Re: [PATCH] spi: bcm2835: do not unregister controller in shutdown handler

On Mon, Oct 04, 2021 at 10:17:56AM -0300, Jason Gunthorpe wrote:

Shutdown is supposed to quiet the HW so it is not doing DMAs any
more. This is basically an 'emergency' kind of path, the HW should be
violently stopped if available - ie clearing the bus master bits on
PCI, for instance.

When something like kexec happens we need the machine to be in a state
where random DMA's are not corrupting memory.

That's all well and good but there's no point in implementing something
half baked that's opening up a whole bunch of opportunities to crash the
system if more work comes in after it's half broken the device setup.  

Due to the emergency sort of nature it is not appropriate to do
locking complicated sorts of things like struct device unregistrations
here.

That's just not what's actually implemented in a bunch of places, nor
something one would infer from the documentation ("Called at shut-down
to quiesce the device", no mention of emergency cases which I'd guess
would just be kdump) - there's a bunch of locks in shutdown paths, and
drivers on sleeping buses with shutdown callbacks.  Never mind the few
of them that use a shutdown callback to power the system down, though
that's a different thing and definitely abusing the API.  I would guess
that a good proportion of people implementing it are more worried about
clean system shutdown than they are about kdump.