linux-kernel - Re: [PATCH 14/28] nios2: DMA mapping API

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Message-Id: <201404221552.00501.arnd@arndb.de>
Date:	Tue, 22 Apr 2014 15:52:00 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Ley Foon Tan <lftan@...era.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, lftan.linux@...il.com,
	cltang@...esourcery.com
Subject: Re: [PATCH 14/28] nios2: DMA mapping API

On Friday 18 April 2014, Ley Foon Tan wrote:

> +static inline int dma_supported(struct device *dev, u64 mask)
> +{
> +	/* we fall back to GFP_DMA when the mask isn't all 1s,
> +	 * so we can't guarantee allocations that must be
> +	 * within a tighter range than GFP_DMA.
> +	 */
> +	if (mask < 0x00ffffff)
> +		return 0;
> +
> +	return 1;
> +}

> +void *dma_alloc_coherent(struct device *dev, size_t size,
> +			    dma_addr_t *dma_handle, gfp_t gfp)
> +{
> +	void *ret;
> +
> +	/* ignore region specifiers */
> +	gfp &= ~(__GFP_DMA | __GFP_HIGHMEM);
> +
> +	/* optimized page clearing */
> +	gfp |= __GFP_ZERO;
> +
> +	if (dev == NULL || (dev->coherent_dma_mask < 0xffffffff))
> +		gfp |= GFP_DMA;

What is the significance of ZONE_DMA on this architecture?

Do you actually have DMA masters with a 0x00ffffff mask?

	Arnd

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