linux-kernel - Re: [PATCH v2] clocksource: exynos-mct: Register the timer for stable udelay

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Date:	Sat, 21 Jun 2014 15:23:39 +0530
From:	amit daniel kachhap <amit.daniel@...sung.com>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Will Deacon <will.deacon@....com>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	David Riley <davidriley@...omium.org>,
	Jonathan Austin <Jonathan.Austin@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] clocksource: exynos-mct: Register the timer for stable udelay

On Fri, Jun 20, 2014 at 11:17 PM, Doug Anderson <dianders@...omium.org> wrote:
> Hi,
>
> On Fri, Jun 20, 2014 at 4:19 AM, amit daniel kachhap
> <amit.daniel@...sung.com> wrote:
>> On Fri, Jun 20, 2014 at 1:45 PM, Will Deacon <will.deacon@....com> wrote:
>>> On Thu, Jun 19, 2014 at 05:40:49PM +0100, Tomasz Figa wrote:
>>>> On 19.06.2014 18:31, Doug Anderson wrote:
>>>> >>> My personal vote would be to submit a patch to change "cycles_t" to
>>>> >>> always be 32-bits.  Given that 32-bits was fine for udelay() for ARM
>>>> >>> that seems sane and simple.  If someone later comes up with a super
>>>> >>> compelling reason why we need 64-bit timers for udelay (really??) then
>>>> >>> they can later add all the complexity needed.
>>>> >>
>>>> >> Yes, this could work. I'm not sure what else cycles_t is used for, though.
>>>> >
>>>> > True, it is a bit questionable to change this since it's a type that's
>>>> > not obviously just for udelay().  Perhaps a better option would be to
>>>> > make a new typedef for the result of read_current_timer().  ...or just
>>>> > change it to return a u32?
>>>> >
>>>>
>>>> Sounds good to me, but let's hear other opinions. I'm adding Will and
>>>> Jonathan as they wrote the ARM delay timer code.
>>>
>>> I think cycles_t is only used for small interval calculations at the moment,
>>> but I remember Ted mentioning something about using it (or something
>>> similar) as a source of early entropy, in which case the more bits the
>>> better.
>>>
>> Will,
>> Thanks for the clarification that cycles_t is used for short
>> intervals. So it is safe to return lower 32 bit
>> counter for read_current_timer.
>
> As I looked at it more, I realized that we have two types in Linux.
> There's cycle_t and cycles_t.  Whoa, confusing!
>
> I'd perhaps advocate a wholesale rename of cycles_t to avoid the
> confusion.  I don't have a good name for it, though.  cycle32_t?  Or
> we could just use u32 for the function...  :-/
or cycles_t can be renamed as short_cycle_t
>
>
>> Tomasz, Doug,
>> As of now let me send a minimal implementation of this read delay
>> timer to fix the broken udelay for exynos platforms so that it goes to
>> upstream in rc releases. I will also prepare a fix for all
>> raw_readl/writel in mct to relaxed version to make it consistent.
>
> I'm reworking my 32-bit conversion patches right now and it's getting
> messy to intermingle this with yours.  I'm going to pick up your patch
> and include it in my series.  I hope that's OK.
its fine and thanks for posting the series.
>
> My plan is:
>
> 1. For 3.16 I think it's important to fix the udelay() problems and
> trying to rework cycle_t there doesn't seem like it makes sense.  I'll
> just use Amit's original code that uses exynos_frc_read().  It might
> not be quite as optimal but it's good as a safe bugfix.
>
> 2. I'll post the cleanup patch moving away from the __raw_readl / __raw_writel
>
> 3. I'll post a patch moving to 32-bit, including moving Amit's code to
> 32-bit but with a compile time warning for now.  I'll add a KConfig
> depends to keep it from compiling on ARM64.  We can improve this once
> we change the delay timer to always request 32-bits.
>
> -Doug
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