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

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Date:   Mon, 4 Oct 2021 10:17:56 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Mark Brown <broonie@...nel.org>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, f.fainelli@...il.com,
        rjui@...adcom.com, sbranden@...adcom.com,
        bcm-kernel-feedback-list@...adcom.com, nsaenz@...nel.org,
        linux-spi@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        p.rosenberger@...bus.com, linux-integrity@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] spi: bcm2835: do not unregister controller in shutdown
 handler

On Mon, Oct 04, 2021 at 01:49:21PM +0100, Mark Brown wrote:

> This still leaves a potential race where something (eg, an interrupt
> handler) could come in and try to schedule more SPI transfers on the
> shut down hardware.  I'm really not sure we can do something that's
> totally robust here without also ensuring that all the client drivers
> also have effective shutdown implementations (which seems ambitious) or
> doing what we have now and unregistering the clients.  I am, however,
> wondering if we really need the shutdown callback at all - the commit
> adding it just describes what it's doing, it doesn't explain why it's
> particularly needed.  I guess there might be an issue on reboot with
> reset not completely resetting the hardware?

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.

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

Jason