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

[Ray Jui <rjui@broadcom.com>]

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!