linux-kernel - Re: [RFC v2] dma-mapping: Use unsigned long for dma

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Date:	Wed, 01 Jun 2016 08:08:11 +0200
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Russell King <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Joerg Roedel <joro@...tes.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Hocko <mhocko@...e.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Arnd Bergmann <arnd@...db.de>,
	Andy Lutomirski <luto@...nel.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	xen-devel@...ts.xenproject.org, dri-devel@...ts.freedesktop.org,
	linux-samsung-soc@...r.kernel.org,
	iommu@...ts.linux-foundation.org, hch@...radead.org,
	sstabellini@...nel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [RFC v2] dma-mapping: Use unsigned long for dma_attrs

On 05/31/2016 08:15 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, May 30, 2016 at 01:54:06PM +0200, Krzysztof Kozlowski wrote:
>> The dma-mapping core and the implementations do not change the
>> DMA attributes passed by pointer.  Thus the pointer can point to const
>> data.  However the attributes do not have to be a bitfield. Instead
>> unsigned long will do fine:
>>
>> 1. This is just simpler.  Both in terms of reading the code and setting
>>    attributes.  Instead of initializing local attributes on the stack and
>>    passing pointer to it to dma_set_attr(), just set the bits.
>>
>> 2. It brings safeness and checking for const correctness because the
>>    attributes are passed by value.
> 
> 
> .. why not go the next step a do an enum? Perhaps that should be mentioned
> as part of the description?

These are additive flags so to me this would look a little bit weird:
enum dma_attr {
	DMA_ATTR_WRITE_BARRIER	= 0x1,
	DMA_ATTR_WEAK_ORDERING	= 0x2,
	DMA_ATTR_WRITE_COMBINE	= 0x4,
	DMA_ATTR_NON_CONSISTENT	= 0x8,
	...
}

It doesn't really look like enumeration.

Best regards,
Krzysztof