linux-kernel - Re: [PATCH 1/1]mmc: fix division by zero when calculate mmc erase time

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Date:	Thu, 24 Feb 2011 13:34:14 +0100
From:	Marc Koschewski <marc@...nowledge.org>
To:	"Dong, Chuanxiao" <chuanxiao.dong@...el.com>
Cc:	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"cjb@...top.org" <cjb@...top.org>,
	"adrian.hunter@...ia.com" <adrian.hunter@...ia.com>,
	"linus.walleij@...aro.org" <linus.walleij@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1]mmc: fix division by zero when calculate mmc erase
 time

* Dong, Chuanxiao <chuanxiao.dong@...el.com> [2011-02-24 20:25:21 +0800]:

> > -----Original Message-----
> > From: Marc Koschewski [mailto:marc@...nowledge.org]
> > Sent: Thursday, February 24, 2011 8:23 PM
> > To: Dong, Chuanxiao
> > Cc: linux-mmc@...r.kernel.org; cjb@...top.org; adrian.hunter@...ia.com;
> > linus.walleij@...aro.org; linux-kernel@...r.kernel.org
> > Subject: Re: [PATCH 1/1]mmc: fix division by zero when calculate mmc erase time
> > 
> > * Dong, Chuanxiao <chuanxiao.dong@...el.com> [2011-02-24 20:09:59 +0800]:
> > 
> > >
> > >
> > > > -----Original Message-----
> > > > From: Marc Koschewski [mailto:marc@...nowledge.org]
> > > > Sent: Thursday, February 24, 2011 7:38 PM
> > > > To: Dong, Chuanxiao
> > > > Cc: linux-mmc@...r.kernel.org; cjb@...top.org; adrian.hunter@...ia.com;
> > > > linus.walleij@...aro.org; linux-kernel@...r.kernel.org
> > > > Subject: Re: [PATCH 1/1]mmc: fix division by zero when calculate mmc erase
> > time
> > > >
> > > > Hi,
> > > >
> > > > * Chuanxiao Dong <chuanxiao.dong@...el.com> [2011-02-24 19:18:01 +0800]:
> > > >
> > > > > Since if clock gating feature is enabled, the clock frequency may be zero when
> > > > > host clock is gated. In such scenario, mmc_set_mmc_erase_timeout() may
> > have a
> > > > > division by zero bug.
> > > > >
> > > > > So this patch used mmc_host_clk_rate() to fix this.
> > > > >
> > > > > Signed-off-by: Chuanxiao Dong <chuanxiao.dong@...el.com>
> > > > > ---
> > > > >  drivers/mmc/core/core.c |   10 ++++++++--
> > > > >  1 files changed, 8 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> > > > > index 34a7e8c..12d0eb8 100644
> > > > > --- a/drivers/mmc/core/core.c
> > > > > +++ b/drivers/mmc/core/core.c
> > > > > @@ -1201,8 +1201,14 @@ static void mmc_set_mmc_erase_timeout(struct
> > > > mmc_card *card,
> > > > >  		 * less but not that much less, so fudge it by multiplying by 2.
> > > > >  		 */
> > > > >  		timeout_clks <<= 1;
> > > > > -		timeout_us += (timeout_clks * 1000) /
> > > > > -			      (card->host->ios.clock / 1000);
> > > > > +
> > > > > +		/*
> > > > > +		 * at this moment, host controller maybe clock gated, so make
> > > > > +		 * sure we can get a correct host clock freq.
> > > > > +		 */
> > > > > +		if (mmc_host_clk_rate(card->host))
> > > > > +			timeout_us += (timeout_clks * 1000) /
> > > > > +				      (mmc_host_clk_rate(card->host) / 1000);
> > > >
> > > > Why don't you just reuse mmc_host_clk_rate()'s result instead of calling it
> > twice?
> > > This is a incline function and just return host->ios.clock. Reuse
> > mmc_host_clk_rate()'s result need to add a new variable to save the value.
> > 
> > It's not inline on trunk and it spinlocks.
> > 
> > drivers/mmc/core/host.c:195
> > 
> >     194  */
> >     195 unsigned int mmc_host_clk_rate(struct mmc_host *host)
> >     196 {
> OK. With the clock gating framework enabled... I agree. So, what do you think? Add a new variable is better?

I personally would prefer the variable over the spinlock and function call, yes.

And calling the same method with the same parameters on a line and another
time on the next line is generally a bad idea I think. But maybe that's kind
of a 'taste', moreover. It just hit my eye when I saw it...

Cheers,
Marc

> 
> Thanks
> Chuanxiao
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-- 
Marc Koschewski
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