the value of i2c_base here? If it is MAP_FAILED, then mmap
failed. And one reason for failing would be that sizeof(i2c_base) is not
a multiple of the page size.

-- 
                                                                Gilles.