linux-kernel - Re: [PATCH] Documentation DMA-API-HOWTO.txt Add dma mapping error check usage examples

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Date:	Thu, 18 Oct 2012 08:30:59 -0600
From:	Shuah Khan <shuah.khan@...com>
To:	mtk.manpages@...il.com
Cc:	Dan Carpenter <dan.carpenter@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	konrad.wilk@...cle.com, rob@...dley.net, joerg.roedel@....com,
	devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, shuahkhan@...il.com
Subject: Re: [PATCH] Documentation DMA-API-HOWTO.txt Add dma mapping error
 check usage examples

On Thu, 2012-10-18 at 12:13 +0200, Michael Kerrisk (man-pages) wrote:
> On Thu, Oct 18, 2012 at 11:47 AM, Dan Carpenter
> <dan.carpenter@...cle.com> wrote:
> > On Sun, Oct 14, 2012 at 09:54:24AM -0600, Shuah Khan wrote:
> >> diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
> >> index a0b6250..cf1adb4 100644
> >> --- a/Documentation/DMA-API-HOWTO.txt
> >> +++ b/Documentation/DMA-API-HOWTO.txt
> >> @@ -468,11 +468,46 @@ To map a single region, you do:
> >>       size_t size = buffer->len;
> >>
> >>       dma_handle = dma_map_single(dev, addr, size, direction);
> >> +     if (unlikely(dma_mapping_error(dma_handle))) {
> >
> > Don't encourage people to put unlikely() and likely() into their
> > driver code.  It should only be used after benchmarking both with
> > and without.  I can't imagine how it would make a measurable
> > difference here.
> 
> (Hi Dan)
> 
> Quite. See http://blog.man7.org/2012/10/how-much-do-builtinexpect-likely-and.html
> 
> I wonder how many blind uses of likely() and unlikely() in the kernel
> are actually degrading performance.
> 
> Cheers,
> 
> Michael


Dan, Michael,

Thanks. I see unlikely() and likely() used widely and didn't realize the
negatives of it. Will fix the examples without the use of unlikely() and
send v2 patch.

Thanks,
-- Shuah


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