linux-kernel - Re: [PATCH 2/2 v3] mmc: dw_mmc: Fix the CTO overflow calculation for 32-bit systems

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Message-ID: <20180227115243.7ca84b3b@xhacker.debian>
Date:   Tue, 27 Feb 2018 11:52:43 +0800
From:   Jisheng Zhang <Jisheng.Zhang@...aptics.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:     "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dianders@...omium.org" <dianders@...omium.org>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "Eugeniy.Paltsev@...opsys.com" <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "Evgeniy.Didin@...opsys.com" <Evgeniy.Didin@...opsys.com>,
        "ulf.hansson@...aro.org" <ulf.hansson@...aro.org>
Subject: Re: [PATCH 2/2 v3] mmc: dw_mmc: Fix the CTO overflow calculation
 for 32-bit systems

On Mon, 26 Feb 2018 20:27:22 +0000 Alexey Brodkin wrote:

> Hi Andy,
> 
> On Mon, 2018-02-26 at 20:30 +0200, Andy Shevchenko wrote:
> > On Mon, Feb 26, 2018 at 7:14 PM, Evgeniy Didin
> > <Evgeniy.Didin@...opsys.com> wrote:  
> > > On Mon, 2018-02-26 at 18:53 +0200, Andy Shevchenko wrote:  
> > > > On Mon, Feb 26, 2018 at 5:14 PM, Evgeniy Didin
> > > > <Evgeniy.Didin@...opsys.com> wrote:  
> > > > > On Mon, 2018-02-26 at 16:39 +0200, Andy Shevchenko wrote:  
> > > > > > On Mon, Feb 26, 2018 at 4:34 PM, Evgeniy Didin
> > > > > > <Evgeniy.Didin@...opsys.com> wrote:  
> > > > > > > In commit 4c2357f57dd5 ("mmc: dw_mmc: Fix the CTO timeout calculation")
> > > > > > > have been made changes which can cause multiply overflow for 32-bit systems.
> > > > > > > The value of cto_ms is lower the drto_ms, but nevertheless overflow can occur.
> > > > > > > Lets cast this multiply to u64 type which prevents overflow.
> > > > > > > -       cto_ms = DIV_ROUND_UP(MSEC_PER_SEC * cto_clks * cto_div, host->bus_hz);
> > > > > > > +
> > > > > > > +       cto_ms = DIV_ROUND_UP((u64)MSEC_PER_SEC * cto_clks * cto_div, host->bus_hz);  
> > > > > > 
> > > > > > IIRC, someone commented on this or similar, i.e.
> > > > > > 
> > > > > > DIV_ROUND_UP_ULL() ?  
> > > > > 
> > > > > Switch DIV_ROUND_UP macro to DIV_ROUND_UP_ULL is not reasonable
> > > > > because overflow happens on multiply and DIV_ROUND_UP_ULL helps
> > > > > with sum overflow.  
> > > > 
> > > > Did you try to compile your code for 32-bit target?  
> > > 
> > > Yes, we have compiled code for 32-bit system.
> > > I am wondering why are you asking that?  
> > 
> > Because I simple didn't believe you.  
> 
> Well world around us is much more complicated than it sometimes looks like :)
> 
> > ERROR: "__udivdi3" [drivers/mmc/host/dw_mmc.ko] undefined!
> > ...scripts/Makefile.modpost:92: recipe for target '__modpost' failed
> > make[2]: *** [__modpost] Error 1  
> 
> That's right __udivdi3() is not defined for some architectures but it is defined for
> some others including ARC, Xtensa etc, see
> https://elixir.bootlin.com/linux/latest/ident/__udivdi3
> 
> What happens __udivdi3() is implemented in libgcc in one form or another form
> be it pure assembly or generic implementation written in C.
> 
> So maybe we need to add export of __udivdi3() for other arches, what do you think?

Per my understanding, Linux kernel prefer to make use of do_div or implementations
in <linux/math64.h> for 64bit divide

Thanks