linux-kernel - Re: [RFC] ARC: allow to use IOC and non-IOC DMA devices simultaneously

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Date:   Fri, 15 Jun 2018 15:17:10 +0200
From:   Christoph Hellwig <hch@....de>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
Cc:     linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>, hch@....de
Subject: Re: [RFC] ARC: allow to use IOC and non-IOC DMA devices
        simultaneously

> +#ifndef ASM_ARC_DMA_MAPPING_H
> +#define ASM_ARC_DMA_MAPPING_H
> +
> +#define arch_setup_dma_ops arch_setup_dma_ops
> +
> +#include <asm-generic/dma-mapping.h>
> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +			const struct iommu_ops *iommu, bool coherent);

Can you keep the #define for arch_setup_dma_ops right next to the
prototype?

> index 000000000000..9d0d310bbf5a
> --- /dev/null
> +++ b/arch/arc/mm/dma-mapping.c
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// (C) 2018 Synopsys, Inc. (www.synopsys.com)
> +
> +#include <linux/dma-mapping.h>
> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +			const struct iommu_ops *iommu, bool coherent)
> +{
> +	const struct dma_map_ops *dma_ops = &dma_noncoherent_ops;
> +
> +	/*
> +	 * IOC hardware snoops all DMA traffic keeping the caches consistent
> +	 * with memory - eliding need for any explicit cache maintenance of
> +	 * DMA buffers - so we can use dma_direct cache ops.
> +	 */
> +	if (is_isa_arcv2() && ioc_enable && coherent)
> +		dma_ops = &dma_direct_ops;
> +
> +	set_dma_ops(dev, dma_ops);
> +}


Why not just write the routines as:

void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
			const struct iommu_ops *iommu, bool coherent)
{
	if (is_isa_arcv2() && ioc_enable && coherent)
		set_dma_ops(dev, &dma_direct_ops);
	else
		set_dma_ops(dev, &dma_noncoherent_ops);
}

Also a new file just for this routine seems a little wasteful.  Can't
you just squeeze it into setup.c or some other file?