linux-kernel - Re: [RFC PATCH v3 1/2] clk: samsung: Add a clock lookup function

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Message-ID: <20150331014013.25195.26862@quantum>
Date:	Mon, 30 Mar 2015 18:40:13 -0700
From:	Michael Turquette <mturquette@...aro.org>
To:	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	"Tomasz Figa" <tomasz.figa@...il.com>
Cc:	"Stephen Boyd" <sboyd@...eaurora.org>,
	"Sylwester Nawrocki" <s.nawrocki@...sung.com>,
	"Kukjin Kim" <kgene@...nel.org>, "Olof Johansson" <olof@...om.net>,
	"Doug Anderson" <dianders@...omium.org>,
	"Krzysztof Kozlowski" <k.kozlowski@...sung.com>,
	"Kevin Hilman" <khilman@...aro.org>,
	"Tyler Baker" <tyler.baker@...aro.org>,
	"Abhilash Kesavan" <kesavan.abhilash@...il.com>,
	"Chanwoo Choi" <cw00.choi@...sung.com>,
	"linux-arm-kernel" <linux-arm-kernel@...ts.infradead.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v3 1/2] clk: samsung: Add a clock lookup function

Quoting Javier Martinez Canillas (2015-03-30 09:08:40)
> Hello Tomasz,
> 
> On 03/30/2015 06:02 PM, Tomasz Figa wrote:
> > Hi Javier,
> > 
> > 2015-03-31 0:53 GMT+09:00 Javier Martinez Canillas
> > <javier.martinez@...labora.co.uk>:
> >> The Samsung helpers functions to register clocks, add the clock instance
> >> returned by the common clock framework to a lookup table that is used by
> >> OF to lookup the clocks.
> >>
> >> But this table could also be useful to clock drivers if they need to get
> >> a clock instance since the helper functions don't return them.
> >>
> >> The common clock framework __clk_lookup() function from the clk provider
> >> API could be used by drivers as well. But it's more efficient to use the
> >> Samsung specific lookup table that returns the clock instance in constant
> >> time, than using the __clk_lookup() function that uses the clock name as
> >> an index so it has a linear search time.
> > 
> > Is this really something we should be concerned about? If so, maybe
> > the generic look-up function should be rewritten to use something
> > faster, such as tree or hash table?
> >
> 
> I don't performance is a big issue here. I just thought that since the
> lookup table is already filled by the driver and the lookup function
> is one line, we could use that instead to get the performance benefit.
> 
> But I don't mind to drop this patch and use the generic lookup function
> from the CCF API if that is preferred.

Hello,

I am not a fan of __clk_lookup and I don't like to see it used more and
more outside of drivers/clk/clk.c. You mentioned that performance wasn't
really the problem here.  The real method for a driver to get a clock is
with clk_get(). Any reason to not use that?

Regards,
Mike

>  
> > Best regards,
> > Tomasz
> >
> 
> Best regards,
> Javier
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