Messages in this thread | | | Date | Thu, 27 Sep 2018 15:49:23 +0200 | From | Christoph Hellwig <> | Subject | Re: [PATCH 3/5] dma-direct: refine dma_direct_alloc zone selection |
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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.
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