Question about V4L2_MEMORY_USERPTR

Question about V4L2_MEMORY_USERPTR

[Hans Petter Selasky]

Hi,

I have a question about V4L2_MEMORY_USERPTR:

>From which context are the kernel's "copy_to_user()" functions called in 
relation to V4L2_MEMORY_USERPTR ? Can this be a USB callback function or is it 
only syscalls, like read/write/ioctl that are allowed to call "copy_to_user()" 
?

The reason for asking is that I am maintaining a userland port of the media 
tree's USB drivers for FreeBSD. At the present moment it is not allowed to 
call copy_to_user() or copy_from_user() unless the backtrace shows a syscall, 
so the V4L2_MEMORY_USERPTR feature is simply removed and disabled. I'm 
currently thinking how I can enable this feature.

--HPS

Re: Question about V4L2_MEMORY_USERPTR

[Sakari Ailus]

Hi Hans,

On Fri, Mar 23, 2012 at 08:19:45AM +0100, Hans Petter Selasky wrote:
> Hi,
> 
> I have a question about V4L2_MEMORY_USERPTR:
> 
> From which context are the kernel's "copy_to_user()" functions called in 
> relation to V4L2_MEMORY_USERPTR ? Can this be a USB callback function or is it 
> only syscalls, like read/write/ioctl that are allowed to call "copy_to_user()" 
> ?
> 
> The reason for asking is that I am maintaining a userland port of the media 
> tree's USB drivers for FreeBSD. At the present moment it is not allowed to 
> call copy_to_user() or copy_from_user() unless the backtrace shows a syscall, 
> so the V4L2_MEMORY_USERPTR feature is simply removed and disabled. I'm 
> currently thinking how I can enable this feature.

I hope this is still relevant --- I just read your message the first time.

I don't know how V4L2 is being used in FreeBSD userland, but the intent of
copy_to_user() function is to copy the contents of kernel memory to
somewhere the user space has a mapping to (and the other way around for
copy_from_user()).

Are your video buffers allocated by the kernel or not? How is USB accessed
when you don't have the Linux kernel USB framework around?

Kind regards,

-- 
Sakari Ailus
e-mail: sakari.ailus@iki.fi	jabber/XMPP/Gmail: sailus@retiisi.org.uk

Re: Question about V4L2_MEMORY_USERPTR

[Laurent Pinchart]

On Sunday 01 July 2012 17:00:58 Sakari Ailus wrote:
> On Fri, Mar 23, 2012 at 08:19:45AM +0100, Hans Petter Selasky wrote:
> > Hi,
> > 
> > I have a question about V4L2_MEMORY_USERPTR:
> > 
> > From which context are the kernel's "copy_to_user()" functions called in
> > relation to V4L2_MEMORY_USERPTR ? Can this be a USB callback function or
> > is it only syscalls, like read/write/ioctl that are allowed to call
> > "copy_to_user()" ?
> > 
> > The reason for asking is that I am maintaining a userland port of the
> > media tree's USB drivers for FreeBSD. At the present moment it is not
> > allowed to call copy_to_user() or copy_from_user() unless the backtrace
> > shows a syscall, so the V4L2_MEMORY_USERPTR feature is simply removed and
> > disabled. I'm currently thinking how I can enable this feature.
> 
> I hope this is still relevant --- I just read your message the first time.
> 
> I don't know how V4L2 is being used in FreeBSD userland, but the intent of
> copy_to_user() function is to copy the contents of kernel memory to
> somewhere the user space has a mapping to (and the other way around for
> copy_from_user()).

copy_(to|from)_user(), by definition, require a userspace memory context to 
perform the copy operation. They can't be called from interrupt context, 
kernel threads, or any other context where no userspace memory context is 
present.

> Are your video buffers allocated by the kernel or not? How is USB accessed
> when you don't have the Linux kernel USB framework around?

-- 
Regards,

Laurent Pinchart

Re: Question about V4L2_MEMORY_USERPTR

[Hans Petter Selasky]

Hi Laurent and Sakari,

For the sake of the matter, here is the driver in question:
http://www.freshports.org/multimedia/webcamd/

Under native-Linux (kernel mode):

I've looked at the linux-media code a bit and it appears that video data is 
copied directly from the USB callback functions to the destination process in 
userspace. This works because the userspace buffer is mapped into kernel 
memory it appears. Correct me if I'm wrong:

video/videobuf-core.c:

        err = __videobuf_mmap_setup(q, count, size, V4L2_MEMORY_USERPTR);

Under FreeBSD where the Linux kernel code is running in user-space as a driver 
daemon, this part cannot be done exactly like this, so I've just patched out 
the V4L2_MEMORY_USERPTR feature until further.

Am I clear?

--HPS

On Monday 02 July 2012 11:24:15 Laurent Pinchart wrote:
> On Sunday 01 July 2012 17:00:58 Sakari Ailus wrote:
> > On Fri, Mar 23, 2012 at 08:19:45AM +0100, Hans Petter Selasky wrote:
> > > Hi,
> > > 
> > > I have a question about V4L2_MEMORY_USERPTR:
> > > 
> > > From which context are the kernel's "copy_to_user()" functions called
> > > in relation to V4L2_MEMORY_USERPTR ? Can this be a USB callback
> > > function or is it only syscalls, like read/write/ioctl that are
> > > allowed to call "copy_to_user()" ?
> > > 
> > > The reason for asking is that I am maintaining a userland port of the
> > > media tree's USB drivers for FreeBSD. At the present moment it is not
> > > allowed to call copy_to_user() or copy_from_user() unless the backtrace
> > > shows a syscall, so the V4L2_MEMORY_USERPTR feature is simply removed
> > > and disabled. I'm currently thinking how I can enable this feature.
> > 
> > I hope this is still relevant --- I just read your message the first
> > time.
> > 
> > I don't know how V4L2 is being used in FreeBSD userland, but the intent
> > of copy_to_user() function is to copy the contents of kernel memory to
> > somewhere the user space has a mapping to (and the other way around for
> > copy_from_user()).
> 
> copy_(to|from)_user(), by definition, require a userspace memory context to
> perform the copy operation. They can't be called from interrupt context,
> kernel threads, or any other context where no userspace memory context is
> present.
> 
> > Are your video buffers allocated by the kernel or not? How is USB
> > accessed when you don't have the Linux kernel USB framework around?

Re: Question about V4L2_MEMORY_USERPTR

[Laurent Pinchart]

Hi Hans-Petter,

On Monday 02 July 2012 21:07:56 Hans Petter Selasky wrote:
> Hi Laurent and Sakari,
> 
> For the sake of the matter, here is the driver in question:
> http://www.freshports.org/multimedia/webcamd/
> 
> Under native-Linux (kernel mode):
> 
> I've looked at the linux-media code a bit and it appears that video data is
> copied directly from the USB callback functions to the destination process
> in userspace. This works because the userspace buffer is mapped into kernel
> memory it appears. Correct me if I'm wrong:
> 
> video/videobuf-core.c:
> 
>         err = __videobuf_mmap_setup(q, count, size, V4L2_MEMORY_USERPTR);

It's hard to tell from that line only. V4L2_MEMORY_USERPTR takes a memory 
pointer from userspace and uses it in the kernel. How the memory is used 
depends on the driver, it can be converted to a scatter list of physical 
memory and passed to the hardware, mapped to the kernel, be accessed using 
copy_from_user/copy_to_user, ...

BTW videobuf1 is outdated, drivers will eventually be ported to videobuf2.

> Under FreeBSD where the Linux kernel code is running in user-space as a
> driver daemon, this part cannot be done exactly like this, so I've just
> patched out the V4L2_MEMORY_USERPTR feature until further.
> 
> Am I clear?

-- 
Regards,

Laurent Pinchart