linux-kernel - Re: [PATCH] i2c: exynos5: Properly use the "noirq" variants of suspend/resume

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Date:	Mon, 23 Jun 2014 15:42:01 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Kevin Hilman <khilman@...aro.org>
Cc:	Wolfram Sang <wsa@...-dreams.de>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	naveen krishna <ch.naveen@...sung.com>,
	Jingoo Han <jg1.han@...sung.com>,
	Jean Delvare <jdelvare@...e.de>, Simon Glass <sjg@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Masanari Iida <standby24x7@...il.com>,
	Sachin Kamat <sachin.kamat@...aro.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i2c: exynos5: Properly use the "noirq" variants of suspend/resume

Kevin,

On Mon, Jun 23, 2014 at 3:23 PM, Kevin Hilman <khilman@...aro.org> wrote:
>> So I guess in this case the truly correct way to handle it is:
>>
>> 1. i2c controller should have Runtime PM even though (as per the code
>> now) there's nothing you can do to it to save power under normal
>> circumstances.  So the runtime "suspend" code would be a no-op.
>>
>> 2. When the i2c controller is told to runtime resume, it should
>> double-check if a full SoC poweroff has happened since the last time
>> it checked.  In this case it should reinit its hardware.
>>
>> 3. If the i2c controller gets a full "resume" callback then it should
>> also reinit the hardware just so it's not sitting in a half-configured
>> state until the first peripheral uses it.
>>
>> If later someone finds a way to power gate the i2c controller when no
>> active transfers are going (and we actually save non-trivial power
>> doing this) then we've got a nice place to put that code.
>>
>> NOTE: Unless we can actually save power by power gating the i2c
>> peripheral when there are no active transfers, we would also just have
>> the i2c_xfer() init the hardware if needed.  Maybe that's kinda gross,
>> though.
>
> Yes, this is how we manage the i2c controller on OMAP.
>
> Essentially, between every xfer, the hw is disabled and can potentially
> lose context, so eveery xfer requires a hw init.  We use the runtime PM
> "autosuspend" feature so that it stays alive for X milliseconds so
> bursty i2c xfers are not punished.
>
> Have a look at drivers/i2c/busses/i2c-omap.c.
>
> You'll notice there are not callbacks for system suspend/resume, it's
> only doing runtime PM.

OK, cool!  That might be a bit too aggressive of a change for what I
can take on right now.  I've filed http://crbug.com/388007 to see if
Samsung can take a look at this.

-Doug
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