linux-kernel - Re: [PATCH 1/5] bus: arm-cci: use asm unreachable

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Date:   Wed, 21 Mar 2018 09:22:47 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     ard.biesheuvel@...aro.org, arnd@...db.de, 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: [PATCH 1/5] bus: arm-cci: use asm unreachable

On 21.03.2018 00:30, Russell King - ARM Linux wrote:
> On Wed, Mar 21, 2018 at 12:02:02AM +0100, 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();
>>         ^
>>
>> Instead of using the unreachable() macro use the assember variant
>> ASM_UNREACHABLE.  This will no longer emit __builtin_unreachable(),
>> but since the function is naked and its return type is void it seems
>> not to have aversive effects.
> 
> I think that unreachable() there is rather silly - this function
> *does* return, and the comments say as much.  Just delete the silly
> "unreachable()", there's no need to put an ASM_UNREACHABLE in there.
> 
> The function is not declared as not returning, and nothing in this
> file uses it anyway - it's called from the mcpm code, which also
> _does_ expect this function to return (if it doesn't, then we're
> basically saying the CPU that called it is dead.)
> 

Hm, that makes sense. Will just drop unreachable() in the next revision.

Thanks for reviewing!

--
Stefan

>>
>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> ---
>>  drivers/bus/arm-cci.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/bus/arm-cci.c b/drivers/bus/arm-cci.c
>> index 5426c04fe24b..ee9da86fec47 100644
>> --- a/drivers/bus/arm-cci.c
>> +++ b/drivers/bus/arm-cci.c
>> @@ -2084,6 +2084,7 @@ asmlinkage void __naked cci_enable_port_for_self(void)
>>
>>  "	mov	r0, #0 \n"
>>  "	bx	lr \n"
>> +	ASM_UNREACHABLE
>>
>>  "	.align	2 \n"
>>  "5:	.word	cpu_port - . \n"
>> @@ -2103,8 +2104,6 @@ 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();
>>  }
>>
>>  /**
>> --
>> 2.16.2
>>