linux-kernel - Re: [PATCH] Add option to build with -O3

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Date:	Wed, 5 Mar 2014 20:43:37 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Jon Ringle <jon@...gle.org>
Cc:	"Ringle, Jonathan" <Jonathan.Ringle@...dpoint.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Add option to build with -O3

On Wed, Mar 05, 2014 at 02:32:26AM -0500, Jon Ringle wrote:
> On Wed, 5 Mar 2014, Greg KH wrote:
> 
> > On Tue, Mar 04, 2014 at 07:01:49PM -0500, Jon Ringle wrote:
> > > +config CC_OPTIMIZE_FOR_SPEED
> > > +    bool "Optimze for speed (-O3)"
> > > +    help
> > > +      Enabling this option will pass "-O3" to gcc
> > > +      resulting in a larger kernel (but possibly faster)
> > 
> > Are you sure about that?  Have you measured it?
> 
> (Resending this message, since it was "destroyed". Hopefully, this is now 
> an an acceptable form :)
> 
> I do know that there is an improvement performance-wise for my particular 
> use-case.
> 
> My target is an ARM board being built with gcc-4.8.2. My board has on it a 
> sc16is740 that is used as an RS-485 port. The sc16is740 is on the i2c bus, 
> so when an interrupt comes in to indicate that there is data available to 
> be read, I need to get the data over the i2c bus. I do this on a kthread 
> to do this work. The i2c transactions (using i2c-davinci driver) are also 
> interrupt driven. I was seeing a lot of lost packets when receiving data 
> at only 19200. Adding the -O3 compile option helped in this regard in that 
> I am now rarely seeing packet loss.

I'd take this up with the linux-arm developers, that sounds like
something is wrong with your system that a compiler change fixes
problems.  Something else must be wrong...

good luck,

greg k-h
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