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Date:   Thu, 27 Sep 2018 15:49:23 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:     Christoph Hellwig <hch@lst.de>, iommu@lists.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 3/5] dma-direct: refine dma_direct_alloc zone selection
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References: <20180920185247.20037-1-hch@lst.de> <20180920185247.20037-4-hch@lst.de> <1811156d5a1df1166c7ab7522525619b951f047d.camel@kernel.crashing.org>
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On Thu, Sep 27, 2018 at 11:45:15AM +1000, Benjamin Herrenschmidt wrote:
> I'm not sure this is entirely right.
> 
> Let's say the mask is 30 bits. You will return GFP_DMA32, which will
> fail if you allocate something above 1G (which is legit for
> ZONE_DMA32).

And then we will try GFP_DMA further down in the function:

		if (IS_ENABLED(CONFIG_ZONE_DMA) &&
		    dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
		    !(gfp & GFP_DMA)) {
			gfp = (gfp & ~GFP_DMA32) | GFP_DMA;
			goto again;
		}

This is and old optimization from x86, because chances are high that
GFP_DMA32 will give you suitable memory for the infamous 31-bit
dma mask devices (at least at boot time) and thus we don't have
to deplete the tiny ZONE_DMA pool.