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

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Date:   Wed, 14 Jun 2017 12:40:39 +0100
From:   Will Deacon <will.deacon@....com>
To:     John Garry <john.garry@...wei.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        anurup.m@...wei.com, tanxiaojun@...wei.com, xuwei5@...ilicon.com,
        sanil.kumar@...ilicon.com, gabriele.paoloni@...wei.com,
        shiju.jose@...wei.com, huangdaode@...ilicon.com,
        linuxarm@...wei.com, dikshit.n@...wei.com, shyju.pv@...wei.com,
        anurupvasu@...il.com
Subject: Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon
 Djtag driver

On Wed, Jun 14, 2017 at 12:35:07PM +0100, John Garry wrote:
> On 14/06/2017 12:01, Will Deacon wrote:
> >On Wed, Jun 14, 2017 at 11:42:30AM +0100, Mark Rutland 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
> >>>
> >>>
> >>>Given that we're dealing with iomem, I think it will work, but I could be
> >>>missing something obvious.
> >>
> >>Don't we have the race below where both threads can enter the critical
> >>section?
> >>
> >>	// flag f initial zero (unlocked)
> >>
> >>	// t1, flag 1			// t2, flag 2
> >>	readl(f); // reads 0		l = readl(f); // reads 0
> >>
> >>	<thinks lock is free>		<thinks lock is free>
> >>
> >>	writel(1, f);
> >>	readl(f); // reads 1
> >>	<thinks lock owned>
> >>					writel(2, f);
> >>					readl(f) // reads 2
> >>					<thinks lock owned>
> >>
> >>	<crticial section>		<critical section>
> >
> >Urgh, yeah, of course and *that's* what the udelay is trying to avoid,
> >by "ensuring" that the <thinks lock is free> time and subsequent write
> >propagation is all over before we re-read the flag.
> >
> >John -- how much space do you have on this device? Do you have, e.g. a byte
> >for each CPU?
> 
> Hi Will,
> 
> To be clear, the agents in our case are the kernel and UEFI. Within the
> kernel, we use a kernel spinlock to lock the same djtag between threads, for
> these reasons:
> - kernel has a native spinlock

If we only have to effectively deal with two threads, then we might be able
to use something like Dekker's.

> - we are limited in locking values, as the lock flag is only a 8b field in
> v2 hw (called module select)

By 8b do you mean 8 bits or 8 bytes? If the latter, does it support sub-word
accesses?

Will