linux-kernel - Re: [PATCH 06/17] arm64: arch_timer: Get rid of erratum_workaround_set

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Date:   Mon, 20 Mar 2017 16:59:19 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Scott Wood <oss@...error.net>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Hanjun Guo <hanjun.guo@...aro.org>,
        Ding Tianhong <dingtianhong@...wei.com>
Subject: Re: [PATCH 06/17] arm64: arch_timer: Get rid of
 erratum_workaround_set_sne

On 20/03/17 14:06, Mark Rutland wrote:
> On Mon, Mar 06, 2017 at 11:26:11AM +0000, Marc Zyngier wrote:
>> Let's move the handling of workarounds affecting set_next_event
>> to the affected function, instead of overwriding the pointers
> 
> Nit: overriding
> 
>> as an afterthough. Yes, this is an extra indirection on the
>> erratum handling path, but the HW is busted anyway.
> 
> [...]
> 
>> +#define erratum_set_next_event_tval_virt(...)		({BUG_ON(1); 0;})
>> +#define erratum_set_next_event_tval_phys(...)		({BUG_ON(1); 0;})
> 
> Nit: BUG()
> 
> [...]
> 
>>  {
>> +	if (needs_unstable_timer_counter_workaround())
>> +		return erratum_set_next_event_tval_virt(evt, clk);
>> +
>>  	set_next_event(ARCH_TIMER_VIRT_ACCESS, evt, clk);
>>  	return 0;
>>  }
>> @@ -487,6 +490,9 @@ static int arch_timer_set_next_event_virt(unsigned long evt,
>>  static int arch_timer_set_next_event_phys(unsigned long evt,
>>  					  struct clock_event_device *clk)
>>  {
>> +	if (needs_unstable_timer_counter_workaround())
>> +		return erratum_set_next_event_tval_phys(evt, clk);
>> +
>>  	set_next_event(ARCH_TIMER_PHYS_ACCESS, evt, clk);
>>  	return 0;
>>  }
> 
> I take it that the new conditionals are NOPs in the !erratum case, so
> this doesn't adversely affect the usual case.

Indeed, we only get an extra NOP on the normal path. The erratum path
triggers the insertion of a jump targeting the end of the function,
where the workaround gets handled.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...