From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
Subject: Re: [PATCH 3/5] dma-direct: refine dma_direct_alloc zone selection
Date: Thu, 27 Sep 2018 15:49:23 +0200
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To: Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>
Cc: Greg Kroah-Hartman <gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org>, linuxppc-dev-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>, Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.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.