Messages in this thread | | | Subject | Re: [PATCH] mtd: ubi: avoid expensive do_div() on 32-bit machines | From | Zhihao Cheng <> | Date | Wed, 13 Mar 2024 20:10:54 +0800 |
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在 2024/3/13 19:53, Arnd Bergmann 写道: > On Wed, Mar 13, 2024, at 12:29, Zhihao Cheng wrote: >> 在 2024/3/13 16:46, Arnd Bergmann 写道: >>> From: Arnd Bergmann <arnd@arndb.de> >>> >>> The use of do_div() in ubi_nvmem_reg_read() makes calling it on >>> 32-bit machines rather expensive. Since the 'from' variable is >>> known to be a 32-bit quantity, it is clearly never needed and >>> can be optimized into a regular division operation. >>> >> Do you meet a performance problem on a 32-bit machine? There are too >> many places invoking do_div, why do you optimize this one? >> Have you tested the influence on a x86_64 platform after this patch >> applied? Looks like that do_div is more efficient in x86. > > This one was just introduced. The call site looks like a fast > path and it caused a build regression that Daniel addressed with > an suboptimal commit b8a77b9a5f9c ("mtd: ubi: fix NVMEM over > UBI volumes on 32-bit systems"). > > The way it usually goes is that someone adds an open-coded > 64-bit division that causes a link failure, which prompts I'm a little confused, what kind of link failure? Could you show an example? > the original developer to either rewrite the code to avoid > the long division if possible, or add do_div() after showing > that it is now performance critical, e.g. only called at > probe time. > > Arnd > . >
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