linux-kernel - Re: [PATCH 2/2] ARM: omap5/dra7xx: Fix counter frequency drift for AM572x errata i856.

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Date:	Tue, 16 Dec 2014 13:33:24 -0600
From:	Nishanth Menon <nm@...com>
To:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
CC:	Lokesh Vutla <lokeshvutla@...com>, <t-kristo@...com>,
	<linux-omap@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Sekhar Nori <nsekhar@...com>
Subject: Re: [PATCH 2/2] ARM: omap5/dra7xx: Fix counter frequency drift for
 AM572x errata i856.

On 12/16/2014 01:27 PM, Lennart Sorensen wrote:
>> I see why arch_timer_freq might skip the rounding error of 39, 15 and
>> 55 Vs existing logic which is possibly at a truncation error risk
>> (without errata for sysclk 13, 26 and 27MHz).
>>
>> all you'd probably need to do is cast rate, num and den to unsigned
>> long and have a common computation logic.
> 
> If that is acceptable, then sure I can do that.  I liked avoiding casts
> in general though.
> 
>> if you'd really want to handle truncation error, it must be a separate
>> patch of it's own - I would not mix it with the errata fix.
> 
> Well there is no error in the existing code because the rate / den
> is always a clean integer division.  The problem is introduced by the

key is "there is no error in existing code for existing value" :) ->
the same code for new values will fail. and introducing (rate * num) /
den without cast will fail for old values.

> SYSCLK1 / 610 used by the emulated clock which is not a clean division.
> 
> So for the existing logic, the calculation was perfect.  It is only for
> the errata case that it is a problem.
> 
> So I think leaving the existing calculation but moved up works well,
> and then having the alternate order calculation only in the errata case
> seemed cleaner and avoids casts and 64bit math which I thought was
> overall desirable.

In general using DIV_ROUND_UP and family of macros(in kernel.h) is the
right way of doing division in similar cases in Linux kernel. And for
the same functionality, we want a common equation - if it does not fit
well for all values (even if we introduce new values), then we must
come to a better equation that will work for all values. What we do
not do is to have two equations meant for doing the same thing.


-- 
Regards,
Nishanth Menon
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