linux-kernel - Re: DMA mapping fill dma

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Date:   Wed, 28 Apr 2021 12:25:35 +0200
From:   Corentin Labbe <clabbe.montjoie@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     hch@....de, m.szyprowski@...sung.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: DMA mapping fill dma_address to 0

Le Wed, Apr 28, 2021 at 11:06:10AM +0100, Robin Murphy a écrit :
> On 2021-04-28 09:42, Corentin Labbe wrote:
> > Hello
> > 
> > I work on the crypto offloader driver of cortina/gemini SL3516 SoC.
> > I test it by filling a LUKS2 partition.
> > I got a reproductible problem when handling skcipher requests.
> > I use dma_map_sg() and when iterating other the result, sg_dma_address(sg) return 0.
> > But sg_dma_len(sg) is still correct (4096 in my case).
> > 
> > Below is a simplified view of my code:
> > nr_sgs = dma_map_sg(ce->dev, areq->src, sg_nents(areq->src), DMA_TO_DEVICE);
> > (nr_sgs = 1 in my case)
> > sg = areq->src;
> > if (!sg_dma_address(sg))
> > 	FAIL
> 
> What is this check supposed to be for in the first place? 0 is a valid 
> DMA address, because it's also a valid physical address, and I recall 
> RAM at PA 0 on Hikey 960 flushing out some bugs in the past when we 
> tried to use 0 for DMA_MAPPING_ERROR. All the Gemini DTs appear to show 
> RAM starting at PA 0 too, so I'd have to guess that it's simply the case 
> that your DMA buffer happened to end up using that particular page.
> 
> Robin.
> 

Yes, 0 is a valid DMA address.
I just find it by going further and printing mem_map value and testing it against sg_page() return.

So my original problem was not related to this.
Sorry for the noise.
Thanks