Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA

Edward Estabrook
2008-12-06T00:32:40Z

> Well, UIO already rapes the mmap interface by using the "offset" parameter to
> pass in the number of the mapping.

Exactly.

> But I'll NAK the current concept, too. It's a UIO kernel driver's task to tell
> userspace which memory a device has to offer. The UIO core prevents userspace
> as much as possible from mapping anything different. And it should stay that
> way.

The ultimate purpose (I thought) of the UIO driver is to simplify
driver development
by pushing device control into userspace. There is a very real need
for efficient
dynamic control over the DMA allocation of a device. Why not 'allow' this to
happen in userspace if it can be done safely and without breaking anything else?

Remember that for devices employing ring buffers it is not a question of
'how much memory a device has to offer' but rather 'how much system
memory would the
driver like to configure that device to use'.

I don't want to stop my DMA engine and reload the driver to create
more buffers (and I don't
want to pre-allocate more than I need as contingency).

Cheers,
Ed