linux-kernel - Re: [PATCH v2 1/4] ARC: allow to override default mcpu compiler flag

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Date:   Tue, 16 Jun 2020 22:12:27 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
CC:     "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] ARC: allow to override default mcpu compiler flag

On 6/5/20 3:47 AM, Alexey Brodkin wrote:
> Hi Eugeniy,
> 
> A couple of minor notes below.
> 
>> -----Original Message-----
>> From: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
>> Sent: Thursday, June 4, 2020 8:39 PM
>> To: linux-snps-arc@...ts.infradead.org; Vineet Gupta <vgupta@...opsys.com>
>> Cc: linux-kernel@...r.kernel.org; Alexey Brodkin <abrodkin@...opsys.com>; Eugeniy Paltsev
>> <paltsev@...opsys.com>
>> Subject: [PATCH v2 1/4] ARC: allow to override default mcpu compiler flag
>>
>> Kernel builds set their own default -mcpu for a given ISA build.
> 
> We used to use a default "-mcpu" per ARC ISA version (one for ARCompact
> and one for ARCv2).
> 
>> But that gets in the way of "custom" -mcpu flags from propagating
>> into kernel build.
> 
> But with more versions of CPUs & SoCs becoming available we want to
> be able to fine-tune generated code more precise.
> 
>> This will also be used in next patches for HSDK-4xD board support which
>> uses a different -mcpu to effect dual issue scheduling.
> 
> "...for utilization of the new CPU's dual-issue capabilities"?
> 
>> +++ b/arch/arc/Makefile
>> @@ -10,8 +10,25 @@ CROSS_COMPILE := $(call cc-cross-prefix, arc-linux- arceb-linux-)
>>  endif
>>
>>  cflags-y	+= -fno-common -pipe -fno-builtin -mmedium-calls -D__linux__
>> -cflags-$(CONFIG_ISA_ARCOMPACT)	+= -mA7
>> -cflags-$(CONFIG_ISA_ARCV2)	+= -mcpu=hs38
>> +
>> +tune-mcpu-def-$(CONFIG_ISA_ARCOMPACT)	:= -mA7
> 
> I'd suggest to either swap "-mA7" which is being obsoleted with "-mcpu=arc700"
> right here or as a separate change, otherwise we may soon get ATC700 builds
> broken with newer compilers.

FWIW I've added a modified variant of this patch to for-curr as we need it for
ongoing ARC64 port as well.

-Vineet