linux-kernel - Re: [PATCH 3/5] dma-direct: refine dma_direct

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Date:   Fri, 28 Sep 2018 10:06:48 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 3/5] dma-direct: refine dma_direct_alloc zone selection

On Thu, 2018-09-27 at 15:49 +0200, Christoph Hellwig wrote:
> 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.

I see, it's rather confusing :-) Wouldn't it be better to check against
top of 32-bit memory instead here too ?


Cheers,
Ben.