Re: mutual exclusion locks over PCI memory

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On Fri, Feb 20, 2009 at 07:09:51PM +0530, arun c wrote:
> PCI host machine (PPC cpu) writes commands to
> the PCI memory space of the (Coldfire CPU)
> target device. Target device takes the command
> and executes it.
> 
> Target devices SDRAM is exposed over PCI to host.
> A circular buffer residing on target memory is used
> for command exchange.
> 
> I should not allow host and target to play on the
> buffer simultaneously in order to avoid corruption.
> 
> Does anybody know how to implement a lock
> suitable for this issue?
> 
> or any lock less algorithm exists for communication
> over PCI?

I would have thought that a standard head and tail lockless queue would
be perfect for your application.  Expressed in C:

struct queue {
	unsigned head;
	unsigned entries[QUEUE_SIZE];
	unsigned tail;
}

int queue_size(struct queue __iomem *q)
{
	int size = readl(&q->head) - readl(&q->tail);
	if (size < 0)
		size += QUEUE_SIZE;
	return size;
}

int queue_empty(struct queue __iomem *q)
{
	return readl(&q->head) == readl(&q->tail);
}

int queue_push(struct queue __iomem *q, unsigned item)
{
	unsigned head = readl(&q->head) + 1;
	if (head == readl(&q->tail))
		return FAIL;
	writel(&q->entries[head], item);
	writel(&q->head, head);
}

Something like that anyway ... I obviously haven't tested or even
compiled it.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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