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

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Message-ID: <20211004171301.GA3544071@ziepe.ca>
Date:   Mon, 4 Oct 2021 14:13:01 -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 09:55:32AM -0700, Florian Fainelli wrote:
> On 10/4/21 9:51 AM, Jason Gunthorpe wrote:
> > 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?
> 
> Power off, the device is put into a low power state (equivalent to ACPI
> S5) and then a remote control key press, or a GPIO could wake-up the
> device again. While it is in that mode, it consumes less than 0.5W(AC).
> Imagine your stick/cast/broom behind your TV falling in that category.

So really this is more of a very deep sleep that cannot be recovered
from than what other platforms would call a shutdown - eg the
powerdomain of the device under driver control will not loose
power.

I'm kind of surprised a scheme like this didn't involve a FW call
after Linux is done with the CPUs to quiet all the HW and let it
sleep, I've built things that way before at least.

> I am fairly sure that no driver write knows about the being bound in
> time aspect.

Well, it is a logical consequence. The system is shutting down, no
driver should be designed to deadlock the shutdown forever.

I suppose this is why I've occasionally seen Linux just hang at a
black screen and no power off when told to shutdown :)

Jason