Raspberry Pi acces to /dev/mem using PREEMPT_RT
01 Jan 2017 13:18 #85004
by DaBit
Raspberry Pi acces to /dev/mem using PREEMPT_RT was created by DaBit
I am currently building a 3D printer from the ground up.
To control it, I intend to use a Raspberry Pi 3 with either LinuxCNC or MachineKit. Why? Because I can.
So far I am using LinuxCNC; it is a comfortable environment and I am not yet sure what benefits MachineKit would offer over LinuxCNC. Once things are working I will try it, but until then I will stick to LinuxCNC.
Now, to drive stepper motors I attached ST Microelectronics dSpin/L6470 chips. These are real beauties; they communicate with SPI, they incorporate a trapezoidal motion planner, and the 128-ustep voltage-mode driving they use makes the motor REALLY silent. There is also a bigger brother called powerSTEP01 which is capable of 80V/10Amp and either voltage-mode driving or current-mode, so I am seeing potential in those chips even beyond a humble 3D printer.
Now, I wrote a module that communicates with the chips using bit-banged SPI. The Raspberry can toggle it's I/O faster than the L6470's can accept anyway, those chips need a pulse on the chip select every byte, and I like to keep the hardware SPI free for other purposes.
To access the GPIO I used a part of the WiringPi library, which needs access to /dev/mem to be able to map the SoC registers. To gain that access, I took a piece of code from hm2_rpspi.c
The code that does that is:
Full code can be found here: github.com/dabit20
(but it is still very much in an 'under construction / barely working' state)
Now, I started to develop this on a few months old Rasbian install with kernel from www.frank-durr.de/?p=203.
This worked fine.
Recently I decided I would do a fresh Raspbian install on a faster and larger SD card. The 'old' image is a bit polluted due to tinkering with many things and the kernel mentioned above does not support the onboard WiFi on the Pi3. So I took a fresh Raspbian install, the raspbian kernel sources, applied RT patches from kernel.org and configured the kernel for realtime.
4.4.39-rt49 kernel works fine, latency is very acceptable for a servo-only system:
But no access to /dev/mem from within a LinuxCNC module
I tried a lot of things, and at the moment I ended up with:
- The original 8GB card with 4.4.9-rt17 kernel from frank-durr.de, LinuxCNC 2.7.7 compiled for uspace.
- A new 32GB card with fresh Raspbian Jessie, the very same 4.4.9-rt17 kernel and the very same LinuxCNC 2.7.7 as on the 8GB card (I copied the linuxcnc-dev tree from the 8GB card).
On both cards linuxcnc-dev/bin/rtapi_app has the permissions changed by sudo make setuid:
In the end it seems to be a uid/permission issue.
On the 8GB SD card:
EUID=0, UID=1000 (which is user 'pi'). so I can see it working.
On the new 32GB card:
Both EUID and UID are 1000, and opening /dev/mem fails (as it should).
I am pretty sure this is more a Linux problem than a LinuxCNC problem, but I have no idea where to search for a solution.
To control it, I intend to use a Raspberry Pi 3 with either LinuxCNC or MachineKit. Why? Because I can.
So far I am using LinuxCNC; it is a comfortable environment and I am not yet sure what benefits MachineKit would offer over LinuxCNC. Once things are working I will try it, but until then I will stick to LinuxCNC.
Now, to drive stepper motors I attached ST Microelectronics dSpin/L6470 chips. These are real beauties; they communicate with SPI, they incorporate a trapezoidal motion planner, and the 128-ustep voltage-mode driving they use makes the motor REALLY silent. There is also a bigger brother called powerSTEP01 which is capable of 80V/10Amp and either voltage-mode driving or current-mode, so I am seeing potential in those chips even beyond a humble 3D printer.
Now, I wrote a module that communicates with the chips using bit-banged SPI. The Raspberry can toggle it's I/O faster than the L6470's can accept anyway, those chips need a pulse on the chip select every byte, and I like to keep the hardware SPI free for other purposes.
