Messages in this thread | | | Date | Sat, 28 Jan 2012 08:52:25 +0900 | Subject | Re: [PATCH] mtd/nand: use string library | From | Akinobu Mita <> |
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2012/1/28 Brian Norris <computersforpeace@gmail.com>: > On Fri, Jan 27, 2012 at 9:16 AM, Joe Perches <joe@perches.com> wrote: >> On Fri, 2012-01-27 at 23:24 +0900, Akinobu Mita wrote: >>> - Use memchr_inv to check if the data contains all 0xFF bytes. >>> It is faster than looping for each byte. >> >> Stupid question: >> >> Are there any mtd devices modified that are slower >> at 64 bit accesses than repeated 8 bit accesses? > > I believe this patch deals with kernel buffers, not any kind of direct > access to the MTD, so the question (which is not stupid IMO) should be > regarding CPU architectures. And my educated guess is that 64-bit > access should not be any slower. I do know that 8-bit access *is* > slower for some relevant architectures.
It could be slower when the number of bytes scanned is very small (found a unmatched character immediately, or the size of the area is very small), because memchr_inv() needs to generate a 64bit pattern to compare before starting the loop. I recalled that Eric Dumazet pointed out it could generate the 64bit pattern more efficiently. (https://lkml.org/lkml/2011/8/8/480)
Even if that small scanning is slower, this change can be assumed cleanup patch that simplifies the code. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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