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

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Date:	Sat, 16 Aug 2014 01:18:09 +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 11:46:49PM +0200, Stephen Warren wrote:
> On 08/15/2014 03:34 PM, Peter De Schrijver wrote:
> > 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.
> 
> I expect that works by echoing the slow-speed pre-amble to the 
> slow-speed bus segment, then emitting a stop and turning off the echo. 
> For actual slow-speed transactions, the whole thing would be echo'd. 
> That way the slow-speed devices don't ever see any high-speed pulses.
> 

Indeed.

> That all said, that does indeed imply that a master supporting the 
> high-speed transactions would need to emit a varying-speed signal. My 
> assumption would be that this happens inside the I2C HW, rather than 
> under SW control though, since the transition would need to happen 
> mid-protocol. Still, perhaps the selection between low-speed and 
> high-speed-with-a-slow-preamble might need SW clock programming 
> depending on the HW though... Who knows.
> 

That's true if the master wants to do a high-speed transaction. If the master
wants to do a normal-speed transaction to a slave on the same bus, the master
will need to select a lower speed clock under software control I think.

Cheers,

Peter.
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