linux-kernel - Re: [PATCH v2 RFC/RFT 1/5] ARM: dma-mapping: Add fallback normal page allocations

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Date:   Wed, 24 Apr 2019 11:33:11 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     robin.murphy@....com, vdumpa@...dia.com, linux@...linux.org.uk,
        catalin.marinas@....com, will.deacon@....com, joro@...tes.org,
        m.szyprowski@...sung.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        tony@...mide.com
Subject: Re: [PATCH v2 RFC/RFT 1/5] ARM: dma-mapping: Add fallback normal
 page allocations

Hi Christoph,

On Wed, Apr 24, 2019 at 05:06:38PM +0200, Christoph Hellwig wrote:
> On Tue, Mar 26, 2019 at 04:01:27PM -0700, Nicolin Chen wrote:
> >  	page = dma_alloc_from_contiguous(dev, count, order, gfp & __GFP_NOWARN);
> > +	if (!page)
> > +		page = alloc_pages(gfp, order);
> 
> We have this fallback in most callers already.  And with me adding
> it to the dma-iommu code in one series, and you to arm here I think
> we really need to take a step back and think of a better way
> to handle this, and the general mess that dma_alloc_from_contiguous.
> 
> So what about:

Thanks for the suggestion!

>  (1) change the dma_alloc_from_contiguous prototype to be:
> 
> struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp);
> 
>      that is: calculate order and count internally, pass the full gfp_t
>      and mask it internally, and drop the pointless from in the name.
>      I'd also use the oppurtunity to forbid a NULL dev argument and
>      opencode those uses.
>  
>  (2) handle the alloc_pages fallback internally.  Note that we should
>      use alloc_pages_node as we do in dma-direct.

I feel it's similar to my previous set, which did most of these
internally except the renaming part. But Catalin had a concern
that some platforms might have limits on CMA range [1]. Will it
be still okay to do the fallback internally?

[1: https://www.spinics.net/lists/arm-kernel/msg714295.html ]