How to use sendfile with a char device driver's file descriptor

[Vinesh Christopher]

I have a device driver which has to sent a data through network. Currently
the driver's buffer is mmaped to the user mode program which uses the
standard socket sent() call.

To improve the performance and reduce CPU overhead, I tried using sendfile()
Call. But it failed with EINVAL. 

Browsing thru the mm/filemap.c, I found the driver has to provide a readpage
functions in address_space_operations. So I added it. 

Now the sendfile() does not fail, but it returns with 0 bytes sent. Also my
function readpage is never called.

How can I use sendfile() with my driver? Is there any documentation
available on how to add address_space_operations to a driver, so that
sendfile can be used.

Any help will be appreciated

Samvinesh Christopher.