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

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Message-ID: <1cbc4e10-96e6-c222-fb1a-fd7847be5755@gmail.com>
Date:   Tue, 22 May 2018 15:13:58 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Michel Dänzer <michel@...nzer.net>,
        Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Christian König <christian.koenig@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] swiotlb: Fix inversed DMA_ATTR_NO_WARN test

Am 02.05.2018 um 18:59 schrieb Michel Dänzer:
> 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.

Coming back to this topic once more, sorry for the delay but busy as 
usual :)

What exactly do you mean with "dma allocator" here? The TTM allocator 
using the dma_alloc_coherent calls? Or the swiotlb implementation of the 
calls?

Christian.