Re: Mapping kernel memory into user space

Ian Reid (
Thu, 18 Jul 1996 16:14:36 +0100

Linus Torvalds wrote:
> On Tue, 16 Jul 1996 wrote writing for
> Hubert Schreier:
> >
> > I am writing a device driver for a PCI video capture board that writes image
> > data directly into the memory of the computer. As it is impossible to obtain
> > large contiguos memory areas in kernel space, I use blocks of 4k and program
> > the framegrabber's processor to write to them. To obtain fast acces to the
> > image data, I want to use memory mapped acces. However, I cannot figure out
> > how to map these 4k blocks to a contiguos area in user space memory. For
> > allocating the blocks I use
> >
> > Q2: Is there a way to use shared memory, so I could use XShmPutImage()
> > to write to a window?
> No. If you want that, then you shouldn't mmap() the device, instead you
> should just create a shared memory area in your application and do a
> "read()" on the device. The device could look up the physical addresses
> Linus

I just posted a very similar question before I noticed this thread (sorry). I want
to write directly to an XShmImage but I don't know how to "look up the physical
address" of the pages. How is this done??

With reference to the earlier poster's questions, Matt Welsh wrote a simple kernel
patch called "bigphysarea" which reserves a contiguous large block of memory at boot
time for just such uses as yours. I use it to provide a nice big buffer for frame
on the Matrox Meteor. The patch is distributed with the meteor driver at in /pub/linux/system/Meteor (currentl driver version is 1.4).

Cheers, Ian