linux-kernel - Re: [PATCH 22/22] parisc: use generic dma_noncoherent

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Date:   Sat, 21 Apr 2018 22:42:47 +0100
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-arch@...r.kernel.org, Michal Simek <monstr@...str.eu>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        linux-alpha@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-c6x-dev@...ux-c6x.org,
        linux-hexagon@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        nios2-dev@...ts.rocketboards.org, openrisc@...ts.librecores.org,
        linux-parisc@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 22/22] parisc: use generic dma_noncoherent_ops

On Fri, 2018-04-20 at 10:03 +0200, Christoph Hellwig wrote:
> diff --git a/arch/parisc/kernel/setup.c b/arch/parisc/kernel/setup.c
> index 8d3a7b80ac42..4e87c35c22b7 100644
> --- a/arch/parisc/kernel/setup.c
> +++ b/arch/parisc/kernel/setup.c
> @@ -97,14 +97,12 @@ void __init dma_ops_init(void)
>  		panic(	"PA-RISC Linux currently only supports
> machines that conform to\n"
>  			"the PA-RISC 1.1 or 2.0 architecture
> specification.\n");
>  
> -	case pcxs:
> -	case pcxt:
> -		hppa_dma_ops = &pcx_dma_ops;
> -		break;
>  	case pcxl2:
>  		pa7300lc_init();
>  	case pcxl: /* falls through */
> -		hppa_dma_ops = &pcxl_dma_ops;
> +	case pcxs:
> +	case pcxt:
> +		hppa_dma_ops = &dma_noncoherent_ops;
>  		break;
>  	default:
>  		break;

Well, this is wrong: you just made every 32 bit parisc system
unnecessarily use non-coherent.  We actually only have a small small
set of non-coherent systems.  The pxcs and pcxt systems (which are
about 99% of the user base) can use coherent dma ops.  The problem
seems to be in your new world you only have one dma_noncoherent_ops
pointer ... we definitely need two on parisc, so whether
arch_dma_cache_sync is present or not needs to be dynamic not config
defined.

James