To access the GPIO I used a part of the WiringPi library, which needs access to /dev/mem to be able to map the SoC registers. To gain that access, I took a piece of code from hm2_rpspi.c
The code that does that is:
/* Taken from hm2_rpspi.c */
int rtapi_open_as_root(const char *filename, int mode) {
fprintf (stderr, "euid: %d uid: %d\n", geteuid(), getuid());
setfsuid(geteuid());
int r = open(filename, mode);
setfsuid(getuid());
return r;
}
int gpioInitialise(void)
{
int fd;
gpioHardwareRevision(); /* sets piModel, needed for peripherals address */
fd = rtapi_open_as_root("/dev/mem", O_RDWR | O_SYNC) ;
if (fd < 0)
{
fprintf(stderr,
"Connot open /dev/mem. This program needs root privileges. Try using sudo\n");
return -1;
}
gpioReg = initMapMem(fd, GPIO_BASE, GPIO_LEN);
systReg = initMapMem(fd, SYST_BASE, SYST_LEN);
bscsReg = initMapMem(fd, BSCS_BASE, BSCS_LEN);
close(fd);
// and some more..Full code can be found here: github.com/dabit20
(but it is still very much in an 'under construction / barely working' state)
Now, I started to develop this on a few months old Rasbian install with kernel from www.frank-durr.de/?p=203.
This worked fine.
Recently I decided I would do a fresh Raspbian install on a faster and larger SD card. The 'old' image is a bit polluted due to tinkering with many things and the kernel mentioned above does not support the onboard WiFi on the Pi3. So I took a fresh Raspbian install, the raspbian kernel sources, applied RT patches from kernel.org and configured the kernel for realtime.
4.4.39-rt49 kernel works fine, latency is very acceptable for a servo-only system:
But no access to /dev/mem from within a LinuxCNC module
I tried a lot of things, and at the moment I ended up with:
- The original 8GB card with 4.4.9-rt17 kernel from frank-durr.de, LinuxCNC 2.7.7 compiled for uspace.
- A new 32GB card with fresh Raspbian Jessie, the very same 4.4.9-rt17 kernel and the very same LinuxCNC 2.7.7 as on the 8GB card (I copied the linuxcnc-dev tree from the 8GB card).
On both cards linuxcnc-dev/bin/rtapi_app has the permissions changed by sudo make setuid:
pi@raspberrypi:~/linuxcnc-dev/bin $ ls -al rtapi*
-rwsr-x--- 1 root pi 524360 Oct 20 11:27 rtapi_appIn the end it seems to be a uid/permission issue.
On the 8GB SD card:
pi@raspberrypi:~/rpi_cnc/linuxcnc_modules $ uname -a
Linux raspberrypi 4.4.9-rt17-v7+ #1 SMP PREEMPT RT Wed May 11 22:46:14 CEST 2016 armv7l GNU/Linux
pi@raspberrypi:~/rpi_cnc/linuxcnc_modules $ halrun -I dspin_test.hal
Note: Using POSIX realtime
loading dspin module
euid: 0 uid: 1000
joint 0 config register: 2e88
halcmd: quit
dSpin module unloaded
Note: Using POSIX realtime
pi@raspberrypi:~/rpi_cnc/linuxcnc_modules $EUID=0, UID=1000 (which is user 'pi'). so I can see it working.
On the new 32GB card:
pi@raspberrypi:~/rpi_cnc/linuxcnc_modules $ uname -a
Linux raspberrypi 4.4.9-rt17-v7+ #1 SMP PREEMPT RT Wed May 11 22:46:14 CEST 2016 armv7l GNU/Linux
pi@raspberrypi:~/rpi_cnc/linuxcnc_modules $ halrun -I dspin_test.hal
Note: Using POSIX realtime
loading dspin module
euid: 1000 uid 1000
This program needs root privileges. Try using sudo
GPIO initialise went wrong
rtapi_app: caught signal 11 - dumping core
dspin_test.hal:2: waitpid failed /home/pi/linuxcnc-dev/bin/rtapi_app dspin
dspin_test.hal:2: /home/pi/linuxcnc-dev/bin/rtapi_app exited without becoming ready
dspin_test.hal:2: insmod for dspin failed, returned -1
halcmd: quit
Waited 3 seconds for master. giving up.
Note: Using POSIX realtime
pi@raspberrypi:~/rpi_cnc/linuxcnc_modules $Both EUID and UID are 1000, and opening /dev/mem fails (as it should).
I am pretty sure this is more a Linux problem than a LinuxCNC problem, but I have no idea where to search for a solution.
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