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From:   Vineet Gupta <vineet.gupta1@synopsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>,
        "linux-snps-arc@lists.infradead.org" 
        <linux-snps-arc@lists.infradead.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Alexey Brodkin" <alexey.brodkin@synopsys.com>
Subject: Re: [PATCH] ARC: IOC: panic if kernel was started with previously
 enabled IOC
Thread-Topic: [PATCH] ARC: IOC: panic if kernel was started with previously
 enabled IOC
Thread-Index: AQHUW+Pi2l0qN/ga1kOYlHrTlH6D2Q==
Date:   Fri, 5 Oct 2018 22:56:52 +0000
Message-ID: <C2D7FE5348E1B147BCA15975FBA23075012B09B9EF@us01wembx1.internal.synopsys.com>
References: <20181004131212.16405-1-Eugeniy.Paltsev@synopsys.com>
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On 10/04/2018 06:12 AM, Eugeniy Paltsev wrote:=0A=
>  =0A=
> diff --git a/arch/arc/mm/cache.c b/arch/arc/mm/cache.c=0A=
> index f2701c13a66b..ee7b63e9c5e3 100644=0A=
> --- a/arch/arc/mm/cache.c=0A=
> +++ b/arch/arc/mm/cache.c=0A=
> @@ -1144,6 +1144,25 @@ noinline void __init arc_ioc_setup(void)=0A=
>  {=0A=
>  	unsigned int ioc_base, mem_sz;=0A=
>  =0A=
> +	/*=0A=
> +	 * Disabling and reconfiguring of IOC are quite a tricky actions becaus=
e=0A=
> +	 * nobody knows what happens if there're IOC-ahndled tarnsactions in=0A=
> +	 * flight when we're disabling IOC.=0A=
> +	 *=0A=
> +	 * And the problem is external DMA masters [that were initialized and=
=0A=
> +	 * set in a bootlaoder that was executed before we got here] might=0A=
> +	 * continue to send data to memory even at this point and we have=0A=
> +	 * no way to prevent that.=0A=
> +	 *=0A=
> +	 * That said it's much safer to not enable IOC at all anywhere before=
=0A=
> +	 * Linux kernel.=0A=
> +	 */=0A=
> +	if (read_aux_reg(ARC_REG_IO_COH_ENABLE) & ARC_IO_COH_ENABLE_BIT)=0A=
> +		panic("kernel was started with previously enabled IOC!\n");=0A=
=0A=
While I understand the needs, this seems excessive, should we warm the user=
,=0A=
instead of panic ? Did you run into specific issue to warrant this !=0A=
=0A=
OTOH in recent past more than 1 person ran into some hsdk uboot shenanigans=
, where=0A=
we had to upgrade the uboot to get it working with prebuit images - is that=
 what=0A=
you are trying to prevent here - panic early instead of random user errors =
/ hangs=0A=
later ?=0A=
=0A=
-Vineet=0A=