Hello to anyone,
I'm working on a problem with V4L2 on Linux Kernel 2.6.37.
The problem arise when I try to query a video device to list the
controls it provides.
When is call
ioctl(fd, VIDIOC_QUERYCTRL, &queryctrl)
the function doesn't return 0 and errno is set to EINVAL
This happen for every control, even for the controls that the driver
provides (checked in the code) like brightness.
After adding a lot of printk in the videodev.ko module I found that the
problem is caused by a wrong value of VIDIOC_QUERYCTRL, that in the
kernel module is 0xC0485624 while in userspace application is 0xC0445624.
I digged the kernel source to understand what's happening, and
discovered the definition of VIDIOC_QUERYCTRL:
#define VIDIOC_QUERYCTRL _IOWR('V', 36, struct
v4l2_queryctrl)
dove:
#define _IOWR(type,nr,size) \
_IOC(_IOC_READ|_IOC_WRITE,(type),(nr),(_IOC_TYPECHECK(size)))
with
#define _IOC(dir,type,nr,size) \
(((dir) << _IOC_DIRSHIFT) | \
((type) << _IOC_TYPESHIFT) | \
((nr) << _IOC_NRSHIFT) | \
((size) << _IOC_SIZESHIFT))
and
#define _IOC_NRSHIFT 0
#define _IOC_TYPESHIFT (_IOC_NRSHIFT+_IOC_NRBITS)
#define _IOC_SIZESHIFT (_IOC_TYPESHIFT+_IOC_TYPEBITS)
#define _IOC_DIRSHIFT (_IOC_SIZESHIFT+_IOC_SIZEBITS)
#define _IOC_NRBITS 8
#define _IOC_TYPEBITS 8
#define _IOC_SIZEBITS 14
#define _IOC_NONE 0U
#define _IOC_WRITE 1U
#define _IOC_READ 2U
#define _IOC_TYPECHECK(t) (sizeof(t))
thus, the _IOC definition means:
_IOC(dir,type,nr,size) <=> [dir|size|type|nr]
that, for the aftermentioned values means:
0xC0445624 <=> [3|044|56|24] ==> size = 0x44 = 68
0xC0485624 <=> [3|048|56|24] ==> size = 0x48 = 72
To be sure that the number 68 and 72 are correct, I added a printk() in
the videodev.ko module and a println() in the userspace application to
print "sizeof(struct v4l2_queryctrl)", and the outputs are:
Kernel module:
printk("[DBG] %s() ~ sizeof(struct v4l2_queryctrl): %u\n",
__func__, sizeof(struct v4l2_queryctrl));
=> [DBG] videodev_init() ~ sizeof(struct v4l2_queryctrl): 72
Userspace application:
printf("[dbg] sizeof(struct v4l2_queryctrl): %u\n",
sizeof(struct v4l2_queryctrl));
=> [dbg] sizeof(struct v4l2_queryctrl): 68
In both cases (module and application), there is the inclusion of
<linux/videodev2.h>.
When I compiled the application I istructed the compiler to pick the
same kernel header of the module, with a proper CFLAGS += -I<PATH> in
the Makefile.
Any idea about this strangeness?
Regards
Francesco
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