Re: MDIO Debug Interface

On Fri Jul 10, 2020 at 3:18 AM CEST, Vladimir Oltean wrote:

I will let the PHY library maintainers comment about implementation
design choices made by mdio-netlink. However, I want to add a big "+1"
from my side for identifying the correct issues in the existing PHY
ioctls and doing something about it. I think the mainline kernel needs
this.
Please be aware that, if your mdio-netlink module, or something
equivalent to it, lands in mainline, QEMU/KVM is going to be one of its
users (for virtualizing an MDIO bus). So this is going to be more than
just for debugging.
And, while we're at it: context switches from a VM to a host are
expensive. And the PHY library polls around 5 MDIO registers per PHY
every second. It would be nice if your mdio-netlink module had some sort
of concept of "poll offload": just do the polling in the kernel side and
notify the user space only of a change.

The current flow is:

1. User: Send program to kernel.
2. Kernel: Verify program.
3. Kernel: Lock bus.
4. Kernel: Execute program.
5. Kernel: Unlock bus.
6. User: Read back status, including the output buffer.

(3, 5) is what allows for doing complex operations in a race free
manner. (4) is capped with a timeout to make sure that userspace can't
monopolize the bus. A "poll offload" would have to yield (i.e. unlock)
the bus in between poll cycles. Certainly doable, but it complicates
the model a bit.

Thanks,
-Vladimir