Re: Question about V4L2_MEMORY_USERPTR

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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