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

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Date:   Mon, 4 Oct 2021 13:51:27 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Lino Sanfilippo <LinoSanfilippo@....de>, 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 09:36:37AM -0700, Florian Fainelli wrote:

> No please don't, I should have arguably justified the reasons why
> better, but the main reason is that one of the platforms on which this
> driver is used has received extensive power management analysis and
> changes, and shutting down every bit of hardware, including something as
> small as a SPI controller, and its clock (and its PLL) helped meet
> stringent power targets.

Huh? for device shutdown? What would this matter if the next step is
reboot or power off?

> TBH, I still wonder why we have .shutdown() and we simply don't use
> .remove() which would reduce the amount of work that people have to do
> validate that the hardware is put in a low power state and would also
> reduce the amount of burden on the various subsystems.

The difference between remove and shutdown really is that 'emergency'
sense that shutdown is something that must complete in bounded time
and thus only has to concern itself with quieting hardware to a safe
state for the next step in the shutdown/reboot/kexec/kdump sequence.

Many remove handlers happily block until, eg all user files are closed
or something to allow a graceful module unload.

Jason