Re: can device drivers return non-ram via vm_ops->nopage?

[William Lee Irwin III <wli@holomorphy.com>]

On Mon, Mar 22, 2004 at 05:18:30PM -0500, Jeff Garzik wrote:
>> This is burned into silicon, so supporting it's not an option. Frankly
>> I think what's best is another device interface for userspace to fall
>> back to when this coherent userspace mmap() is unimplementable, e.g.
>> read()/write() on some device node.

On Tue, Mar 23, 2004 at 10:57:19AM +1100, Benjamin Herrenschmidt wrote:
> Exactly. We can implement the simple/nice interface discussed here, and
> just not support it on those platforms, they'll have to fall back to
> read/write or simply not support those drivers who require that
> functionality.
> Eventually a nopage variant may be worth for things doing really
> large mappings, but I tend to think that when we need to do that mapping
> to userland, it is because we need short latencies, which is the opposite
> of what a nopage implementation provides, dunno if it's worth the pain
> (though it's not _that_ painful).

More generality in fault handling would be useful in various ways even
beyond fixing ALSA's issues. I'm not sure why Linus doesn't like the
notion. I didn't insist on API but just moved on to trying to push for
a solution to the driver issues to get merged at all so I can get on
with cleaning up drivers using whatever API people want for the solution.

I've already been over every ->nopage() in the kernel once (wrt. what's
been merged anyway; a number of times for other reasons), so I really
think I can do a bit of useful footwork here.


-- wli