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

[Michael Kerrisk (man-pages)]

On Thu, Oct 18, 2012 at 11:47 AM, Dan Carpenter
<dan.carpenter at oracle.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

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/