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

Will Deacon
2017-06-14T11:05:55Z

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