Re: How to avoid data copies in a driver ?

Jeremy Fitzhardinge
2008-05-16T08:10:49Z

Francis Moreau wrote:
> I'm suprised because what I need doens't seem so uncommon, usually
> devices send or
> receive data to/from files. So a helper (system call ?) to achieve
> that other than the basic
> read/write seems needed, no ?
>

It's fairly rare to have an application which requires moving data to
file with absolutely no processing; normally there's at least a bit of
massaging/parsing/etc. If that's really what you want to do, maybe you
can do it with splice? I haven't looked at it at all, but the intention
is that you can splice file descriptors together, so you can splice your
device fd to a file fd and have it all just work...

Alternatively you could read() from your device into a mmaped file.
That's a single copy from device to file, which is about the best you
can do without going to heroic lengths.

Also, it really depends on your application. Is it a high-bandwidth
thing in which the copy is a huge cost? Or do you want to eliminate the
copies because it seems like a nice thing to do?

J