linux-kernel - Re: [PATCH] i2c: exynos5: Initialise Samsung High Speed I2C controller early

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Message-ID: <CAHfPSqDSTyQXPT-NCZ2hE01qpdmaR54tbWCUYC_zSDGFp58+7g@mail.gmail.com>
Date:	Fri, 9 May 2014 20:12:47 +0530
From:	Naveen Krishna Ch <naveenkrishna.ch@...il.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	Naveen Krishna Chatradhi <ch.naveen@...sung.com>,
	linux-i2c@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>, sjg@...omium.org,
	grundler@...omium.org, linux-kernel@...r.kernel.org,
	wsa@...-dreams.de, cpgs@...sung.com
Subject: Re: [PATCH] i2c: exynos5: Initialise Samsung High Speed I2C
 controller early

Hello Mark,

On 9 May 2014 19:21, Mark Brown <broonie@...nel.org> wrote:
> On Fri, May 09, 2014 at 05:50:00PM +0530, Naveen Krishna Ch wrote:
>> On 24 April 2014 21:55, Mark Brown <broonie@...nel.org> wrote:
>
>> >> Such solution has been proposed by Mark Brown to fix the problem of
>> >> the regulators not beeing available on the peripheral device probe():
>> >> http://lists.infradead.org/pipermail/linux-arm-kernel/2010-March/011971.html
>
>> > What specifically is this needed for?  We *should* be able to use
>> > deferred probe for most things, but I know that not all subsystems are
>> > able to yet.
>
>> DRM related drivers like DP, FIMD, HDMI, Mixer wants to be probed ASAP
>> during the boot.
>> The real problem comes when, one of these drivers do a regulator_get().
>
>> If the physical supply  is not enabled/hookedup the regulator_get() call
>> assumes that physical supply is present and returns a
>> "dummy_regulator" (But, not an error).
>
>> Because of which, Display and several other devices fails to work.
>
> These drivers are buggy, if they geniunely expect and handle a missing
> supply then they should be using regulator_get_optional() to request the
> regulator and even if they don't the use of subsys_initcall() is not
> going to fix anything here - if a dummy regulator is going to be
> returned the time things are probed won't make a difference.
We have a situation where.a PMIC is sitting under I2C_TUNNEL
with I2C, SPI and SPI uses DMA.

PMIC --- I2C_TUNNEL ---- I2C/SPI ---- DMA

This PMIC is setting some FETS required for Backlight, LCD and G3D modules.

If all the I2C, SPI, DMA, I2C_TUNNEL, DRM based LCD are all mod_probes()
DRM drivers are probing a head of the PMIC probe. Which is causing the
display drivers to get "dummy_regulators" instead of the real supplies.

We couldn't implement -PROBE_DEFER because, the probes are not failing.

Should we should check if the return value of regulator_get() is a
"dummy_regulator"
and then defer the probe /.

>
>> I2C, I2C_TUNNEL, SPI and DMA drivers are required as subsys_initcall()
>> for similar reason.
>
> What makes you say this?  Typically those drivers don't use regulators
> at all.
I mean I2C, I2C_TUNNEL, SPI and DMA drivers have be to subsys_initcall()
to get the PMIC up intime. so, that the system would use the real supplies.

This looks like a board specific solution. I guess, this is quite
frequent with the
buses like I2C, SPI.

Kindly, suggest a nicer way. Thanks



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