UIO: per-handle event counts

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Hi,

I've been using UIO for a userspace driver and noticed a feature when
multiple uio file handles have been opened for the same device (e.g. when
running separate processes).

Since each handle stores its own count of the interrupts and uses this to
determine whether uio_read returns, it's possible for uio_read to return
before a new interrupt has occurred.

Was this the intended behaviour? In my case, the irq is associated with
completion of some hw processing so it seems a little strange to me. I
expected uio_read to return only after an interrupt occurs that hasn't been
already been handled by userspace code.

Thanks
Phil
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