linux-kernel - Re: [PATCH] ARC: IOC: panic if kernel was started with previously enabled IOC

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075012B09B9EF@us01wembx1.internal.synopsys.com>
Date:   Fri, 5 Oct 2018 22:56:52 +0000
From:   Vineet Gupta <vineet.gupta1@...opsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey Brodkin" <alexey.brodkin@...opsys.com>
Subject: Re: [PATCH] ARC: IOC: panic if kernel was started with previously
 enabled IOC

On 10/04/2018 06:12 AM, Eugeniy Paltsev wrote:
>  
> diff --git a/arch/arc/mm/cache.c b/arch/arc/mm/cache.c
> index f2701c13a66b..ee7b63e9c5e3 100644
> --- a/arch/arc/mm/cache.c
> +++ b/arch/arc/mm/cache.c
> @@ -1144,6 +1144,25 @@ noinline void __init arc_ioc_setup(void)
>  {
>  	unsigned int ioc_base, mem_sz;
>  
> +	/*
> +	 * Disabling and reconfiguring of IOC are quite a tricky actions because
> +	 * nobody knows what happens if there're IOC-ahndled tarnsactions in
> +	 * flight when we're disabling IOC.
> +	 *
> +	 * And the problem is external DMA masters [that were initialized and
> +	 * set in a bootlaoder that was executed before we got here] might
> +	 * continue to send data to memory even at this point and we have
> +	 * no way to prevent that.
> +	 *
> +	 * That said it's much safer to not enable IOC at all anywhere before
> +	 * Linux kernel.
> +	 */
> +	if (read_aux_reg(ARC_REG_IO_COH_ENABLE) & ARC_IO_COH_ENABLE_BIT)
> +		panic("kernel was started with previously enabled IOC!\n");

While I understand the needs, this seems excessive, should we warm the user,
instead of panic ? Did you run into specific issue to warrant this !

OTOH in recent past more than 1 person ran into some hsdk uboot shenanigans, where
we had to upgrade the uboot to get it working with prebuit images - is that what
you are trying to prevent here - panic early instead of random user errors / hangs
later ?

-Vineet