Re: Question about V4L2_MEMORY_USERPTR
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 BTW videobuf1 is outdated, drivers will eventually be ported to videobuf2.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 -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Question about V4L2_MEMORY_USERPTR
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 -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Question about V4L2_MEMORY_USERPTR
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? -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Question about V4L2_MEMORY_USERPTR
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.ai...@iki.fi jabber/XMPP/Gmail: sai...@retiisi.org.uk -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
