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From: Finn Thain <fthain@telegraphics.com.au>
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Stephen N Chivers <schivers@csc.com.au>,
	Thomas Gleixner <tglx@linutronix.de>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	John Stultz <john.stultz@linaro.org>,
	linux-m68k@lists.linux-m68k.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 01/13] arm: Fix mutual exclusion in arch_gettimeoffset
Date: Wed, 14 Nov 2018 14:17:09 +1100 (AEDT)
Message-ID: <alpine.LNX.2.21.1811141142490.8@nippy.intranet> (raw)
In-Reply-To: <20181113234336.GP30658@n2100.armlinux.org.uk>

On Tue, 13 Nov 2018, Russell King - ARM Linux wrote:

> 
> A clocksource provides a cycle counter that monotonically changes and 
> does not wrap between clockevent events.
> 
> A clock event is responsible for providing events to the system when 
> some work is needing to be done, limited by the wrap interval of the 
> clocksource.
> 
> Each time the clock event triggers an interrupt, the clocksource is
> read to determine how much time has passed, using:
> 
> 	count = (new_value - old_value) & available_bits
> 	nanosecs = count * scale >> shift;
> 
> If you try to combine the clocksource and clockevent because you only
> have a single counter, and the counter has the behaviour of:
> - counting down towards zero
> - when reaching zero, triggers an interrupt, and reloads with N
> 
> then this provides your clockevent, but you can't use this as a clock
> source, because each time you receive an interrupt and try to read the
> counter value, it will be approximately the same value.  This means
> that the above calculation fails to register the correct number of
> nanoseconds passing.  Hence, this does not work.
> 
> Also note where I said above that the clock event device must be able
> to provide an interrupt _before_ the clocksource wraps - clearly with
> such a timer, that is utterly impossible.
> 
> The simple solution is to not use such a counter as the clocksource,
> which means you fall back to using the jiffies clocksource, and your
> timekeeping has jiffy resolution - so 10ms, or possibly 1ms if you
> want a 1kHz timer interval.  For most applications, that's simply way
> to coarse, as was realised in the very early days of Linux.
> 
> If only there was a way to interpolate between timer interrupts...
> which is exactly what arch_gettimeoffset() does, and is a historical
> reminant of the pre-clocksource/clockevent days of the kernel - but
> it is the only way to get better resolution from this sort of setup.
> 

Both of the platforms in question (RPC and EBSA110) have not 
defined(CONFIG_GENERIC_CLOCKEVENTS) and have not defined any struct 
clock_event_device, AFAICT.

So, even assuming that you're right about the limitations of single-timer 
platforms in general, removal of arch_gettimeoffset wouldn't require the 
removal of any platforms, AFAICT.

-- 

  parent reply index

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-12  4:12 [RFC PATCH 00/13] m68k: Drop arch_gettimeoffset and adopt clocksource API Finn Thain
2018-11-12  4:12 ` [RFC PATCH 08/13] m68k: atari: Convert to " Finn Thain
2018-11-12  4:12 ` [RFC PATCH 04/13] m68k: mac: Clean up unused timer definitions Finn Thain
2018-11-12  4:12 ` [RFC PATCH 10/13] m68k: hp300: Convert to clocksource API Finn Thain
2018-11-12  4:12 ` [RFC PATCH 11/13] m68k: mac: " Finn Thain
2018-11-12  4:12 ` [RFC PATCH 01/13] arm: Fix mutual exclusion in arch_gettimeoffset Finn Thain
2018-11-12  8:34   ` Christoph Hellwig
2018-11-13  3:39     ` Finn Thain
2018-11-13  9:20       ` Russell King - ARM Linux
2018-11-13 21:55         ` Finn Thain
2018-11-13 23:43           ` Russell King - ARM Linux
2018-11-14  1:35             ` Michael Schmitz
2018-11-14  7:58               ` Russell King - ARM Linux
2018-11-15  1:34                 ` Michael Schmitz
2018-11-14  3:17             ` Finn Thain [this message]
2018-11-14 14:16               ` Russell King - ARM Linux
2018-11-14 14:58                 ` Geert Uytterhoeven
2018-11-14 18:11                   ` Russell King - ARM Linux
2018-11-15  4:12                 ` Finn Thain
2018-11-16 17:47                   ` Russell King - ARM Linux
2018-11-16 22:49                     ` Finn Thain
2018-11-17  3:00                       ` Michael Schmitz
2018-11-14 12:05   ` Russell King - ARM Linux
2018-11-12  4:12 ` [RFC PATCH 02/13] m68k: " Finn Thain
2018-11-12  4:12 ` [RFC PATCH 07/13] m68k: amiga: Convert to clocksource API Finn Thain
2018-11-12  9:02   ` Geert Uytterhoeven
2018-11-12  9:21     ` Finn Thain
2018-11-12  4:12 ` [RFC PATCH 09/13] m68k: bvme6000: " Finn Thain
2018-11-12  4:12 ` [RFC PATCH 06/13] m68k: Drop ARCH_USES_GETTIMEOFFSET Finn Thain
2018-11-12  9:00   ` Geert Uytterhoeven
2018-11-12  9:06     ` Finn Thain
2018-11-13  2:29       ` Michael Schmitz
2018-11-13  3:14         ` Finn Thain
2018-11-13  4:50           ` Michael Schmitz
2018-11-13  6:15             ` Finn Thain
2018-11-13  8:24               ` Michael Schmitz
2018-11-13 22:11                 ` Finn Thain
2018-11-14  1:08                   ` Michael Schmitz
2018-11-14  2:58                     ` Michael Schmitz
2018-11-14 23:54                       ` Michael Schmitz
2018-11-15  4:37                         ` Finn Thain
2018-11-15  6:35                         ` Michael Schmitz
2018-11-16  0:04                           ` Finn Thain
2018-11-12  4:12 ` [RFC PATCH 13/13] m68k: mvme16x: Convert to clocksource API Finn Thain
2018-11-12 18:37   ` Thomas Gleixner
2018-11-13  0:11     ` Finn Thain
2018-11-13 22:04       ` Finn Thain
2018-11-13 22:10         ` [RFC PATCH 13/13] m68k: mvme16x: Convert to clocksource APIy Thomas Gleixner
2018-11-13 22:33           ` Finn Thain
2018-11-18 17:12             ` Thomas Gleixner
2018-11-12  4:12 ` [RFC PATCH 03/13] m68k: mac: Fix VIA timer counter accesses Finn Thain
2018-11-12  4:12 ` [RFC PATCH 12/13] m68k: mvme147: Convert to clocksource API Finn Thain
2018-11-12  4:12 ` [RFC PATCH 05/13] m68k: apollo, q40, sun3, sun3x: Remove arch_gettimeoffset implementations Finn Thain

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