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

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Message-ID: <YVs0Vk9TJqHt8R4K@sirena.org.uk>
Date:   Mon, 4 Oct 2021 18:05:26 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        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 01:51:27PM -0300, 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?

On some embedded systems, especially ultra low cost ones, the system
power off state might not actually involve removing all the physical
power supplies for the all the chips in the system so any residual
leakages or active functions will continue to consume power.

Ideally the system power on/off will be triggered by a PMIC which is
able to physically remove power to most other parts of the system which
avoids this issue (much like the PSU in a server) but that's not always
the case.

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