linux-kernel - Re: [RESEND PATCH v2] bus: arm-cci: use asm unreachable

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Date:   Tue, 08 May 2018 15:01:53 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     Robin Murphy <robin.murphy@....com>
Cc:     arm@...nel.org, arnd@...db.de, linux@...linux.org.uk,
        ard.biesheuvel@...aro.org, nicolas.pitre@...aro.org,
        marc.zyngier@....com, behanw@...verseincode.com,
        keescook@...omium.org, Bernhard.Rosenkranzer@...aro.org,
        mka@...omium.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v2] bus: arm-cci: use asm unreachable

On 08.05.2018 14:19, Robin Murphy wrote:
> On 08/05/18 12:32, Stefan Agner wrote:
>> Mixing asm and C code is not recommended in a naked function by
>> gcc and leads to an error when using clang:
>>    drivers/bus/arm-cci.c:2107:2: error: non-ASM statement in naked
>>    function is not supported
>>          unreachable();
>>          ^
>>
>> While the function is marked __naked it actually properly return
>> in asm. There is no need for the unreachable() call.
> 
> The commit title is a bit out-of-date now (I guess it could just be
> something like "remove unnecessary unreachable()" now), but the rest
> looks OK to me - even GCC clearly doesn't expect anything beyond the
> asm to be reachable anyway since the lack of epilogue includes the
> lack of any compiler-generated return. I've checked that GCC 7.2
> generates identical object files before and after, other than (for
> obvious reasons) the line numbers generated by WANT_WARN_ON_SLOWPATH
> for all the WARN()s appearing later in the file.

Title: Oops, good catch! Will send a v3.

Thanks for checking gcc.

--
Stefan

> 
> Reviewed-by: Robin Murphy <robin.murphy@....com>
> 
>> Suggested-by: Russell King <linux@....linux.org.uk>
>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> Acked-by: Nicolas Pitre <nico@...aro.org>
>> ---
>>   drivers/bus/arm-cci.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/drivers/bus/arm-cci.c b/drivers/bus/arm-cci.c
>> index 443e4c3fd357..b8184a903583 100644
>> --- a/drivers/bus/arm-cci.c
>> +++ b/drivers/bus/arm-cci.c
>> @@ -371,8 +371,6 @@ asmlinkage void __naked cci_enable_port_for_self(void)
>>   	[sizeof_struct_cpu_port] "i" (sizeof(struct cpu_port)),
>>   	[sizeof_struct_ace_port] "i" (sizeof(struct cci_ace_port)),
>>   	[offsetof_port_phys] "i" (offsetof(struct cci_ace_port, phys)) );
>> -
>> -	unreachable();
>>   }
>>     /**
>>