linux-kernel - Re: [PATCH] arm: remove unused code in delay.S

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Date:	Mon, 14 Sep 2009 18:14:08 +0300
From:	Felipe Contreras <felipe.contreras@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jamie Lokier <jamie@...reable.org>,
	Marek Vasut <marek.vasut@...il.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm: remove unused code in delay.S

On Mon, Sep 14, 2009 at 5:40 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Mon, Sep 14, 2009 at 05:38:32PM +0300, Felipe Contreras wrote:
>> On Mon, Sep 14, 2009 at 5:00 PM, Russell King - ARM Linux
>> <linux@....linux.org.uk> wrote:
>> > On Mon, Sep 14, 2009 at 03:58:24PM +0300, Felipe Contreras wrote:
>> >> On Mon, Sep 14, 2009 at 11:10 AM, Russell King - ARM Linux
>> >> <linux@....linux.org.uk> wrote:
>> >> > On Mon, Sep 14, 2009 at 01:21:00AM +0100, Jamie Lokier wrote:
>> >> >> Russell King - ARM Linux wrote:
>> >> >> > On Sun, Sep 13, 2009 at 11:28:47PM +0200, Marek Vasut wrote:
>> >> >> > > >                 bhi     __delay
>> >> >> > > >                 mov     pc, lr
>> >> >> > > >  ENDPROC(__udelay)
>> >> >> > > >
>> >> >> > > Hi
>> >> >> > >
>> >> >> > > why was this code there in the first place ?
>> >> >> >
>> >> >> > To make the delay loop more stable and predictable on older CPUs.
>> >> >>
>> >> >> So why has it been commented out, if it's needed for that?
>> >> >
>> >> > We moved on and it penalises later CPUs, leading to udelay providing
>> >> > shorter delays than requested.
>> >> >
>> >> > So the choice was either stable and predictable on older CPUs but
>> >> > buggy on newer CPUs, or correct on all CPUs but gives unnecessarily
>> >> > longer delays on older CPUs.
>> >>
>> >> Why not add an #ifdef CPU_V4 or whatever?
>> >
>> > Because then you get it whenever you configure for V4 as the lowest
>> > denominator CPU, which leads to the buggy behaviour on better CPUs.
>> > It's far better to leave it as is and just accept that the old CPUs
>> > will have longer than necessary delays.  If people really really
>> > care (and there's likely to only be a small minority of them now)
>> > changing the '0' to a '1' is a very simple change for them to carry
>> > in their local tree.  Unlike getting the right unrolling etc.
>>
>> Well, they can also 'git revert' this patch. If somebody really cares
>> I think they should shout now and provide a better patch, otherwise
>> this one should be merged.
>
> On the other hand, having the code there as it currently stands is not
> harmful in any way, so leaving it there is just as easy.

It makes the code less understandable. I'm not sure about linux's
practices, but an #if 0 generally means somebody is being lazy.

-- 
Felipe Contreras
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