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

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Date:	Mon, 14 Sep 2009 19:54:54 +0300
From:	Felipe Contreras <felipe.contreras@...il.com>
To:	Marek Vasut <marek.vasut@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Jamie Lokier <jamie@...reable.org>,
	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 7:25 PM, Marek Vasut <marek.vasut@...il.com> wrote:
> Dne Po 14. září 2009 17:50:15 Felipe Contreras napsal(a):
>> On Mon, Sep 14, 2009 at 6:21 PM, Russell King - ARM Linux
>>
>> <linux@....linux.org.uk> wrote:
>> > On Mon, Sep 14, 2009 at 06:14:08PM +0300, Felipe Contreras wrote:
>> >> 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.
>> >
>> > I would agree with you if it was a complicated bit of code, but it
>> > isn't.  It is a simple count to zero (or overflow) and terminate
>> > loop.  And it's certainly not about me being lazy.
>
> Russell, what about adding a comment somewhere explaining why it's there? That'd
> be a fine fix I think.
>>
>> Maybe it's not complicated to you, but not everyone is so literate
>> about ARM assembly code (e.g. me). When I first looked at the code I
>> didn't even realize there was an #if 0 there, which yes, I grant is a
>> problem of my editor, but the issue wouldn't have happened if the code
>> wasn't there in the first place.
>
> This is still religious argument and or your problem -- fix your editor.

I'm an example user. I'm pretty sure there's other vim users out there
:) Since I'm not touching the 'asm' configurations I'm assuming many
other people will see the code as I do. Not my main argument, anyway.

>> And I'm not saying your are being lazy, if anything it's probably the
>> people using this code. They should figure a way to avoid patching the
>> code. However, these users are hypothetical at this point.
>
> If you could invest your time into investigating how to make this configurable
> instead of arguing about #if 0, it'd be awesome.

That's what I'm doing, so far I haven't heard any suggestions on how to do that.

-- 
Felipe Contreras
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