Re: [PATCH] test/cxl-xor-region.sh: remove redundant waitting

[<dan.j.williams@intel.com>]

ruansy.fnst wrote:
> From: Ruan Shiyang <ruansy.fnst@fujitsu.com>
> 
> Now that cxl_wait_probe() has been added[1] to wait for udev queue
> empty, the `udevadm settle` here is no longer necessary.
> 
> [1] b231603 cxl/lib: Add cxl_wait_probe()
> 
> Signed-off-by: Ruan Shiyang <ruansy.fnst@fujitsu.com>
> 
> ===
> Question to Dan:
> 
> I understand how cxl_wait_probe() work, but I have some questions about
> the motivation of adding this function:  Firstly, is it function added
> for simply waiting for new added CXL device been ready before cxl
> command does the actual work?  Just for replacing `udevadm settle`'s
> work?

No the motivation is that a command like "cxl list" wants to be to
determine that a given object is "enabled", i.e. driver attached. At any
given point of the time the cxl_pci driver could be going through async
attach, and the arrival of the cxl_acpi driver may trigger
cxl_bus_rescan().

So the assumption is that interfactive invocation of cxl commands should
arrange for a stablized snaphshot of the topology state.

> Now I am facing a problem that cxl command takes a long time to complete
> when I run it in a udev rule(do some configuration when CXL memdev is
> added).  I found it is caused by this function: waitting for udev
> queue's endding but itself is in the queue.  The cxl_wait_probe()
> function does not seem to allow me to do that.

Yes, that is a problem.

> So, the 2nd question is: is it against the spec to run cxl command in
> a udev rule?

No we need to find a way to make that work, so I consider this a bug
report. My initial reaction is that when called from a udev rule cxl-cli
should honor a new environment variable, perhaps "UDEV_CXL", that
disables this waiting that is only meant for human interactive mode.

The udev rule would need to know the potential asynchronous states of
the topology.  However, as long as it is performing object local
operations it should be fine. For example "UDEV_CXL=1; cxl list -m
$memdev", where $memdev just experienced the "add" uevent, should just
dump that object's details without waiting for the rest of the topology
to settle.