linux-kernel - Re: [PATCH v6 2/3] i2c: iproc: Add Broadcom iProc I2C Driver

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Date:	Mon, 19 Jan 2015 13:31:25 -0800
From:	Ray Jui <rjui@...adcom.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Wolfram Sang <wsa@...-dreams.de>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Arend van Spriel <arend@...adcom.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"Grant Likely" <grant.likely@...aro.org>,
	Christian Daudt <bcm@...thebug.org>,
	Matt Porter <mporter@...aro.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Scott Branden <sbranden@...adcom.com>,
	<linux-i2c@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<bcm-kernel-feedback-list@...adcom.com>,
	<devicetree@...r.kernel.org>
Subject: Re: [PATCH v6 2/3] i2c: iproc: Add Broadcom iProc I2C Driver



On 1/19/2015 11:44 AM, Russell King - ARM Linux wrote:
> To see why atomic_t is pure obfuscation:
> 
> typedef struct {
>         int counter;
> } atomic_t;
> 
> So, counter is a plain int.
> 
> On Mon, Jan 19, 2015 at 11:23:47AM -0800, Ray Jui wrote:
>> +static irqreturn_t bcm_iproc_i2c_isr(int irq, void *data)
>> +{
>> +	struct bcm_iproc_i2c_dev *iproc_i2c = data;
>> +	u32 status = readl(iproc_i2c->base + IS_OFFSET);
>> +
>> +	status &= ISR_MASK;
>> +
>> +	if (!status)
>> +		return IRQ_NONE;
>> +
>> +	writel(status, iproc_i2c->base + IS_OFFSET);
>> +	atomic_set(&iproc_i2c->xfer_is_done, 1);
> 
> #define atomic_set(v,i) (((v)->counter) = (i))
> 
> So, this is the same as doing:
> 
> 	iproc_i2c->xfer_is_done.counter = 1;
> 
> which is merely setting the 'int' to 1.
> 
>> +	time_left = wait_for_completion_timeout(&iproc_i2c->done, time_left);
>> +
>> +	/* disable all interrupts */
>> +	writel(0, iproc_i2c->base + IE_OFFSET);
>> +
>> +	if (!time_left && !atomic_read(&iproc_i2c->xfer_is_done)) {
> 
> #define atomic_read(v)  ACCESS_ONCE((v)->counter)
> 
> This is practically the same as:
> 
> 	if (!time_left && !iproc_i2c->xfer_is_done.counter) {
> 
> except that this access will be guaranteed to happen just once at this
> location (see ACCESS_ONCE() in include/linux/compiler.h).
> 
> However, complete()..wait_for_completion() ensures that there are
> barriers in the way: complete takes a spinlock on the waiter, so the
> write to iproc_i2c->xfer_is_done.counter will be visible by the time
> wait_for_completion() returns, and wait_for_completion() also does.
> The same spinlock is also manipulated by wait_for_completion(), which
> means there's barriers there as well, so it can't cache the value of
> "counter" across that call.
> 
> So, the "volatile" access guaranteed by ACCESS_ONCE() isn't even
> needed here.
> 
> (It would be needed if you were spinning in a loop, calling no other
> functions - but then you're supposed to use cpu_relax() in that
> circumstance, which has a compiler barrier in it, which ensures that
> it will re-read such a variable each time.)
> 
I really learned a good lesson here. Thanks for the thorough explanation!
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