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SubjectRe: [RFC] libusb / in-kernel usb driver criteria
> > > So, to get the ball rolling, here are some factors that IMHO
> > > help decide in which side to implement a driver:
> > >
> > > - if the driver ties a hardware device to an existing
> > > in-kernel interface (network, block, serial, bluetooth,
> > > video4linux, etc.), it should probably be implemented
> > > in-kernel.
> >
> > Agreed, I think this is clear.
>
> Yes, this the primary decision point, everything after this depends on
> lots of variables :)

Including a pragmatic concern: performance requirements.

Today's usbfs-based userspace drivers don't get any zerocopy
benefits. So if you're passing around enough data that your
target environment needs a zerocopy I/O model (maybe it's got
to run on an embedded system with a small battery and not much
spare CPU power), that can argue in favor of a kernel driver.

I don't know whether the "usbfs2" work addresses that issue.

- Dave


> Agreed. It all depends on the situation, we have kernel drivers for
> devices that can be done in userspace, but not as cleanly or nicely, and
> so, they stay as kernel drivers.
>
> In the end, it comes down to individual cases, so let's handle them at
> that level, it's easier that way.


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