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

On Mon, Oct 04, 2021 at 03:12:20PM +0100, Mark Brown wrote:

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

quoted

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.

quoted

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.  

Well, that is up to the driver implementing this. It looks like device
shutdown is called before the userspace is all nuked so yes,
concurrency with userspace is a possible concern here.

quoted

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) - 

Drivers mis understanding stuff is not new..

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.

The other important case is to get the device cleaned up enough to
pass back to firmware for platforms that use a firmware
shutdown/reboot path.

Jason

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