need for mmap?

Hi list,

I'm a newbie, and I just read about the mmap system call. I have a
very basic confusion regarding why would a driver ever want to
implement mmap.

1) The purpose of mmap is to map the device memory to user space
programs, right? Isn't that alreay available via /dev/mem? The
interested userspace programs can directly mmap (the required offset
in) /dev/mem into their address space, and the task is done? No?

2) Is it that mmapping /dev/mem works only for memory (RAM)? Are all
physical addresses not available via /dev/mem?

Thanks,

Dan
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Re: need for mmap?

On Wed, 2007-03-21 at 09:37 +0530, Daniel Rodrick wrote:
> Hi list,
>
> I'm a newbie, and I just read about the mmap system call. I have a
> very basic confusion regarding why would a driver ever want to
> implement mmap.
>
> 1) The purpose of mmap is to map the device memory to user space
> programs, right? Isn't that alreay available via /dev/mem?

no /dev/mem doesn't allow convenient access to scatter-gather memory etc
while your own mmap can make the memory look linear


> 2) Is it that mmapping /dev/mem works only for memory (RAM)? Are all
> physical addresses not available via /dev/mem?

/dev/mem is a HORRIBLE interface, and it won't work in the future or
even today on some machines.
The reason for this is that this interface doesn't tell the kernel what
device you want to talk to, so there is no way the kernel can set up the
IOMMU correctly to allow access to the memory. Today only high end
machines have an IOMMU, but I suspect that within a few years all PCs
will have one as well....

--
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