linux-kernel - Re: [PATCH] spi: imx: Revert "spi: imx: enable runtime pm support"

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Date:   Mon, 12 Oct 2020 15:28:21 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Christian Eggers <ceggers@...i.de>
Cc:     Mark Brown <broonie@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        linux-spi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Clark Wang <xiaoning.wang@....com>
Subject: Re: [PATCH] spi: imx: Revert "spi: imx: enable runtime pm support"

On Mon, Oct 12, 2020 at 12:59:34PM +0200, Christian Eggers wrote:
> Hi Sascha,
> 
> On Friday, 9 October 2020, 09:39:44 CEST, Sascha Hauer wrote:
> > On Fri, Oct 09, 2020 at 06:27:38AM +0200, Christian Eggers wrote:
> > > This reverts commit 525c9e5a32bd7951eae3f06d9d077fea51718a6c.
> > >
> > > If CONFIG_PM is disabled, the system completely freezes on probe as
> > > nothing enables the clock of the SPI peripheral.
> >
> > Instead of reverting it, why not just fix it?
> >
> > Normally the device should be brought to active state manually in probe
> > before pm_runtime takes over, then CONFIG_PM disabled doesn't hurt.
> > Using pm_runtime to put the device to active state initially has the
> > problem you describe.
> 
> prior introducing runtime pm for spi-imx, the clock was "manually" enabled and
> disabled around each transfer (so the power usage should already have been
> optimal). If we would manually enable the clock in probe() as you suggested,
> for users without CONFIG_PM there would be a drawback compared with the
> previous state (as the clock will always be on now).
> 
> What is the benefit of controlling the SPI clock with runtime PM instead of
> doing it manually?

The clocks are reconfigured less frequently with pm_runtime. Especially
when enabling/disabling PLLs are involved pm_runtime can increase
performance.

Also you can put other stuff you need to handle for your device into
pm_runtime, think of regulators for example. All this is then abstracted
behind a common kernel API.

Generally when you disable CONFIG_PM then you are not interested in
power consumption and in that case you shouldn't be interested in
disabling your SPI clocks.

Sascha

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