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Subject: Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon
 Djtag driver
To: Will Deacon <will.deacon@arm.com>, Mark Rutland <mark.rutland@arm.com>
References: <1495457312-237127-1-git-send-email-zhangshaokun@hisilicon.com>
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CC: Shaokun Zhang <zhangshaokun@hisilicon.com>,
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        <anurup.m@huawei.com>, <tanxiaojun@huawei.com>, <xuwei5@hisilicon.com>,
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From: John Garry <john.garry@huawei.com>
Message-ID: <53af9b5b-ac93-eaf9-8551-75fb25a243aa@huawei.com>
Date: Wed, 14 Jun 2017 12:35:07 +0100
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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
- we are limited in locking values, as the lock flag is only a 8b field 
in v2 hw (called module select)

Thanks,
John

>
> Will
>
> .
>