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

[Zhihao Cheng]

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@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
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
> .