linux-kernel - Re: [PATCH v1] RISC-V: separate Zbb optimisations requiring and not requiring toolchain support

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Message-ID: <60EFC387-B19B-46A6-AD1B-D6DAC8108836@ventanamicro.com>
Date: Thu, 16 May 2024 12:29:08 +0200
From: Andrew Jones <ajones@...tanamicro.com>
To: Conor Dooley <conor.dooley@...rochip.com>
CC: Conor Dooley <conor@...nel.org>, linux-riscv@...ts.infradead.org,
 xiao.w.wang@...el.com, pulehui@...wei.com,
 Charlie Jenkins <charlie@...osinc.com>,
 Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] RISC-V: separate Zbb optimisations requiring and not requiring toolchain support

On May 16, 2024 10:59:44 AM GMT+02:00, Conor Dooley <conor.dooley@...rochip.com> wrote:
>On Thu, May 16, 2024 at 09:59:44AM +0200, Andrew Jones wrote:
>> On Wed, May 15, 2024 at 04:27:40PM GMT, Conor Dooley wrote:
>> 
>> So the new hidden config is a shorthand for
>> 
>> #if defined(CONFIG_RISCV_ISA_ZBB) && \
>>     defined(CONFIG_TOOLCHAIN_HAS_ZBB) && \
>>     defined(CONFIG_RISCV_ALTERNATIVE)
>> 
>> which is reasonable to add, since that's a mouthful, but I'm not sure the
>> name, RISCV_ISA_ZBB_ALT, does a good job conveying all that.
>> 
>> If we instead just dropped the 'depends on TOOLCHAIN_HAS_ZBB' from
>> config RISCV_ISA_ZBB (keeping the 'depends on RISCV_ALTERNATIVE',
>> since nobody is really complaining about that), then we could change
>> this to
>> 
>> #if defined(CONFIG_RISCV_ISA_ZBB) && defined(CONFIG_TOOLCHAIN_HAS_ZBB)
>
>Yeah, I think this is a cleaner solution.
>
>> >  	asm goto(ALTERNATIVE("j %l[legacy]", "nop", 0,
>> >  				      RISCV_ISA_EXT_ZBB, 1)
>> >  			  : : : : legacy);
>> > diff --git a/arch/riscv/include/asm/bitops.h b/arch/riscv/include/asm/bitops.h
>> > index 880606b0469a..3ed810a6123d 100644
>> > --- a/arch/riscv/include/asm/bitops.h
>> > +++ b/arch/riscv/include/asm/bitops.h
>> > @@ -15,7 +15,7 @@
>> >  #include <asm/barrier.h>
>> >  #include <asm/bitsperlong.h>
>> >  
>> > -#if !defined(CONFIG_RISCV_ISA_ZBB) || defined(NO_ALTERNATIVE)
>> > +#if !defined(CONFIG_RISCV_ISA_ZBB_ALT) || defined(NO_ALTERNATIVE)
>> 
>> nit: It's sufficient to check !defined(CONFIG_RISCV_ISA_ZBB), so no need
>> for this change or its #endif comment change below.
>
>Are you sure? I did test leaving this as-was and it broke the build for
>llvm-14.

Oops, sorry. I didn't look at the full context. You were right.

Thanks,
drew

>
>Cheers,
>Conor.