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

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Message-ID: <MWHPR1201MB0046338E4B38598FFB5DBCB9A1860@MWHPR1201MB0046.namprd12.prod.outlook.com>
Date:   Fri, 5 Jun 2020 10:47:57 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Eugeniy Paltsev" <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: RE: [PATCH v2 1/4] ARC: allow to override default mcpu compiler flag

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.

-Alexey