linux-kernel - Re: [PATCH] watchdog: dw

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Date:	Tue, 12 Apr 2016 11:01:28 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	Sudip Mukherjee <sudipm.mukherjee@...il.com>,
	Wim Van Sebroeck <wim@...ana.be>,
	Haavard Skinnemoen <hskinnemoen@...il.com>,
	Hans-Christian Egtvedt <egtvedt@...fundet.no>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kernel-testers@...r.kernel.org, linux-watchdog@...r.kernel.org
Subject: Re: [PATCH] watchdog: dw_wdt: dont build for avr32

On Tue, Apr 12, 2016 at 05:58:20PM +0300, Andy Shevchenko wrote:
> On Tue, Apr 12, 2016 at 4:39 PM, Sudip Mukherjee
> <sudipm.mukherjee@...il.com> wrote:
> > On Tuesday 12 April 2016 06:36 PM, Guenter Roeck wrote:
> >>
> >> On 04/11/2016 10:51 PM, Sudip Mukherjee wrote:
> >>>
> >>> The build of avr32 allmodconfig fails with the error:
> >>> ERROR: "__avr32_udiv64" [drivers/watchdog/kempld_wdt.ko] undefined!
> >>>
> >> This means there is a direct 64 bit divide operation in the driver,
> >> which we should identify and fix.
> 
> This driver will quite likely never be used on AVR32. Do we need to
> fix this due to some other architectures?
> 
> > yes, there is.
> >
> > in function: kempld_wdt_set_stage_timeout()
> > remainder = do_div(stage_timeout64, prescaler);
> 
> > Any idea how to fix it?
> 
> Not easy, however, prescaler value is ((1 << 21) - 1) which someone
> might consider as (1 << 21) with lost in precision.
> 
> Thus, shift on 20 bits right, add last bit to the value and shift on 1
> bit right more.
> 
Sorry, I am missing something. do_div() should work fine with any 32 bit value
as divisor, no matter what that value is. Why does it fail here ? And why does
it work if I pass 0x1fffff as second parameter to do_div() directly ?

Thanks,
Guenter

> -- 
> With Best Regards,
> Andy Shevchenko
> --
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