linux-kernel - Re: [PATCH] swiotlb: Fix inversed DMA_ATTR_NO

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Date:   Wed, 2 May 2018 18:59:42 +0200
From:   Michel Dänzer <michel@...nzer.net>
To:     Christoph Hellwig <hch@....de>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        iommu@...ts.linux-foundation.org,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH] swiotlb: Fix inversed DMA_ATTR_NO_WARN test

On 2018-05-02 06:21 PM, Christoph Hellwig wrote:
> On Wed, May 02, 2018 at 04:31:09PM +0200, Michel Dänzer wrote:
>>> No.  __GFP_NOWARN (and gfp_t flags in general) are the wrong interface
>>> for dma allocations and just cause problems.  I actually plan to
>>> get rid of the gfp_t argument in dma_alloc_attrs sooner, and only
>>> allow either GFP_KERNEL or GFP_DMA passed in dma_alloc_coherent.
>>
>> How about GFP_TRANSHUGE_LIGHT? TTM uses that to opportunistically
>> allocate huge pages (GFP_TRANSHUGE can result in unacceptably long
>> delays with memory pressure).
> 
> Well, that is exactly what I don't want drivers to do - same for
> __GFP_COMP in some drm code.  This very much assumes the page allocator
> is used to back dma allocations, which very often it actually isn't, and
> any use of magic gfp flags creates a tight coupling of consumers with a
> specific implementation.
> 
> In general I can't think of a good reason not to actually use
> GFP_TRANSHUGE_LIGHT by default in the dma allocator unless
> DMA_ATTR_ALLOC_SINGLE_PAGES is set.  Can you prepare a patch for that?

I'm afraid I'll have to leave that to somebody else.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer