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

[Will Deacon <will.deacon@arm.com>]

On Wed, Jun 14, 2017 at 11:48:07AM +0100, Russell King - ARM Linux wrote:
> 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.

I think that it also needs to account for write propagation delays.

> 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.

If buffering is a concern, then I think the non-relaxed write has the
barrier on the wrong side, so relaxed + mb() would be better.

Will