linux-kernel - Re: [PATCH 2/2] Convert PowerPC macro spin_event_timeout() to architecture independent macro

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Message-ID: <20130731071630.GI8868@codeaurora.org>
Date:	Wed, 31 Jul 2013 00:16:30 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Arpit Goel <B44344@...escale.com>
Cc:	linux@....linux.org.uk, takata@...ux-m32r.org, philb@....org,
	geert@...ux-m68k.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, linux390@...ibm.com,
	davem@...emloft.net, rob.herring@...xeda.com, arnd@...db.de,
	swarren@...dia.com, john.stultz@...aro.org,
	jesper.nilsson@...s.com, gregkh@...uxfoundation.org,
	sam@...nborg.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-m32r@...linux-m32r.org,
	linux-m32r-ja@...linux-m32r.org, linux-m68k@...r.kernel.org,
	linux-s390@...r.kernel.org, sparclinux@...r.kernel.org,
	mattw@...eaurora.org
Subject: Re: [PATCH 2/2] Convert PowerPC macro spin_event_timeout() to
 architecture independent macro

On 07/30, Arpit Goel wrote:
> This patch ports PowerPC implementation of spin_event_timeout() for generic
> use. Architecture specific implementation can be added to asm/delay.h, which
> will override the generic linux implementation.
> 
> Signed-off-by: Arpit Goel <B44344@...escale.com>
> ---

We use something similar internally but it's tied specifically to
readl.

>  
> +#ifndef spin_event_timeout
> +/**
> + * spin_event_timeout - spin until a condition gets true or a timeout elapses
> + * @condition: a C expression to evalate
> + * @timeout: timeout, in microseconds
> + * @delay: the number of microseconds to delay between each evaluation of
> + *         @condition
> + *
> + * The process spins until the condition evaluates to true (non-zero) or the
> + * timeout elapses.  The return value of this macro is the value of
> + * @condition when the loop terminates. This allows you to determine the cause
> + * of the loop terminates.  If the return value is zero, then you know a
> + * timeout has occurred.
> + *
> + * This primary purpose of this macro is to poll on a hardware register
> + * until a status bit changes.  The timeout ensures that the loop still
> + * terminates even if the bit never changes.  The delay is for devices that
> + * need a delay in between successive reads.
> + *
> + * gcc will optimize out the if-statement if @delay is a constant.
> + *
> + * This is copied from PowerPC based spin_event_timeout() implementation
> + * and modified for generic usecase.
> + */
> +#define spin_event_timeout(condition, timeout, delay)		\
> +({								\
> +	typeof(condition) __ret;				\
> +	unsigned long __loops = timeout/USECS_PER_JIFFY;	\
> +	unsigned long __start = jiffies;			\
> +	while (!(__ret = (condition)) &&			\
> +		time_before(jiffies, __start + __loops + 1))	\
> +		if (delay)					\
> +			udelay(delay);				\
> +		else						\
> +			schedule();				\
> +	if (!__ret)						\
> +		__ret = (condition);				\
> +	__ret;							\
> +})

What do you do here if jiffies aren't incrementing (i.e
interrupts are disabled). The time_before() check won't work
there and it would be nice if we were able to use this in such
situations. I think powerpc gets around this by reading the
hardware timer directly?

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