linux-kernel - Re: [RFC PATCH v2 06/11] dma-direct: Support PCI P2PDMA pages in dma-direct map

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Date:   Tue, 16 Mar 2021 08:56:58 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Minturn Dave B <dave.b.minturn@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jason Ekstrand <jason@...kstrand.net>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        Stephen Bates <sbates@...thlin.com>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Christoph Hellwig <hch@....de>,
        Xiong Jianxin <jianxin.xiong@...el.com>
Subject: Re: [RFC PATCH v2 06/11] dma-direct: Support PCI P2PDMA pages in
 dma-direct map_sg

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.