linux-kernel - Re: [PATCH 2/2] spi: Add Qualcomm QUP SPI controller support

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Date:	Tue, 11 Feb 2014 17:46:27 +0200
From:	"Ivan T. Ivanov" <iivanov@...sol.com>
To:	Courtney Cavin <courtney.cavin@...ymobile.com>
Cc:	Andy Gross <agross@...eaurora.org>,
	Mark Brown <broonie@...nel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	"linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Alok Chauhan <alokc@...eaurora.org>,
	Gilad Avidov <gavidov@...eaurora.org>,
	Kiran Gunda <kgunda@...eaurora.org>,
	Sagar Dharia <sdharia@...eaurora.org>
Subject: Re: [PATCH 2/2] spi: Add Qualcomm QUP SPI controller support


Hi, 

On Mon, 2014-02-10 at 12:29 -0800, Courtney Cavin wrote: 
> On Mon, Feb 10, 2014 at 08:41:44PM +0100, Ivan T. Ivanov wrote:
> > 
> > Hi,
> > 
> > On Mon, 2014-02-10 at 11:47 -0600, Andy Gross wrote:
> > > On Mon, Feb 10, 2014 at 06:55:02PM +0200, Ivan T. Ivanov wrote:
> > > 
> > > [....]
> > > 
> > > > > > > Bail here?
> > > > > > 
> > > > > > I don't know. What will be the consequences if controller continue to
> > > > > > operate on its default rate?
> > > > > > 
> > > > > 
> > > > > It is unclear.  But if you can't set the rate that is configured or if there is
> > > > > a misconfiguration, it's probably better to exit the probe and catch it here.
> > > > 
> > > > 
> > > > My preference is to delay clock speed change till first
> > > > SPI transfer. And use wherever transfer itself mandate.
> > > > 
> > > 
> > > That works.  My only concern is that it might be nice to catch a configuration
> > > problem early rather than wait for the SPI transfer to fail continuously.
> > 
> > If developer is skilled enough to know which version controller is,
> > (s)he will be able to put the right frequency constrain here :-)
> 
> A developer doesn't have to have much skill at all to copy-paste DT
> configurations around and muck with numbers....  I agree with Andy here,
> early validation is a good idea here, at the very least, some sanity
> checks.

Actually, thinking more on this. Supplying SPI controller with, 
let say 50MHz, which is what success of clk_set_rate() means, doesn't
necessarily guaranteer that controller will be able to do transfers
properly, right? Setting frequency at this point didn't bring any
benefit. 

Regards,
Ivan


> 
> -Courtney


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