Re: [PATCH] mtd: ubi: avoid expensive do_div() on 32-bit machines

[Arnd Bergmann]

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@xxxxxxxx>
>> 
>> 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