linux-kernel - Re: [PATCH] i2c: i2c-tegra: Move clk_prepare/clk_set

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Message-ID: <20140815213442.GJ1626@tbergstrom-lnx.Nvidia.com>
Date:	Sat, 16 Aug 2014 00:34:42 +0300
From:	Peter De Schrijver <pdeschrijver@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Mikko Perttunen <mperttunen@...dia.com>,
	Laxman Dewangan <ldewangan@...dia.com>,
	"wsa@...-dreams.de" <wsa@...-dreams.de>,
	"thierry.reding@...il.com" <thierry.reding@...il.com>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i2c: i2c-tegra: Move clk_prepare/clk_set_rate to probe

On Fri, Aug 15, 2014 at 09:45:46PM +0200, Peter De Schrijver wrote:
> On Fri, Aug 15, 2014 at 08:07:01PM +0200, Stephen Warren wrote:
> > >> However, the new code sets the clock rate after the clock is prepared. I
> > >> think the rate should be set first, then the clock prepared. While this
> > >> likely doesn't apply to the Tegra clock controller, prepare() is allowed
> > >> to enable the clock if enable() can't be implemented in an atomic
> > >> fashion (in which case enable/disable would be no-ops), and we should
> > >> make sure that the driver correctly configures the clock before
> > >> potentially enabling it.
> > >>
> > >> I'm not sure if a similar change to our SPI drivers is possible; after
> > >> all, the SPI transfer rate can vary per message, so if clk_set_rate()
> > >> acquires a lock, it seems there's no way to avoid the issue there.
> > >
> > > Even for i2c this could be the case I think if you use the highspeed (3.4Mhz)
> > > mode? From what I remember, a highspeed i2c transaction starts with a lower
> > > speed preamble to make sure non highspeed slaves don't get confused? Which
> > > means you could change the bus speed depending on the slave you're addressing.
> > 
> > Since there's no separate chip-select for I2C, I believe all I2C devices 
> > need to be able to understand the entire transaction, so the I2C bus 
> > speed is fixed.
> > 
> 
> Does it? I would assume the slave only needs to check if the address matches
> its own address after a START condition and if not can just wait until the
> STOP condition appears on the bus?
> 

http://www.nxp.com/documents/user_manual/UM10204.pdf says you can mix them by
using an interconnect bridge between the highspeed and the non-highspeed
capable slaves. The bridge uses the special preamble to disconnect the non-
highspeed part of the bus when a highspeed transaction is ongoing. It's afaics
transparent to the master.

Cheers,

Peter.

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