linux-kernel - Re: [PATCH 2/3] i8k: Autodetect maximal fan speed and fan RPM multiplier

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Message-Id: <201412101250.13891@pali>
Date:	Wed, 10 Dec 2014 12:50:13 +0100
From:	Pali Rohár <pali.rohar@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Jean Delvare <jdelvare@...e.de>,
	Gabriele Mazzotta <gabriele.mzt@...il.com>,
	Steven Honeyman <stevenhoneyman@...il.com>,
	Jochen Eisinger <jochen@...guin-breeder.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] i8k: Autodetect maximal fan speed and fan RPM multiplier

On Tuesday 09 December 2014 23:42:08 Guenter Roeck wrote:
> On Tue, Dec 09, 2014 at 09:23:22PM +0100, Pali Rohár wrote:
> > On Tuesday 09 December 2014 21:20:23 Guenter Roeck wrote:
> > > On Tue, Dec 09, 2014 at 09:07:00PM +0100, Pali Rohár wrote:
> > > > This patch adds new function i8k_get_fan_nominal_rpm()
> > > > for doing SMM call which will return nominal fan RPM
> > > > for specified fan speed. It returns nominal RPM value
> > > > at which fan operate when speed is set. It looks like
> > > > RPM value is not accurate, but still provides very
> > > > useful information.
> > > > 
> > > > First it can be used to validate if certain fan speed
> > > > could be accepted by SMM for setting fan speed and we
> > > > can use this routine to detect maximal fan speed.
> > > > 
> > > > Second it returns RPM value, so we can check if value
> > > > looks correct with multiplier 30 or multiplier 1 (until
> > > > now only these two multiplier was used). If RPM value
> > > > with multiplier 30 is too high, then multiplier 1 is
> > > > used.
> > > > 
> > > > In case when SMM reports that new function is not
> > > > supported we will fallback to old hardcoded values.
> > > > Maximal fan speed would be 2 and RPM multiplier 30.
> > > > 
> > > > Signed-off-by: Pali Rohár <pali.rohar@...il.com>
> > > > ---
> > > > I tested this patch only on my Dell Latitude E6440 and
> > > > autodetection worked fine Before appying this patch it
> > > > should be tested on some other dell machines too but if
> > > > machine does not support i8k_get_fan_nominal_rpm()
> > > > driver should fallback to old values. So patch should
> > > > be without regressions.
> > > 
> > > It looks like many of your error checks are unnecessary.
> > > Why did you add those ?
> > > 
> > > Please refrain from adding unnecessary code.
> > > 
> > > Guenter
> > 
> > Which error checks do you mean?
> 
> There are several you added. I noticed the ones around
> 'index', which would only be hit on coding errors. At that
> point I stopped looking further and did not verify which of
> the other added error checks are unnecessary as well.
> 
> A quick additional check reveals that the fan variable range
> check in i8k_get_fan_nominal_rpm is completely unnecessary -
> if the range was wrong, the calling code would fail as well,
> since you unconditionally write into an array indexed by the
> very same variable. Given the simplicity of the calling code,
> it can even be mathematically proven that the error condition
> you are checking can never happen.
> 
> With that I really stopped looking further.
> 
> Guenter
> 

Should I remove those access out-of-array checks?

-- 
Pali Rohár
pali.rohar@...il.com

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