17.16. UIO
UIO is a kernel subsystem that allows to do certain types of driver operations from userland.
This would be awesome to improve debuggability and safety of kernel modules.
VFIO looks like a newer and better UIO replacement, but there do not exist any examples of how to use it: https://stackoverflow.com/questions/49309162/interfacing-with-qemu-edu-device-via-userspace-i-o-uio-linux-driver
TODO get something interesting working. I currently don’t understand the behaviour very well.
TODO how to ACK interrupts? How to ensure that every interrupt gets handled separately?
TODO how to write to registers. Currently using /dev/mem and lspci.
This example should handle interrupts from userland and print a message to stdout:
./uio_read.sh
TODO: what is the expected behaviour? I should have documented this when I wrote this stuff, and I’m that lazy right now that I’m in the middle of a refactor :-)
UIO interface in a nutshell:
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blocking read / poll: waits until interrupts
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write: callirqcontrolcallback. Default: 0 or 1 to enable / disable interrupts. -
mmap: access device memory
Sources:
Bibliography:
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https://stackoverflow.com/questions/15286772/userspace-vs-kernel-space-driver
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https://01.org/linuxgraphics/gfx-docs/drm/driver-api/uio-howto.html
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https://stackoverflow.com/questions/7986260/linux-interrupt-handling-in-user-space
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https://yurovsky.github.io/2014/10/10/linux-uio-gpio-interrupt/
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https://github.com/bmartini/zynq-axis/blob/65a3a448fda1f0ea4977adfba899eb487201853d/dev/axis.c
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https://yurovsky.github.io/2014/10/10/linux-uio-gpio-interrupt/
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http://nairobi-embedded.org/uio_example.html that website has QEMU examples for everything as usual. The example has a kernel-side which creates the memory mappings and is used by the user.
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userland driver stability questions: