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Date:   Tue, 16 Mar 2021 08:56:58 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        Minturn Dave B <dave.b.minturn@intel.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Dan Williams <dan.j.williams@intel.com>,
        Stephen Bates <sbates@raithlin.com>,
        Jakowski Andrzej <andrzej.jakowski@intel.com>,
        Christoph Hellwig <hch@lst.de>,
        Xiong Jianxin <jianxin.xiong@intel.com>
Subject: Re: [RFC PATCH v2 06/11] dma-direct: Support PCI P2PDMA pages in
 dma-direct map_sg
Message-ID: <20210316075658.GA15949@lst.de>
References: <20210311233142.7900-1-logang@deltatee.com> <20210311233142.7900-7-logang@deltatee.com> <215e1472-5294-d20a-a43a-ff6dfe8cd66e@arm.com> <d7ead722-7356-8e0f-22de-cb9dea12b556@deltatee.com> <a8205c02-a43f-d4e8-a9fe-5963df3a7b40@arm.com>
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On Fri, Mar 12, 2021 at 06:11:17PM +0000, Robin Murphy wrote:
> Sure, that's how things stand immediately after this patch. But then 
> someone comes along with the perfectly reasonable argument for returning 
> more expressive error information for regular mapping failures as well 
> (because sometimes those can be terminal too, as above), we start to get 
> divergent behaviour across architectures and random bits of old code subtly 
> breaking down the line. *That* is what makes me wary of making a 
> fundamental change to a long-standing "nonzero means success" interface...

Agreed.  IMHO dma_map_sg actually needs to be switched to return
unsigned to help root this out, going the other way is no helpful.