Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon Djtag driver

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Wed, Jun 14, 2017 at 11:06:58AM +0100, Will Deacon wrote:
> Apologies, I misunderstood your algorithm (I thought step (a) was on one CPU
> and step (b) was on another). Still, I don't understand the need for the
> timeout. If you instead read back the flag immediately, wouldn't it still
> work? e.g.
> 
> 
> lock:
>   Readl_relaxed flag
>   if (locked)
>     goto lock;
> 
>   Writel_relaxed unique ID to flag
>   Readl flag
>   if (locked by somebody else)
>     goto lock;
> 
> <critical section>
> 
> unlock:
>   Writel unlocked value to flag

I think the delay is to counter this:

	Agent 1			Agent 2
	read flag
	not locked
				read flag
				not locked
	write unique ID
	read back
	not locked by someone else
				write unique ID
				read back
				not locked by someone else

With the delay present, this becomes:

	Agent 1			Agent 2
	read flag
	not locked
				read flag
				not locked
	write unique ID
	delay
				write unique ID
				delay
	read back
	locked by agent 2
				read back
				not locked by someone else

For this to work, the delay has to be guaranteed to be greater than
the maximum duration that any agent takes between the initial read
and the write of its unique ID.  The delay doesn't even have to be
identical between each agent, it just has to satisfy that condition.

The key thing though is that the reads and writes must happen when
the program intends them to, so I don't think the _relaxed variants
should be used here.  If they're buffered, then the delay doesn't
have the desired effect.

-- 
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