Messages in this thread | | | Date | Wed, 13 Mar 2024 12:53:08 +0100 | From | "Arnd Bergmann" <> | Subject | Re: [PATCH] mtd: ubi: avoid expensive do_div() on 32-bit machines |
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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 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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